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AN ASSESSMENT OF THE ECOSYSTEM SERVICES PROVIDED BY THE STREET TREES OF OLYMPIA WASHINGTON
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2020
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Zarghami, Heidi
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Thesis_MES_2021_Zarghami
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AN ASSESSMENT OF THE ECOSYSTEM SERVICES
PROVIDED BY THE STREET TREES
OF OLYMPIA WASHINGTON
by
Heidi Zarghami
A Thesis
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
May 2020
©2020 by Heidi Zarghami. All rights reserved.
This Thesis for the Master of Environmental Studies Degree
by
Heidi Zarghami
has been approved for
The Evergreen State College
by
________________________
Kathleen Saul
Member of the Faculty
________________________
Date
ABSTRACT
An assessment of the ecosystem services provided by the street trees
within the City of Olympia, Washington.
Heidi Zarghami
Urban forests provide a number of ecosystem services, including improved air
quality, stormwater processing, cooling, and health benefits to the community and are
considered by the literature to be an important component of city planning to become
more sustainable and resilient. To date, the City of Olympia has not yet assessed the
ecosystem services of their street trees, specifically how much these trees contribute to
the health of the community and the environment in a given year. This study resolves this
gap in research by answering the question: What are the annual ecosystem services
provided by the street trees in Olympia, Washington? This study quantified the air
quality, stormwater, energy savings, and carbon storage and sequestration services
provided every year by street trees in both quantitative and monetary amounts. The study
utilizes GIS tree inventory data from the City of Olympia’s Urban Forestry Department
on the conditions and dimensions of the 2,483 street trees. My results quantified the
annual fiscal benefits and ecosystem services of the city’s street trees using iTree Streets
and iTree Eco software and include an ArcGIS geospatial analysis of the street trees
population. The results of this thesis provide a more comprehensive assessment,
including a cost-benefit analysis, of street trees for the City of Olympia. This study found
that the annual ecosystem service benefits provided by the street trees outweighed the
annual urban forest program costs, and determined that the energy reduction services of
street trees were the most important service they provide to the City of Olympia and its
residents.
Table of Contents
Introduction................................................................................................................ 1
Lay of the Land ............................................................................................................... 2
Regional tree studies and study significance.................................................................. 4
Literature Review ...................................................................................................... 8
Urban environments as ecosystems .................................................................................... 8
Trees as a green network................................................................................................... 13
Urban trees in a changing world ....................................................................................... 18
How valuable is a tree? ..................................................................................................... 25
Methodology ............................................................................................................. 32
Stage I: Emission rates and regional data
33
Emission rates .............................................................................................................. 34
Regional data................................................................................................................ 35
Stage II: iTree Analysis .................................................................................................... 37
iTree Streets software methodology ............................................................................. 38
iTree Eco software methodology .................................................................................. 40
Stage III: ArcGIS Methods ............................................................................................... 41
Results ....................................................................................................................... 44
Urban Forest Structure ................................................................................................ 44
iTree Streets Software Results ...................................................................................... 48
iTree Eco Software Results ........................................................................................... 54
ArcGIS Results.............................................................................................................. 60
Discussion and Recommendations ......................................................................... 68
Trees and carbon services ............................................................................................ 69
Street trees and energy reduction ................................................................................. 72
Street trees and air quality ........................................................................................... 77
Street trees and stormwater .......................................................................................... 80
Limitations and recommendations ............................................................................... 83
Conclusion ................................................................................................................ 87
iv
Bibliography ............................................................................................................. 90
Appendix A. Emission Rates ................................................................................... 94
Appendix A1. Research methods for acquiring rates. .................................................. 94
Appendix A2. Olympia City Data ................................................................................ 95
Appendix B. Regional Data ..................................................................................... 96
Appendix B1. Olympia Tree Inventory ......................................................................... 96
Appendix B2. Olympia City sidewalk and street design standards .............................. 97
Appendix B3. Annual expenses by Olympia’s Urban Forestry program. .................... 98
Appendix C. iTree Methods ................................................................................... 99
Appendix C1. Land use data entered into iTree programs .......................................... 99
Appendix C2. Tree measurement variables entered into iTree programs. ................ 100
Appendix C3. Eco Canopy Health Classes ................................................................ 101
Appendix D. ArcGIS workflow ............................................................................ 102
Appendix E. iTree Results Comparison .............................................................. 103
v
List of Figures
Figure 1. Tree plot locations and the tree conditions map. These plots are maintained by the City of
Olympia. ......................................................................................................................................... 3
Figure 2. Thermal infrared image indicating the daytime difference in radiative surface temperature
between exposed and tree-shaded pavement. (Pearlmutter et al., 2017) ...................................... 20
Figure 3. Thurston Climate Action Team report showing that the top emitters of greenhouse gases are
building and vehicle emissions. .................................................................................................... 21
Figure 4. Legion Way tree “topping” diagram of effects on long term tree health and maintenance. ......... 28
Figure 5. The 2015 Energy Flow Diagram is based on actual and estimated data obtained from Puget
Sound Energy (electricity) and TRPC estimates (transportation ................................................. 34
Figure 6. Tree dimension measurements diagram ....................................................................................... 36
Figure 7. Climate zones used in iTree Streets to determine average tree growth rate data and climate
conditions (Vargas, 2018) ............................................................................................................ 38
Figure 8. This workflow diagram illustrates the methods undertaken in GIS to analyze the urban forest
structure using the City of Olympia’s tree inventory dataset. ...................................................... 42
Figure 9. Diagram showing how satellite imagery sensors can detect leaf vitality using the near infrared
response of healthy leaves using the Normalized Difference Vegetation Index (NDVI) (Earth
Observing System, n.d.). .............................................................................................................. 43
Figure 10. iTree Eco results for the top ten species size distribution within the street tree population. ...... 46
Figure 11. iTree Streets results for DBH distribution within the street tree population. Note that the BDL
Other (Broadleaf Deciduous Large trees) category was attributed to tree species not recognized
by iTree Streets. ............................................................................................................................ 46
Figure 12. Water interception and evaporation provided by trees per month for the weather data year 2016.
...................................................................................................................................................... 57
Figure 13. iTree Eco results for monthly pollutant removal trends by Olympia’s street trees. .................... 58
Figure 14. GIS energy benefits maps of trees to buildings in Olympia ....................................................... 61
Figure 15. Heat Index GIS map showing the Downtown Olympia area and the heat response of buildings
in bright red and orange, and the tree locations shown in the black circles. This image
demonstrates the cooling potential of street trees in dense urban areas like Olympia. ................ 62
Figure 16. GIS NDVI raster image of Legion Way showing the near-red infrared response (red color) from
vegetation during the Winter (leaf-off) season. The dark circles indicate where street trees are
located........................................................................................................................................... 63
Figure 17. Planting sites (growspace) examples of street trees in Olympia. ................................................ 64
Figure 18. ArcGIS Insights results for growspace trends and relative DBH distributions of street trees. ... 65
Figure 19. ArcGIS Insights results for DBH trends showing the location and sizes of the street trees. ...... 65
Figure 20. ArcGIS Insights results for Land Use and Tree Condition trends .............................................. 66
vi
Figure 21. Outreach and education storymap using Esri Storymaps............................................................. 67
Figure 22. ArcGIS Insights results for growspace trends.............................................................................. 70
Figure 23. iTree Streets graph results from iTree Streets showing the total amount of ecosystem services
provided by each street tree in an average year ............................................................................ 72
Figure 24. Heat Index GIS maps of Westside Olympia and Downtown Olympia........................................ 74
Figure 25. All trees within 60-feet of buildings are here colored purple, and all trees 60-feet from a tree are
colored blue. Visual assessment of tree energy benefits to buildings may be possible from this
GIS method of analysis. ................................................................................................................ 75
Figure 26. All trees within 60-feet of buildings are purple, and all trees 60-feet from a tree are blue. Visual
assessment of tree energy benefits to buildings may be possible from this GIS method of
analysis. ......................................................................................................................................... 76
Figure 27. Tree energy range in blue across the City of Olympia, showing the range of cooling, and air
quality services provided by street trees. ...................................................................................... 77
Figure 28. iTree Eco results for monthly pollutant removal trends by Olympia’s street trees. ……...........79
Figure 29. iTree Eco result of water interception and evaporation provided by trees each month for the
weather data year of 2016, showing a dramatic rise in stormwater benefits in leaf-on seasons. .. 80
Figure 30. NDVI analysis using aerial imagery of Downtown Olympia in winter, showing red chloryphil
response of grass and conifer trees, but no response from street trees. ........................................ 81
Figure 31. ArcGIS Insights results for DBH trends showing the location and sizes of the street trees. ....... 82
vii
List of Tables
Table 1. Status report for TRPC in spring 2018 shows upward trend in emissions by year. ....................... 22
Table 2. Ten most important species, collectively accounting for 60% of Olympia’s street tree population.
...................................................................................................................................................... 45
Table 3. iTree Eco growspace distributions ............................................................................................... 47
Table 4. iTree Streets results for annual stormwater intercepted by species, and citywide total in gallons
and dollars. ................................................................................................................................... 48
Table 5. iTree Streets results for annual air quality shown in total pounds of deposition (pollutants
intercepted by the trees), total pounds avoided (energy emissions reduced thereby reducing air
pollutants), and total biogenic volatile compound (BVOC) emissions (natural emissions from
trees) here shown as a negative value........................................................................................... 49
Table 6. iTree Streets results for annual energy savings citywide from the street trees shown as total
savings of megawatt hours and dollars, and total savings of natural gas in Therms and dollars. 50
Table 7. iTree Streets results for annual carbon dioxide sequestration in pounds and dollars, and avoided
emissions in pounds and dollars. .................................................................................................. 51
Table 8. iTree Streets results for the total stored carbon dioxide by the street trees in pounds and dollars. 52
Table 9. Report of the annual cost-benefit breakdown for Olympia. As shown, the overall net ecosystem
service benefits surpass the costs of tree maintenance. ................................................................ 53
Table 10. Excerpt from iTree Eco results for street tree benefits (ex. carbon storage and sequestration) by
tree species.................................................................................................................................... 56
Table 11. iTree Streets results for annual air quality shown in total pounds of deposition ......................... 78
viii
KEYWORDS
Anthropocene: a proposed geological epoch dating from the commencement of significant human impact
on Earth's geology and ecosystems, including, but not limited to, anthropogenic climate change.
Ecosystem services: the broad range of beneficial services to humans as provided by natural systems.
GIS: geographical information systems mapping software used for geospatial analysis of landscapes.
Green infrastructure: refers to biotic systems—such as street trees, parks, and living shorelines—that
replace or support the services provided by grey infrastructure.
Grey infrastructure: Man-made infrastructure, often within an urban environment, engineered to manage
natural processes such as water flow, stormwater, and erosion.
iTree Eco (Eco): a software program designed to quantify forest structure, environmental effects, and
value to communities.
iTree Streets (iTree Streets): a software program designed to analyze urban street trees by determining
the ecosystem services and the cost-benefits of urban forestry planning and maintenance.
LID: low impact development is a form of green infrastructure related to site development and on-site
stormwater processing using natural filtration processes.
NDVI: Normalized Difference Vegetation Index: a method to determine vegetation health using aerial
imagery within GIS.
Street tree: a tree planted in the public right-of-way, usually in the planting strip between sidewalk and
road, or approximately 10 feet from the curb or roadside if a sidewalk is not present.
Urban Forest: trees within an urban landscape growing on both public and private lands.
UHI: urban heat island effect. A measured increase in average temperature in urban areas dominated by
non-porous surfaces and industrial processes.
Urban Resilience: the measurable ability of an urban system or community to withstand and quickly
recover from stressors and shock.
Urban Sustainability: the theoretical perspective that natural resources and waste production within urban
areas should be efficiently managed in order to support and enable the well-being of current and
future populations of humans and other living things.
USFS: United States Forest Service.
Water interception: rainfall stored temporarily on tree leaves which then drips down the body of the tree,
falls off the leaves into the ground, or is evaporated into the atmosphere.
ix
Acknowledgments
This thesis was not created in a vacuum. The idea for this work came about while
enjoying a beer with a friend and looking out over Downtown Olympia. I told him “I
wonder what our trees are doing for us right now— how much stormwater are they
processing? How much carbon are they storing?” My excitement about the thought, and
the conversation that followed, led me down the rabbit hole of this research, which is
now laid out in detail on the following pages. Along the way many people held space for
me as I thought aloud; guiding me as I structured my research design, and listened to me
lament about the challenges I faced as I progressed further towards the finish line. I am
thankful to you all.
The skills to create my research design and GIS maps were gained over the two
years of the Master of Environmental Studies program at Evergreen State College, and I
want to thank Kevin Francis and the entire faculty for accepting me into this program.
I would like to thank my thesis reader, Kathleen Saul for her guidance and
unrelenting encouragement throughout my thesis process. Each time I left our meetings I
felt a renewed sense of purpose in my work, and inspired to consider new possibilities.
Thank you to Julian Wischniewski and Anna Duron, for being an amazing peerreview team, and to Susan McCleary for taking me on as an intern and for cheering me
on as I worked to complete my research while working with the City of Olympia. To
Michelle Bentley for having tea with me and encouraging me along while patiently
answering my million-and-one questions, and to the Woody Schaufler for providing me
with the vital information needed to make this research possible.
Last, but never least, I dedicate this thesis to the one person without whom
graduate school would have been out of reach: my best friend, adopted family-member,
and favorite human-robot-alien hybrid, Michael Zarghami.
Thank you.
x
INTRODUCTION
Like many other cities today, the City of Olympia is dealing with climate-related
stressors and faced with the difficult task of creating and implementing climate
adaptation plans for the health and sustainability of the urban environment and their
residents (Haub, Harrington, McGowan, & Reed, 2007). The natural benefits of urban
trees can help to mitigate the stressors of climate change within the City of Olympia,
though it is still underrepresented in regional climate mitigation plans (Meerow, Newell,
& Stults, 2016; Pearlmutter et al., 2017; TRPC, 2017). At the same time, the approaches
to urban forestry and tree valuation in Olympia have focused on aesthetic amenities and
the traditional metrics of tree appraisal and tree maintenance costs— what has not yet
been researched are the many benefits of their trees for the social, economic, and
environmental health of the city and its residents, referred to as ecosystem services
(AMEC, 2011; CFC, 2016; Roush & McFarland, 2006). Ecosystem services are the broad
range of services provided by trees including the ability of urban trees to improve air
quality, process stormwater, store and sequester carbon, and reduce energy demands by
cooling the surrounding environment (Grant, 2012; Young, 2011).
This study highlights the importance of the many services provided by trees to the
health and well-being of city residents, and to the long term sustainability of the City of
Olympia in the face of the complex challenges of regional climate change and climate
adaptation. Determining the exact content and distribution of the benefits, and their
1
extent, is the key to understanding the important contributions of our urban trees. By
utilizing the existing street tree inventory maintained by the City of Olympia’s urban
forestry program, regional satellite imagery in Geographical Information System (GIS)
software, and iTree Streets and iTree Eco (ECO) urban forestry software, this study has
determined the costs and benefits of street tree ecosystem services. In addition, this study
was able to determine the annual contribution of street trees to improve air quality,
reduce stormwater flows and energy demands, and sequester and store carbon in both
weight and dollar amount. The exciting results of this study further illustrate the
importance of including street trees in urban planning for climate change adaptation
within the City of Olympia and Thurston County, and the need for more research on the
contributions of our entire urban forest.
Lay of the Land
The 2,500 street tree plots maintained by Olympia’s Urban Forestry department
collectively make up only part of the urban forest. Urban forests consist of trees on both
public and private lands, which together create a green network that offers the community
the array of services referred to as ecosystem services (Young, 2011). The broad range of
services includes the ability of urban trees to regulate air temperatures, shade sidewalks
and nearby buildings, store and filter stormwater, and store and sequesters carbon (Grant,
2012). The ecosystem services of urban forests are considered by many cities throughout
the United States to be an important component of urban planning in efforts to become
2
more sustainable and resilient (Hastie, 2003; Kuehler, Hathaway, & Tirpak, 2017;
Young, 2011).
In this study, I use a sample of street trees to represent the urban forest within the
City of Olympia to determine these beneficial services. This sample is composed mainly
of a deciduous species, with an average trunk size measured at diameter breast height
(DBH) of less than 9 inches. Taking these factors into account, the methodology outlined
in this study illustrates how we can begin to use existing street tree inventory data to
quantify, monetize, and thus understand in a number of different ways their beneficial
ecosystem contributions.
Figure 1. Tree plot locations and the tree conditions map. These plots are maintained by the City of Olympia.
The street tree inventory of Olympia as seen in Figure 1 is fairly new, having been
created in GIS with funds from a Department of Natural Resources (DNR) grant in 2016
(CFC, 2016). The inventory grant funds allowed for a maximum of 2,500 trees to be
3
assessed by a trained arborist and entered into the inventory database, so the main arteries
throughout the City and downtown Olympia were chosen for inclusion in the GIS
inventory for seasonal tree maintenance. These areas were prioritized primarily because
the City recognized that downed limbs in these areas, especially during extreme weather
events and natural disasters, would block the roads, bus routes, and emergency vehicles
from being able to move throughout the city to provide essential services (W. Schaufler,
pers. comm. Mar. 5, 2020). City workers needed information about the trees to maintain
them properly. Additionally, since part of the Downtown Strategy for Olympia includes
street trees as an integral part of their plan for beautification and community
enhancement, tree care was prioritized for that area, which in 2016 included
approximately 660 street trees (CFC, 2016; Roush & McFarland, 2006).
Regional tree studies and study significance
Traditional methods of valuation rely on calculations which determines the dollar
value of a tree based on the potential costs of treatment, replacement, and property value
increase (Roush & McFarland, 2006). In 2016, the Department of Natural Resources
estimated the entire urban forest of Olympia to be worth approximately $6,100,000
(CFC, 2016). Although impressive, this value did not include the annual fiscal and
environmental benefits provided by the street trees. This research adds the value of those
benefits provided by street trees by using iTree software and visualizes them using
4
ArcGIS mapping software (AMEC Earth & Environmental Inc., 2011; Roush &
McFarland, 2006).
To determine the value of Olympia street trees, this study utilizes urban forestry
software that has embedded peer-reviewed scientific models that calculate the annual
contributions that urban trees provide. iTree Streets and iTree Eco are publically
available urban forestry software created in partnership with the USDA Forest Service
and the Arbor Day Foundation, to support the efforts of urban forestry programs to secure
funding and support urban tree conservation. These peer-reviewed tools quantify the
ecosystem services in quantitative and monetary terms provided by trees in a given
region based on collected tree health and dimensions (Vargas, 2018). iTree software has
been used in many peer-reviewed studies and urban forestry assessments, and considered
by many arborists, and this author, to be reliable for estimating the services trees provide
in urban environments such as the City of Olympia (American Forests, 2008; Asselmeier,
et al., 2019; Grant, 2012). A further discussion of iTree Streets and iTree Eco can be
found in the Methodology and Results Chapter of this thesis.
Thurston County has conducted an urban tree canopy (UTC) assessment to
support sustainable planning for future urban growth, but the scope of their investigation
did not include the developed urban areas of Olympia. The UTC assessment focused
solely on the unincorporated urban growth areas of Thurston County— areas just beyond
the current city boundaries projected to be urbanized and developed within the next 20years (AMEC Earth & Environmental Inc., 2011). My study will be focusing on the
urban core and main thoroughfares of Olympia to fulfill this gap in research.
5
To date, the Master Street Tree Plan (2001-2011) provides the most
comprehensive report on the urban forest of Olympia, but this report focused on the
logistics of urban forestry planting, maintenance costs, and current policies for street
trees, and does not include an assessment of the ecosystem services provided by trees
(Roush & McFarland, 2006). Although trees are considered important for healthy water,
air, and communities, the report does not include supporting data on the trees actual
contribution to local air and water quality, or to public health in a given year (Roush &
McFarland, 2006, pp. 4). Additionally, the Master Street Tree Plan is now dated and
requires new research and an updated street tree assessment (M. Bentley, pers. comm.
Jan. 16, 2020). This study could help to fill this gap with new research using the street
tree inventory data provided by the City of Olympia’s urban forestry department.
The City of Olympia is listed as a Tree City USA Community, and has met the
Arbor Day Foundation program requirements for the past 26 years (“Tree City,” 2019).
Olympia may be proud of its urban trees; however, their ongoing urban forestry efforts
could be supported and strengthened by new research on the street trees and the services
they provide. To that end, my research responds to this question: What are the ecosystem
services of the street trees in the City of Olympia, Washington? The research
concentrated on annual air quality benefits, stormwater processing potential, carbon
storage, sequestration, and carbon emission mitigation, and the reduction of building
energy demands. Throughout this investigation I also investigated any potential
disservices associated with these trees. The research and results from this study have also
been tailored into education and outreach materials for general public audiences for the
6
City of Olympia’s urban forestry department to raise awareness of this topic, and to
garner support for future urban forestry program costs.
Before reaching these conclusions however, we must first understand Olympia’s
street trees and urban trees in the larger context of existing scientific literature, and how
cities shape (and are shaped by) the natural world. By extension, we will also consider
how traditional value systems dictate how we perceive and interact with urban trees in
urban environments. In the next section we will take a closer look at the urban landscape
and the role that trees play in this manufactured environment.
7
LITERATURE REVIEW
Urban environments as ecosystems
The world, we are told, was made especially for man
— a presumption not supported by all the facts.
― John Muir, A Thousand-Mile Walk to the Gulf
Some people would hardly consider the city in which they live an ecosystem. And
yet, despite the dominance of concrete material and industrial processes, the urban
environment behaves much like a natural ecosystem. It may differ qualitatively from a
natural ecosystem, but the system dynamics of energy and material exchanges mirror
those of the natural world (Samson Ch.1; Pearlmutter et al., 2017). The city has its own
urban hydrologic cycle, weather patterns, and localized climate. It has its own familiar
flora and fauna, and systems of transit that move energy and materials from one place to
another. We cannot take cities out of the natural landscape in the same way that we
cannot separate ourselves from the natural world in which we all live (Roszak, 1992). If
we were to walk through a city, we could see all around us the ways we are rethinking
and redesigning our natural and man-made surroundings. In this section we will discuss
8
the city environment and the theories of urban sustainability as it relates to urban
planning and forestry for the long-term health of the city, its residents, and its trees.
An increasing number of academics and practitioners approach the urban
environment as an ecosystem; the theory has even prompted entirely new disciplines of
study, such as Urban Ecology (Gaston, Davies, & Edmondson, 2013). The attention to
the city as a complex system comes at a time when urbanization is happening at a pace
and scale never before seen (O’Neill et al., 2010). With this trend comes the growing
need to make our cities cleaner, more resilient, and sustainable (Fitzgerald, 2010; Grant,
2012; Young, 2011). As Theodore Roszak (1992) said about cities, “Nothing has
absorbed more energy; nothing projects more of our aspirations” (pg. 215). The city is a
socio-ecological conglomeration, and the human is a unique character that affects the
biophysical behavior of the system by its participation (Meerow, Newell, & Stultz, 2016).
We humans are an integral part of, and actively change our urban environments, simply
by existing within them. Every decision we make to add or take away elements in our
homes and on our properties collectively change our cities and how we see ourselves
within that collective system.
Researchers, and now city professionals, are increasingly examining urban
dynamics on large spatial scales and embracing complex systems theories, such as
resilience theory, embedded ecological-based services, and the theory of sustainability
(Turner, Gardner, O’Neill & O’Neill, 2001. Alberti et al., 2003). In turn, urban forestry
dynamics, management, and design continue to evolve as new research is conducted on
the ecology of the city and on the concept of green infrastructure within urban planning
as a tactic for urban sustainability (Grant, 2012). Urban sustainability is generally
9
considered to be urban planning and governance that support the health of socioecological systems and considers the well-being of current inhabitants without
compromising future generations’ ability to access the same resources (Baharash, 2017;
Suzuki et al., 2010). Planning sustainable healthy urban ecosystems emphasizes three
priorities: social, economic, and ecological, which are (ideally) considered in tandem
(Seitzinger et al., 2012). When city planners come together to build climate mitigation
plans, improve public transit systems, or work to plant trees in lower-income
communities, we are seeing this theory of urban sustainability in action (Haub et al.,
2007; Vogel et al., 2016).
The concept of green infrastructure as an essential component to the long-term
health of the urban ecosystem is a growing trend across the disciplines of landscape
architecture, urban planning, urban ecology, and urban forestry (Fitzgerald, 2010; Grant,
2012; Pearlmutter et al., 2017). As you may recall, green infrastructure, a type of
sustainable infrastructure, refers to living systems—such as urban trees, and green spaces
within the city—that replace or support the services provided by engineered
infrastructure. Grey infrastructure uses engineered structures to control and manage
natural processes, such as pipes and culverts to manage the urban hydrologic cycle.
Where traditional grey infrastructure addresses a single function, green infrastructure
typically provides ecological services that serve multiple functions. When used in tandem
for stormwater management, these two approaches have the effect of reducing water
runoff and pollution, and increasing water retention/aquifer replenishment (Gill, Handley,
Ennos, & Pauleit, 2007). Urban green infrastructure, such as street trees, rain gardens,
and non-porous surfaces, can be used to strengthen urban sustainability against climate-
10
related challenges, but some argue that it is still underrepresented in the planning stages
of urban design at large scales (Young, 2011). Unlike the built grey infrastructure of
drainage systems and sea walls, the biotic system of green infrastructure is alive and is, to
some extent, self-regulating and fragile, as are all living things (Grant, 2012).
These systems are all around us if we just take a look. In my backyard an apple
tree grows. I don’t tell it to grow leaves and bear fruit, and yet it does anyways. I can
prune the limbs and leaves to encourage a certain shape, and tend to its soil to prolong its
health into the cold months, but it is the tree that tirelessly takes in the sunlight and
through photosynthesis produces the energy it needs to survive and produce fruit,
exchange nutrients with the soil, and exhale the oxygen I breathe. In this way the network
of living things within the urban environment work to clean the air, improve the soil, and
shade their surroundings whether we ask them to or not—they do it as a consequence of
their existence. Clean air, soil nutrient cycling, and shading are examples of the services
provided by urban forests within the urban ecosystem, known collectively as ecosystem
services. Green infrastructure provides these ecosystem services all around the urban
environment, and if we pay close enough attention to look for them we can begin to see
their importance.
Much like the tending of the apple tree to improve the health of the plant in order
to benefit from the fruit it bears, a city needs to tend to its garden for the benefit and
health of the system and its inhabitants. Some argue that a “broad view” of the many
urban ecosystem dynamics is key to the undertaking of this stewardship of our green
infrastructure of the modern city (Grant, 2012; Pearlmutter et al., 2017). There is a
general consensus in the literature that a broad view or “systems thinking” approach to
11
green infrastructure management (Krosinsky, 2016) has the potential to broaden the
scope of how complex systems like urban ecosystems are planned, maintained, and
experienced (Young, 2011). In line with this approach, the services provided by green
infrastructure can be considered in four broad but interconnected ways: 1. cultural
services, 2. regulating services, 3. provisioning services, and 4. habitat services
(Pearlmutter et al., 2017 pg. 4).
Considering the many ecosystem services of trees as a form of sustainable
infrastructure in this holistic manner can inform our urban forestry plans, goals, and
actions within the City of Olympia. Understanding each of these ecosystem services;
what they are, and how they interact, is the first link in this network approach to
sustainable urban forestry planning. This examination will expand our understanding of
how these natural services support the sustainability and resilience of our urban
environments, and the importance of reexamining our traditional systems of tree
valuation in light of these insights.
12
Trees as a green network
If the land mechanism as a whole is good, then every part is good,
whether we understand it or not. If the biota, in the course of aeons,
has built something we like but do not understand, then who but a
fool would discard seemingly useless parts? To keep every cog and
wheel is the first precaution of intelligent tinkering.
—Aldo Leopold
All systems depend on the continuous cycling of resources and why trees, rivers,
and circulatory systems have branching growth structures. Everything lives by the
exchanges of these resources; not inherently from the resources themselves (Glanzberg,
2020). Thinking of the urban forest like a branching circulatory system, it is this
continuous flow and exchange of resources throughout the living network that makes the
system valuable, sustainable, and resilient (Meerow, Newell, & Stults, 2016). This green
matrix of vegetation that branches through the human-built environment supports the
exchange of resources from the air, soil, and rain.
When considering the street tree network in Olympia, I argue here that the true
value lies not in the appraised dollar value of an individual tree, but in their collective
ability to improve the social, economic, and environmental health and resilience of the
urban environment and its inhabitants by this process of exchange. Among many other
benefits, urban trees play an important role in improving air quality, modulating city
temperatures, and reducing stormwater flows, atmospheric carbon, and carbon emissions.
13
Trees have been studied extensively for their abilities to improve air quality in
urban settings by intercepting pollutants such as ozone (O3) and fine particulate matter
(Pearlmutter et al., 2017; Sicard et al., 2018). Tree canopies intercept gaseous air
pollutants and polluting particulate matter (PM) when the air-born particulates fall back
to earth in the form of either wet or dry deposition and collect on leaf and bark surfaces.
The gaseous pollutants can also be absorbed through small openings in the surface of tree
leaves called stomata (Pearlmutter et al., 2017, p. 22). Additionally, trees slow the
movement of airflow, reducing the dispersal of pollutant particles in urban areas (Grote et
al., 2016).
The ecosystem service of air quality improvement by urban trees has both social
and economic benefits. For instance, air pollution exposure, including from fine
particulate matter, nitrogen oxides, carbon monoxide, sulfur dioxide, and diesel exhaust,
exacerbate asthmatic symptoms. Hospitals in Washington State charged about $73
million in 2010 for asthma-related hospitalizations, $43 million of which was charged to
Medicaid and Medicare, and approximately $5 million of hospitalization bills was paid
for by Washington State residents (Tran, Aldrich, & McDermot, 2013). Studies also show
that asthma rates are higher for lower-income and minority populations, therefore the
argument has been made that the widespread distribution of urban tree planting can help
to improve the health and safety of these more vulnerable populations within our
community (Tran et al., 2013; Young, 2011).
Conversely, trees can actually contribute to particulate ozone concentrations in
cities through a process called biogenic emissions (Sicard et al., 2018). Many trees emit
small biogenic volatile hydrocarbons (BVOCs), though the amount varies widely by
14
species and microclimate conditions. The BVOCs may contribute to ozone levels in
urban environments, but despite this, the benefits of trees for urban air quality outweigh
the BVOCs emitted by some species (Gaston et al., 2013; Hastie, 2003; Mcpherson et al.,
1997). BVOC emissions of Olympia’s street trees have been included in this research in
order to address this potential disservice by our street trees. Also included in this research
are estimates for the important role of stormwater management by urban trees.
Green infrastructure as a tactic for stormwater management may be still fairly
new as a management approach by city stormwater departments, but it is growing in
interest and popularity (Berland et al., 2017). Grey infrastructure (such as pipes) has been
the traditional approach for cities to control the movement of water through the urban
landscape, but this system can malfunction or become overwhelmed during extreme
precipitation events (Seattle, 2018). This is in large part due to the very nature of the
urban landscape and what is referred to as the urban hydrologic cycle. Impervious
surfaces effectively convey large volumes of stormwater and pollutants in urban areas
dominated by pavement and cement. This lack of porous surface types typical in
undisturbed landscapes also reduces the rates of infiltration through soil and vegetation to
recharge aquifers and filter pollutants before reaching nearby water bodies (Berland et al.,
2017; City of Olympia, 2016). A network of green infrastructure in an urban area can
support traditional infrastructure in processing stormwater through infiltration through
the soil and evapotranspiration from the leaves.
Thurston County and the City of Lacey recognized the ecosystem service of our
urban trees as a viable and important tactic of reducing urban stormwater, erosion, and
improving local water quality in their urban forest assessments (AMEC Earth &
15
Environmental Inc., 2011; Madden et al., 2013). Using our urban trees as a green
stormwater support system in a region like Olympia, which experiences high annual
precipitation rates, would seem to be a well-suited application for stormwater
management.
Stormwater utilities staff working for the City of Olympia have their doubts about
the effect of street trees diminishing the volume of stormwater because of the
predominance of deciduous street trees (J. Roush, pers. comm. Feb. 10, 2020). Rainfall
patterns in Western Washington create wet winters when deciduous trees drop their
leaves, and dry summers when tree canopies are full (SWMP, 2019; “Weather Atlas,”
n.d.). Some experts argue that inadequate research has been done on the ecosystem
services of urban trees, and this research gap hinders the reliance on trees by stormwater
utility managers as a viable approach to stormwater management (Kuehler, Hathaway, &
Tirpak, 2017). Therefore, this study includes an assessment of stormwater services by
street trees and quantifies the average amount of stormwater processed by Olympia’s
street trees each year and it’s associated dollar value. Also included in this study are the
important contributions by trees to mitigate carbon emissions and sequester carbon.
The contribution of trees to carbon storage and sequestration are widely
recognized and studied as an important ecosystem service (Mcpherson et al.; Glaeser and
Kahn, 2010; US DOE, 2008). Within the urban environment, this is an especially
important ecosystem service since approximately 80% of the U.S. population lives in an
urban area (as of 2020), and research suggests that as much as 80% of global emissions
originate from cities (Hastie, 2003; O’Neill et al., 2010). Gaia Vince argues in
Adventures in the Anthropocene that this could actually be seen as an opportunity for
16
global carbon emissions to decline substantially if we shift our urban-industrial planning
to sustainable urban planning (Vince, 2014, pg. 345). Trees help curb carbon emissions
by reducing the temperature of the urban environment and nearby buildings in the
summer, thereby reducing the energy demands for air-conditioning, and by reducing
wind-chill in winter months, thereby reducing energy demands for heating. Some studies
estimate cooling costs in summer months to be reduced by an average of 27%, and 7%
reduction in heating costs in the winter (Hastie, 2003).
Trees also actively sequester atmospheric carbon as they grow, and store carbon
in their above and belowground material over the life of the tree. In a Chicago tree study,
the urban forest was calculated to sequester 155,000 tons of carbon each year, and
researchers have estimated that the urban forests throughout the United States have
collectively stored 700 million tons of carbon with an associated value of $14.3 billion
(Chicago citation). I argue that part of sustainable urban planning is recognizing these
services in urban forestry city programs and supporting the natural process of carbon
storage and sequestration by keeping our trees healthy and in the ground for as long as
possible.
With these many ecosystem services in mind and the guiding principles of urban
sustainability as a conceptual framework, we can better reimagine our cities as we seek
new ways to adapt to climate change. These changes in climate conditions are not only a
call to action for humans to readjust our current models of behavior and design, but a
force that will require a shift to more holistic management tactics for urban forestry.
17
Urban trees in a changing world
But when the storm is over, and we behold the same forests
tranquil again…and consider what centuries of storms have
fallen upon them since they were first planted… we cease to
deplore the violence of her most destructive gales, or of any
other storm-implement whatsoever.
—John Muir, A Windstorm in the Forest
Like many other regions in the world, the Pacific Northwest (PNW) has observed
an increase in average temperatures over the past decade of 1.5 degrees F. and the IPCC
models project that to increase by about 1.4 degrees F. by 2040 (USGCRP, 2018). The
issues of climate change are dynamic and complex, so it seems appropriate to incorporate
the dynamic and complex network of our urban forest and their ecosystem benefits in our
regional climate mitigation plans. One of the Thurston Climate Mitigation Plan goals is to
reduce carbon emissions by 45% by 2030 in an effort to help minimize global
temperatures (TRPC, 2017). In this section I illustrate how, by using a more holistic
perspective to mitigation planning, we can recognize how our urban forest can help to
reach that goal of reducing energy demands, sequester and mitigate carbon emissions,
and mitigate other climate-related stressors.
Often, the climate mitigation goals of cities rely on isolated projects aimed at
single issues or criteria, such as air pollution or carbon emissions (Turner & Gardner,
18
2015). Although important, some argue that this scope is too narrow and will not be
sufficient to meet the challenge of climate change facing many urban environments
(Meerow, Newell, & Stults, 2016; Seitzinger et al., 2012). As argued above, the issues
are interlinked and work on an array of diverse scales of time and space throughout the
urban landscape, and must be responded to with an equally complex system of planning
(Seitzinger et al., 2012). Sustainable city planning should incorporate conceptual
perspectives that support current and future urban design (Joss, 2011). Normative
perspectives encompass a variety of goals and values such as social, ecological, and
economic goals and values that parallel the three pillars of sustainability. Seitzinger’s
three pillars, considered in combination with a normative perspective to governance,
appear to be the most holistic approach to support urban adaptation simultaneously on
multiple scales of development, maintenance, and governance. The normative perspective
of urban forest planning in Olympia would support the recognition of trees in enhancing
the social, economic, and environmental health of the city.
Scientific research supports the provision of urban trees to reduce air pollution
and regulate air temperature at a time when air quality is becoming a point of concern for
many counties, including Thurston County (TRPC, 2017). Increased summer
temperatures mean an increase in energy demands for air-conditioning, and increased
ozone levels in urban environments as a result of vehicle emissions being exposed to
sunlight and heat (Sicard et al., 2018).
The increase in average annual temperatures is compounded by the urban heat
island (UHI) effect which results from the dominance of non-porous land-cover, energy
outputs of cooling systems and other urban industrial processes, and heat-holding nature
19
of our man-made materials like steel and pavement, which can double the rate of urban
temperature increase (Gaston, Davies, & Edmondson, 2013; Gregory et al., 2002).
Studies have found that tree-lined streets, green-spaces, and rain swales all serve as
multifunctional green infrastructure by cooling the surrounding temperatures and
ameliorating the UHI effect, while also supporting the grey infrastructure stormwater
system by reducing stormwater flows (Meerow, Newell, & Stults, 2017; USGCRP, 2018;
Sicard, 2018).
Figure 2. Thermal infrared image indicating the daytime difference in radiative surface temperature between exposed
and tree-shaded pavement. (Pearlmutter et al., 2017)
Locally, trees also help to cool the urban environment through canopy cover
(Figure 2) and evapotranspiration (the collective evaporation from tree leaves and soil)
and reduce the energy demand on surrounding structures, thereby mitigating carbon
emissions from the air-conditioning units and local power plants, and improving urban air
quality (Gaston et al., 2013; Young, 2011). A study done in 1994 in Chicago using iTree
methodology showed that urban trees reduced energy use by between 5-10% and resulted
20
in a city-wide savings $38 million each year (McPherson et al., 1994; McPherson et al.,
1997). This reduction in energy demand has the effect of reducing carbon emissions from
power plants and local A/C units within the city, therefore improving air quality
(Mcpherson et al., 1997).
Figure 3. Thurston Climate Action Team report showing that the top emitters of greenhouse gases are building and
vehicle emissions.
Typical city residents are exposed to an average of 200 different classes of air
pollutants in a day (Sicard et al., 2018). According to the Thurston Climate Action Team,
the second leading emission class source, behind electricity and gas, was cars and lightduty trucks and the heating and cooling of buildings as seen in Figure 3 (Olympia &
Lacey, 2010). With growing urbanization trends and increasing summer temperatures at
mid-latitudes of the Northern Hemisphere, standards for air quality are becoming a
21
priority issue for local climate mitigation planning (TCAT; Sustainable Thurston, 2013).
As of 2018, the Thurston Regional Planning Council status report shows rising annual
emission rates over time, as seen in Table 1.
Table 1. status report for TRPC in spring 2018 shows upward trend in emissions by year.
One consequence of the increased temperatures and emissions is the potential for
the increase of low-level ozone, created when sunlight reacts with the emission particles
from vehicles and energy supply industries. Trees within our cities have canopy leaves
that intercept these airborne pollutants including ozone, as well as harmful particulate
matter smaller than 10 microns that enter the lungs of city residents and lead to increased
asthma and exacerbate cardiovascular conditions (Grant, 2012; Tran et al., 2013).
Although the services of trees in the urban landscape could be recognized as a
response to mitigate some of the challenges faced by cities due to climate change,
humans and our urban environments are not the only things stressed by climate change—
trees are stressed by the changes in their environment and urban foresters are provoked
into adaptive planning for the long term health of the urban trees under their care.
Increasing temperatures and extended warm periods can stress street trees,
especially during the dry summer months. Restricted planter spaces and the urban heat
22
island effect exacerbate these issues and force urban forestry teams to water street trees.
Trees placed in unrestricted grow spaces like parks are able to seek out moisture in the
surrounding earth more easily than those placed in sidewalk cutouts. Unfortunately, lack
of staffing and urban forest program funding made it so watering crews, for newly
planted and annual street tree watering, were considered too costly by the City of
Olympia (M. Bentley pers. comm. May 20, 2020). Consequently, no new street trees
have been planted since 2015 because of these watering restrictions (M. Bentley, pers.
comm. May 20, 2020). In effort to determine the most cost-effective watering methods
for street trees, the City has taken on a Pilot Street Tree Watering Project whereby 30
trees were planted, watered, and studied to insure proper tree growth and watering
methods can be used in the future (M. Bentley, pers. comm. May 20, 2020).
Increasing temperatures in mid-latitudes are altering the range of historic tree
growth (USGCRP, 2018). This climate-induced shift in tree species affects the types of
trees that have historically grown successfully in a given urban environment, and many
urban foresters agree that urban forestry planning will require an innovative approach to
species selection in the years ahead (M. Bentley, pers. comm. Jan. 16, 2020). In Olympia
for instance, cedar, hemlock, and true fir tree species are showing signs of stress from
changing climate conditions; tree species that currently grow well in Southern Oregon are
being considered for street tree selection in Olympia because of their ability to thrive in
more arid summer conditions (M. Bentley, pers. comm. Jan. 16, 2020). Additionally,
urban foresters face the challenge of increased severity and range of pest infestations.
Within Olympia cherry, ash, and hemlock tree species are no longer planted because of
their susceptibility to infestation (M. Bentley, pers. comm. Nov. 24, 2019). Proactive
23
urban forestry planning will be needed to support social and ecological health and safety
in the face of such growing threats.
Urban forestry planning with a normative perspective of the many interconnected
ecosystem services can provide a conceptual framework to reimagine our cities as we
seek new ways to adapt to a changing climate. However, complex systems such as
forests, even urban forests, are by their very nature webs of relationships that can be
challenging to predict and manage. Even more difficult is attempting to assign economic
values to the complexity of the natural world.
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How valuable is a tree?
For the true nature of things, if we will rightly consider,
every green tree is far more glorious than if it were made
of gold or silver.
—Martin Luther King Jr.
Trees have embedded cultural and social values, but for many urban forestry
departments, justifying the costs of tree maintenance and planting to local government
requires a conversation about what is meant by a tree’s “value”. Many urban foresters
need to defend the (sometimes high) costs of tree maintenance against the hard-toquantify benefits of trees (Grant, 2012; Vargas, 2018). How do you measure the beauty
of an old tree? How do you measure the benefits of the comforting shade from a
sprawling oak tree?
Nevertheless, measuring the world around us and understanding the embedded
systems within our natural world is the primordial soup from which every scientific
discipline has emerged, grown legs, and established itself as a system of philosophical
thought (Roszak, 1992). In order to defend urban trees as a component of green
infrastructure within the city, scientists and urban foresters are rising to the challenge of
measuring the complex and inter-relational benefits of trees, and including the metrics of
monetary values to reflect our capitalist systems of value (Asselmeier et al., 2019;
McPherson et al., 1994). In this section, I will consider how the value of Olympia’s urban
forest has been determined in the past, some of the current theories and challenges of
25
placing value on elements of the natural environment, and how valuation may be
improving with recent scientific research.
According to the Master Street Tree Plan the entire urban forest in Olympia was
valued to be worth approximately $3,000,000 in 2006, and $6,100,000 by 2016, more
than doubling in 10 years (CFC, 2016; Roush & McFarland, 2006). Conventional
appraisal values for trees are determined by adding planting and replacement costs (in
Olympia this would be $480 per tree) with values determined by the tree type and age as
stated in the Council of Tree and Landscape Appraisers (CTLA) Guide for Plant
Appraisal. However, systems of valuation do more than put a price-tag on a tree —they
affect current and future city planning budgets and dictate urban forest management goals
(B. Moulton, pers. comm. Feb. 13, 2020). On a deeper level, these systems reflect our
societal philosophy on the worth of the environment and our obligation to its well-being.
Conventional attitudes about urban trees color how we perceive their value in the
city environment, and, by extension, how we care for them. One of the most common tree
maintenance requests received at the Urban Forestry department at the City of Olympia is
for tree removal because sidewalk cracking, or tree limb removal in order to better see
business signage from passing motorists on the street (M. Bentley, pers. comm. Jan. 16,
2020). Average annual spending for the Urban Forestry department in Olympia for tree
and stump removal is almost $120,000, accounting for 25% of the total budget (Appendix
B). These requests and subsequent city spending highlight how city residents and city
officials see the role of trees in the urban environment--as problematic things to be
removed. If we see trees as passive ornaments to adorn our streets and increase
26
commerce in downtown areas rather than the dynamic living participants of the city's
ecology, then we do not see their true inherent value.
It would seem at times that the man-made city and the ever-changing-evergrowing trees are at odds with one another. Conventional approaches to greenspace
management and urban forestry have often focused on aesthetic amenities, tourism, and
community enhancement (Pearlmutter et al., 2017; Roush & McFarland, 2006). For
instance, the Downtown Strategy of Olympia included street trees as an important part of
its plan for an aesthetic “continuity in the retail core,” while tree selection and placement
focused on size and canopy shape, with consideration for the tree canopy to obscure
business signage (Arai/Jackson Architects & Planners, 2003). Although economic
benefits to tree landscaping of business districts has been shown to increase consumer
spending by 11% on average, and the presence of trees has been shown to decrease crime
rates and therefore can be seen as a social benefit (Hastie, 2003), these guiding principles
for tree planting do not always work in the best interest for the tree’s long-term health.
It can be challenging, under even the best circumstances, for a tree to reach full
maturity (therefore offering the most in ecosystem services to the community) in the
middle of a city (Pearlmutter et al., 2017). Take, for instance, the tragic tale of the Legion
Way trees in Olympia. Planted in 1928 to honor WWI and Spanish-American War
veterans, they settled into gracious 12’ wide planting strips and over time becoming fully
mature Oak and Sweetgum trees beautifying the Eastside, and generally beloved by the
community. But in the 1980’s, on the north side of the street, an electrical power line
hung dangerously close to the swaying leaves and branches of the growing canopies.
Acting in an orderly fashion, Puget Power sent crews through and systematically topped
27
(cutting off the top half) of each tree on the north side, a pruning tactic no longer used
because of the damaging effect it has on the tree’s growth, health and lifespan (CFC,
2016). In order to maintain a city greenspace aesthetic of uniformity, each tree along the
south side was also topped, leading to years of malformation (Figure 4), tree death, and
costly tree maintenance by the City of Olympia (CFC, 2016).
Figure 4. Legion Way tree “topping” diagram of effects on long term tree health and maintenance.
As a result of this approach to tree maintenance, the city now spends thousands of
dollars every year to maintain the Legion Way trees (Allen-Ba, 2010). In 2010, the City
spent $50,600 to remove and replant damaged trees on Legion Way. Expectedly, this
figure does not include the inevitable loss of the ecosystem services these trees had
provided to the surrounding neighborhood. Because the Eastside neighborhood meets the
requirements of a low-income community, the City secured a $10,000 of grant funding
from the Olympia Housing program in 2010 to remove five of the damaged trees (Allen-
28
Ba, 2010). While the City of Olympia has acted accordingly to plan for the care and
resolution of the Legion Way tree situation, it is interesting to pull back a moment and
consider the consequential economic dynamics happening here. In order to replace trees
damaged by an energy company maintenance crew, the City sourced funding from the
City of Olympia Housing and Social Service program.
As we can see from the Legion Way story, our value systems inform our actions,
and it becomes necessary to recalibrate our systems of valuation to advocate for the longterm health of our urban trees, and for the benefits that come with having a healthy urban
forest. Attempts to advocate on behalf of urban trees and the environment have come a
long way in recent years, with experts and researchers working hard to create a system of
valuation for natural processes in monetary terms (Hirabayashi, 2014; McPherson, 2010;
US DOE EIA, 1998). Economic valuation can be an important part of advocating for
program funding, urban planning priorities, or the enactment of policy.
One interesting example of economic valuation on aspects of the environment to
advocate for new federal policy is the highly contested Social Cost of Carbon, a value
attributed to the economic harm of carbon emissions and climate change (Cropper et al.,
2018). This value not only represents the projected economic losses from climate change
on agriculture and other industries, but also creates a price that can be used to support
federal policy measures addressing the growing concern of climate change. The Social
Cost of Carbon emerged from a continually evolving federal cost-benefit regulation that
began under the Reagan administration in 1981 (Epa & Change Division, 2016).
29
In essence, the calculation involves projecting future population growth and
greenhouse gas emissions, modeling the impacts of climate change, and calculating how
the climate change models would affect the growth models and the economic costs
associated with that growth. It has also been implemented in state efforts to create policy
programs to lower emissions (Epa & Change Division, 2016). In 2020, the Social Cost of
Carbon was estimated at $50 per short ton, the value can vary wildly with minor
adjustments to the models and input values, and more importantly, with the change of
political parties calculating these results. For example, in 2020, Executive Order 13783
amends the previous presidential executive order so that only domestic emissions are
included at a reduced economic rate (Cropper et al., 2018). The adjusted rates reduce the
previous $45 per ton to between $6 and even just $1 per ton. This diminution of
associated Social Cost of Carbon reflects the political attitudes regarding climate change
and the philosophical framework that informs federal and state policy (Cropper et al.,
2018).
We still need to consider the benefits of trees that we cannot factor easily into our
economic equations. It is well documented that trees reduce stress, improve health,
reduce crime (Grant, 2012; McPherson et al., 1994; Pearlmutter et al., 2017), and
although these important benefits do not easily factor into tree cost-benefit analyses,
techniques for determining them continue to evolve, including the iTree urban forestry
software.
Created by researchers at the Pacific Southwest Research Station’s Center for
Urban Forest Research and funded by the USDA Forest Service, the iTree Suite is a
software program specifically designed to aid urban foresters and tree advocates in
30
determining the ecosystem services, associated social and economic benefits, as well as
the structure of a city’s tree population (Vargas, 2018). The results from iTree have been
used to defend conservation, maintenance costs, and planting initiatives by attempting to
calculate the social and economic benefits of trees (American Forests, 2008; Asselmeier,
2019; McPherson et al., 1994). I use “attempt” because the embedded algorithms in iTree
are continually being improved and updated as new peer-reviewed science becomes
available and researchers improve the techniques for determining how to measure the
many benefits of trees (Hirabayashi, 2014).
I chose iTree Streets and iTree Eco as my tools for analysis of street trees'
contribution to processing stormwater, reducing carbon emissions, improving air and
water quality, and the health of Olympia City residents. In the following pages, we will
explore the embedded models used in the iTree Streets and iTree Eco software, and show
how both programs with GIS maps were used in this study to analyze the ecosystem
benefits of Olympia’s street trees.
31
METHODOLOGY
My research questions for this study are “what ecosystem services are offered by
Olympia’s street trees?” and “do the benefits provided by street trees outweigh the costs
of their maintenance?” In order to answer these questions, I chose to use two different
urban forestry support programs, iTree Eco and iTree Streets (developed by USFS) to
determine annual environmental and fiscal benefits of street trees. Using the tree
variables from the existing tree inventory and the most current regional data available, I
ran both the iTree Streets and iTree Eco programs and compared the results. Concurrently
with iTree software processing, geospatial analysis in ArcGIS was completed utilizing
regional data and aerial imagery of Olympia to supplement and support the iTree
program results. My research method can be summarized as occurring in four stages:
Stage I: Emission rates and regional data were collected. Data was then
formatted for import into each of the iTree programs and GIS using
Microsoft Excel. The data is broken down by topic in the following
sections.
Stage II: iTree Streets and iTree Eco first draft runs were conducted and
reexamined for data model improvements. Additional data collection and
formatting was completed for final software reports.
Stage III: Geospatial analysis in ArcGIS Pro conducted using the tree inventory
and raster imagery provided by the City of Olympia. An assessment of
32
greater Olympia for tree canopy health and surface cover completed
using ArcGIS Pro, and Esri Insight.
Stage IV: Final iTree Streets and iTree Eco reports were run and GIS maps
analyzed to determine the ecosystem services provided by Olympia
street trees. Results were then curated, compared, and published using
Esri Storymaps.
Stage I: Emission rates and regional data
To determine mitigation costs and the monetary savings due to the presence of
street trees in Olympia, I collected data to determine a baseline of regional costs. The
collected data include an active tree inventory of 2,483 right-of-way street trees, as well
as GIS tree locations and conditions sourced from the City of Olympia’s Urban Forestry
department. To add to the robustness of my study, LiDAR and satellite raster images of
Olympia were geospatially analyzed to provide an assessment of surface types and tree
canopy health. In the sections that follow, I have included short descriptions of the
regional data types used, and the role each of these variables played in the process of
determining street tree benefits. To see a complete list of exact values used for emission
rates please see Appendix A, and for regional data, see Appendix B.
33
Emission rates
Local electricity rates were used to determine the average energy needs for the region in
question, and imperative for defining the fossil emissions mitigated by the
presence of street trees. I calculated the values by first determining consumer
rates, and then factoring in the fuels used to generate the energy for the region.
Currently Thurston County and the City of Olympia receive all their electricity
and natural gas from Puget Sound Energy (PSE). Interestingly, as of 2016, coal
accounted for 37% of PSE’s fuel mix, natural gas accounted for 22% (2016), 31%
was hydroelectric, while the remaining 10% came from wind and other energy
efficient resources as illustrated in Figure 5. According to the Washington
Utilities and Transportation Commission (UTC) the average PSE residential
customer uses 1,000 kilowatt hours-per-month of electricity at a total cost of
$102.56 and 68 therms of gas a month at a cost of $86.09 (Appendix A).
Figure 5. The 2015 Energy Flow Diagram: actual and estimated data obtained from Puget Sound Energy
(electricity) and TRPC estimates (transportation).
34
Carbon dioxide values I sourced from the “Social Cost of Carbon in the US” estimates
published by the EPA and other federal agencies (Epa & Change Division, 2016).
The carbon cost plays a vital role in determining how many pounds of carbon
emissions can be mitigated by the presence of trees, lowering other incurred costs
such as medical expenses paid by city residents. Carbon values ranged widely
between $12-$123 per ton of CO2 (Epa & Change Division, 2016). Based on the
2016 EPA carbon value converted to 2018 dollars (as recommended by iTree
Eco) the cost of carbon used in this study is $170.55 per ton.
SO2, VOC, PM2.5, PM10, NO2 values were determined using EPA’s Environmental
Benefits Mapping and Analysis Program-Community Edition software
(BenMAP-CE). The BenMap-CE software program was designed to calculate the
medical costs due to poor air quality. These medical costs help us understand how
air quality improves in the presence of trees, thereby improving the health of local
residents. Updated in 2018, the iTree Eco emission default values were used in
this study (Appendix A).
Regional data
City layout data allows for the determination of landcover types in Olympia. The total
area of impervious surfaces in the City and the amount of tree canopy can be
input into iTree and GIS to help determine the ecosystem benefits of trees. Along
35
with city size by square miles, variables included in this analysis were sidewalk
and street dimensions, and tree planter types sourced from Olympia’s 2018
Engineering Design and Development Standards (Appendix B).
Olympia urban forestry program expenditures: To conduct a cost-benefit analysis, a
baseline of expenditures by the City of Olympia on behalf of the trees was
needed. I collected the program expenditures from 2014-2019 were collected to
find average annual costs. Unexpectedly, program expenses fluctuated over
multiple years, varying from year-to-year. For example, irrigation expenses for
newly planted trees were done only in 2015 and 2016, and storm litter cleanup
expenses occurred for 2019 only. Therefore iTree programs were run using
average program costs over five years (2014-2019) (Appendix B).
Identified tree variables used in iTree Eco,
iTree Streets, and GIS included the
following tree variables: species, crown
width, total height, and DBH as seen in
Figure 6, and growspace (planter
type/size) data. I sourced this data from
the tree inventory maintained by the City
of Olympia Urban Forestry Department
Figure 6. Tree meaurements diagram
(Appendix C). The Olympia street tree
inventory includes entries for either
existing trees, planting sites, or tree stumps. Because this is an active inventory,
this research provides a “snapshot” of all data fields from the street tree inventory
36
at one point in time. For exact figures entered into iTree Streets and iTree Eco see
Appendix B.
Land use types in Olympia make it possible to determine trends in street tree health
based on the areas where they were planted. Land-use categories such as
industrial, residential, and commercial, were collected and entered into iTree
programs (Appendix C).
Stage II: iTree Analysis
The underlying models differ between the iTree Streets and iTree Eco programs
in a number of significant ways. For instance, the variables required to determine energy
benefits differ between the programs, and the stormwater model in iTree Streets is based
on a different rainfall interception model than the iTree Eco models (Xiao et al 1998;
Wang et al. 2008). Water interception is rainfall stored temporarily on tree canopy
leaves, which then drips down the body of the tree, falls off the leaves into the ground, or
is evaporated into the atmosphere. In addition, iTree Eco uses 2016 weather and pollution
data, whereas iTree Streets models are based on weather information from 2006. For a
further discussion on the differences of these models and their results, please see the
results and discussion section.
37
iTree Streets software methodology
iTree Streets uses the Pacific Northwest climate region to determine the average
rate of tree growth, average tree size, common tree species, and what leaf area is
commonly measured for trees in the chosen climate zone. Regional tree models within
iTree Streets are based on the study measurements from a regional reference city.
Reference cities are chosen from each of the 16 climate zones to determine baseline tree
growth, and climate conditions for that region as seen in Figure 7 (McPherson, 2010).
Figure 7. Climate zones used in iTree Streets to determine average tree growth rate data and climate
conditions (Vargas, 2018).
38
In each reference city, 30 to 60 trees were chosen from 22 major tree species to be
measured and aged to represent tree growth for the region (McPherson, 2010).
Regression analyses of regional tree growth curves were conducted to determine the
estimated tree benefits expected for each year of a tree’s life cycle and to estimate tree
leaf size index (Gregory et al., 2002; Vargas, 2018). The estimated values from each
reference city are included as default values, which have been updated to reflect current
regional values, as seen in Appendix A (Gregory, Qing fo, Scott, Maco, et al., 2002). The
chosen reference city for the Pacific Northwest was Longview, Washington, conducted in
2006.
Longview is in the Pacific Northwest of Western Washington, about 60 miles
south of Olympia, and has a population of 36,646 (as of 2010) and receives an average
annual of 46 inches in rainfall. In comparison, Olympia has 51,609 residents and receives
inches of 50 inches precipitation annually (Weather Atlas, n.d.). Average temperatures
for Longview are 41° and 61° for winter and summer seasons respectively, and
Olympia’s mean winter temperatures are close behind at 38°, with summer temperatures
averaging 64°. Although not an exact match, the use of Longview as the model for data
analysis is, in this author’s opinion, in range of acceptability as a city of comparison, and
certainly better than using models based on national weather and climate data to
determine such things as average tree growth and leaf area.
39
iTree Eco software methodology
Data from 2,346 trees were successfully imported using the iTree Eco software.
iTree Eco uses tree canopy measurements to determine a tree’s leaf size and biomass, and
how these variables affect the ecosystem service estimates. The iTree Eco model
decreases in accuracy with every missing tree variable required to run the reports.
Because of unavailable inventory data, iTree Eco did not assign any ecosystem values for
an additional 93 tree entries. In total, 2,253 trees were successfully analyzed using limited
DBH and associated tree species data.
iTree Eco uses tree canopy variables to estimate energy savings, air quality, and
stormwater benefits of trees. The Olympia tree inventory does not include data on the
percent of crown missing, bottom canopy height, or crown width in two directions. For
energy models, the direction and distance from the nearest building are required, which
are not available using the existing tree inventory. Reports were generated for all
ecosystem services using the minimal requirements for the program using the parameters
of tree BDH, tree species, and tree condition as a percentage (Appendix C). iTree Eco
model results are compared to iTree Streets results in the following chapter.
40
Stage III: ArcGIS Methods
I used ArcGIS Pro to determine the geospatial relationships of the urban
landscape and the street trees, as well as to provide a visual aid for future street tree
planting recommendations. For a simplified ArcGIS methodology workflow please see
Appendix D. GIS data included for geospatial analysis are summarized below:
1. Tree locations and related data (species, size, growspace, and condition)
2. Raster images of Olympia (2015) and (2018) for leaf-on and leaf-off seasons
3. Surface land cover types (impervious surfaces)
4. Olympia building size and locations
Esri Insights Workbook was utilized to aggregate graphics on the tree inventory
data. The functions were performed to assess trends in tree health and planter size, tree
health and local land type such as business or industrial areas. Associations with DBH
and planter size, and map of planter locations were also performed (Figure 8).
41
Figure 8. This workflow diagram illustrates the methods undertaken in GIS to analyze the urban forest structure using
the City of Olympia’s tree inventory dataset.
Using ArcGIS Pro a Heat Index function was performed using the aerial imagery
obtained from the City of Olympia for 2018 to discern the heat response from different
landcover surface types within the city boundary, focusing on the main arterials where
street trees are planted and downtown Olympia. Heat maps are a visual assessment of the
landscape that shows the cooling effects of vegetation, and the heat reflection of
impervious surface types responsible for the heat island effect.
42
Figure 9. Diagram showing how satellite imagery sensors can detect leaf vitality using the near infrared response of
healthy leaves using the Normalized Difference Vegetation Index (NDVI) (Earth Observing System, n.d.).
The canopy health of trees has been studied as a measure of their potential
ecosystem benefits and overall tree health (Grant, 2012; Young, 2011). The aerial
imagery from 2015 and 2018 were analyzed using Normalized Difference Vegetation
Index (NDVI). NDVI is a GIS raster function for determining plant health, using remote
imagery to detect a change in the near infrared response from vegetation as seen in Figure
9. NDVI was used to compare the canopy health of Olympia street trees in warmer
months (leaf-on) to cooler months (leaf-off) and to compare the vegetation response of
deciduous tree to nearby conifer trees during leaf-off seasons. The resulting maps are
shown in the following section.
43
RESULTS
Utilizing the existing street tree inventory and both iTree Streets and iTree Eco
programs I quantified and assigned dollar values associated with the annual ecosystem
services benefits of Olympia’s street trees. Results include values for forest structure,
carbon storage and sequestration, air quality, energy, stormwater, and net benefits. I
outline the results in this chapter, beginning with a discussion of Olympia’s urban forest
structure, followed by the iTree Streets results and iTree Eco results for street tree
function and associated values. In the discussion I delve into the differences between the
two models and the implications of their results. See Appendix E for a table of report
results from both iTree Streets and the Eco program.
Urban Forest Structure
The iTree Streets and Eco results were in general agreement on the forest
structure of Olympia’s street tree population. The distribution of tree species, trunk
sizes, and canopy area are all aspects of the forest structure and help urban foresters
determine future goals for planting and maintenance. For the purposes of this study, these
results help to establish the size, health, and makeup of the street tree population.
44
Olympia considers street trees as those planted in the public right-of-way, usually
in the planting strip between sidewalk and road, or approximately 10 feet from the curb
or roadside if a sidewalk is not present. The trees maintained by the City of Olympia’s
urban forestry department on a 3-year pruning cycle include 83 different tree species, the
most common being Norway Maple, European Hornbeam, Flowering Pear, Hedge
Maple, and Red Oak. These deciduous top five species currently account for 67% of the
population as seen in Table 2.
Table 2. Ten most important species, collectively accounting for 60% of Olympia’s street tree
population.
iTree uses importance values to expand on how to determine the top ten species
that dominate the Olympia inventory. Importance Values (IV) are calculated as the sum
of the total species percent of the urban population and total percent leaf area as seen in
Table 2. Collectively, the ten most important species make up 60% of Olympia’s street
tree population. Among these top ten species, the size distribution across the population
as reported by iTree Eco is shown in Figure 10.
45
Figure 10. iTree Eco results for the top ten species sizes within the street tree population.
We can see in the iTree Streets diameter breast height (DBH) size distribution graph for
the street tree population in Figure 11 that the trends tend to align with each other.
Figure 11. iTree Streets results for DBH distribution within the street tree population. Note that the BDL Other
(Broadleaf Deciduous Large trees) category was attributed to tree species not recognized by iTree Streets.
46
Figure 11 shows a 3D graph illustrating the top 10 street tree species DBH
classifications in Olympia as reported by iTree Streets , which supports the findings of
iTree Eco on DBH distribution trends. As illustrated in both graphs, almost half of the
population (46.5%) have a trunk DBH of less than 6-inches.
Table 3. iTree Eco tree growspaces
iTree Eco also reported the makeup of the growspace allotted to the street tree
population (Table 3). Growspaces are planter types or growing spaces given to the street
tree at a given location. As we can see from this report, smaller growspaces (4’x4’ tree
grates, 0-4’ small, and 4’-8’ medium) make up more than 50% of the planting spaces.
Which could account the small DBH trends for the street tree population.
The forest structure reports from both iTree programs lay the groundwork for
better understanding the street tree population and helps us to become familiar with the
layout of the iTree reports and the ecosystem service results we will see in the next
section. Next, I will consider the results from first iTree Streets and then iTree Ecobefore
concluding with GIS maps.
47
iTree Streets Software Results
As indicated earlier, I successfully imported data from 2,399 trees and analyzed
them using the iTree Streets model developed by the U.S. Forest Service. I then
conducted an assessment of the ecosystem benefits and associated values of the street
trees, and cost-benefit analysis of tree costs and services, as outlined below.
Table 4. iTree Streets results for annual stormwater intercepted by species, and citywide total in
gallons and dollars.
Stormwater: Determining the interception of rainfall by tree canopy was essential since
my study involves a tree population that grows predominantly in confined urban
growspaces with limited potential for soil infiltration of stormwater. In contrast to
the iTree Eco stormwater results provided in the next section, the rainfall
interception model of iTree Streets is more generous in its estimation of street tree
stormwater benefits. As seen in Table 4, iTree Streets estimated total rainfall
interception capabilities of street trees for this region at over 1,500,000 gallons
per year. Using iTree Eco values for stormwater costs valued at $0.0277 per
48
gallon (based on 2016 weather data and City of Olympia utility costs), I can
estimate that street trees provide an annual savings of almost $42,500.
Table 5. iTree Streets results for annual air quality shown in total pounds of deposition
(pollutants intercepted by the trees), total pounds avoided (energy emissions reduced thereby
reducing air pollutants), and total biogenic volatile compound (BVOC) emissions (natural
emissions from trees) here shown as a negative value.
Air Quality: iTree Streets calculates the air pollution removal values based on the
regional rates for health costs related to poor air quality (Vargas, 2018). Using the
U.S. Environmental Protection Agency’s Environmental Benefits Mapping and
Analysis Program, I determined economic values for ozone, sulfur dioxide,
nitrogen dioxide, and particulate matter smaller than 2.5 microns (Vargas, 2018)
and input them into the iTree model. Based on the air pollution iTree Streets
model, Olympia’s street tree population intercepts and prevents almost 2,500
pounds of airborne pollutants every year, including nitrogen dioxide, sulfur
dioxide, ozone, and particulate matter (PM10) (Table 5).
49
Table 6. iTree Streets results for annual energy savings citywide from the street trees shown as
total savings of megawatt hours and dollars, and total savings of natural gas in Therms and
dollars.
Energy Benefits: The energy report in the iTree Streets model (Table 6) calculates the
average benefits of trees from the Pacific Northwest region in terms of the energy
demand reduction of nearby buildings (McPherson, 2010). The reduced demand
of electricity is represented in total megawatt hours and as the sum of the
megawatt hours multiplied with the local rate of $10.36/kWh (PSE, 2016). These
secondary benefits result from trees shading buildings and nearby surroundings in
the summer and protecting from wind in the winter. The values are based on the
total estimated reduction in energy and the local sources of that energy, such as
Puget Sound Energy’s mixture of energy-producing fuels including coal,
hydroelectric, and natural gas. The energy is broken down by dollar values per
therms, kWh, or pound for a given resource. Exact figures can be found in
Appendix A.
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Table 7. iTree Streets results for annual carbon dioxide sequestration in pounds and dollars, and
avoided emissions in pounds and dollars.
Carbon dioxide: The Carbon Dioxide report presents annual reductions in atmospheric
CO2 due to sequestration by trees and reduced emissions from power plants due
to reduced energy use (reported here in pounds). Sequestration is the process of
removing atmospheric carbon dioxide by plants and new plant growth. The model
does account for CO2 released as trees die and decompose, and CO2 released
during the care and maintenance of trees. Using iTree Streets I determined that
Olympia street trees sequester roughly 210 tons (420,000 pounds) of atmospheric
carbon dioxide with an associated annual savings of approximately $36,000
(Table 7).
Avoided carbon refers to the second-hand benefit of trees reducing building energy
demands, thereby reducing the fossil fuel emissions from Puget Sound Energy by
about more than 74 tons (148,000 pounds) each year, with an associated savings
of about $13,000. In total, street trees remove 265 tons (530,000 pounds) of
carbon annually, resulting in a savings of $45,000 every year to the City of
Olympia (Table 7).
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Table 8. iTree Streets results for the total stored carbon dioxide by the street trees in pounds and
dollars.
Carbon Stored: Whereas the above report quantifies annual CO2 reductions, the Carbon
Stored report accounts for all of the carbon dioxide stored in the urban forest over
the life of the trees as a result of sequestration (in pounds). These values were not
added to the Carbon Dioxide value to avoid double-counting. Taken together, the
CO2 avoided and carbon stored values remind us of the important role trees play
in keeping greenhouse gases out of the atmosphere and thereby helping to
mitigate local climate change stressors. To date, Olympia’s street tree forest has
stored more than 2,500 tons (5,023,314 pounds) of carbon, with an associated
value of $430,000, as seen in Table 8 (EPA, 2016).
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Table 9. Report of the annual cost-benefit breakdown for Olympia. As shown, the overall net
ecosystem service benefits surpass the costs of tree maintenance.
The cost benefit analysis of iTree Streets breaks down the total annual values for tree
ecosystem service (estimated at more than $1 million) and the associated cost for
tree maintenance by the City of Olympia (an average of $486,000 in annual
expenses). It then compares the total costs to the total ecosystem benefits of street
trees to find the annual net benefits of Olympia’s street trees: a collective worth of
approximately $616,000 (Table 9). Each tree, therefore, provides an estimated
gross annual ecosystem service worth $450. Accounting for tree maintenance
costs, this equals $260 in net value for each tree every year.
53
iTree Eco Software Results
Because iTree Eco required tree data that was not included in the Olympia
inventory, and because the inventory data entered into the software did not match all
program requirements, the results from iTree Eco on air pollution, stormwater, and
energy savings are considered by this author to be inconclusive. In this section I expand
on the difference in software models and why I have come to this conclusion.
Compared to iTree Streets, the iTree Eco software models rely on more recently
published tree ecosystem-benefit research and on 2016 weather information, but not all of
the values required for “existing inventory import” were available. I imported the
Olympia Street Tree inventory as the “existing tree inventory”, limiting the number of
tree variables analyzed because not all the iTree Eco software inventory import
requirements were met using that data. Because of missing data, 93 tree entries within
iTree Eco showed an ecosystem benefit value of zero for all reports in the results. That
means that iTree Eco analyzed 146 fewer trees than iTree Streets using data from the
existing Olympia tree inventory. This partially explains why the results of the iTree
Streets and iTree Eco software programs were different in their values (Appendix E). For
example, one of the most striking differences was the Net Benefits totals. For example,
the iTree Streets model estimated the total ecosystem benefits provided by the street trees
to be worth more than $1,100,000, whereas the iTree Eco model valued the benefits to be
merely $4,427 (Appendix E). Understanding the different energy models of iTree Eco
and iTree Streets can further our understanding of why the two programs have diverging
net benefit results.
54
First, I will expand on the problems with the iTree Eco required “Crown Health
Condition” parameters, measured on a scale of 0-100% (Appendix C). The existing
Olympia inventory does not include a crown health field; instead, it includes an overall
tree health variable, measured on a qualitative scale of “dead” to “excellent” with
associated coded values (Appendix C). I initially considered this field a potential
substitute for crown health, but according to the GIS specialist for the Urban Forestry
department, Woody Schaufler, the coded values are not necessarily an indication of the
tree's health in terms of a percentage (W. Schaufler, pers. comm. Mar. 5, 2020). This is
because the assignment of tree health conditions are applied by tree maintenance
crewmembers as a subjective assessment of overall tree health (B. Moulton, pers. comm.
Feb. 13, 2020).
The ability to gauge the health of the tree canopy affects the validity of the iTree
Eco results because Eco determines ecosystem services (such as carbon sequestration, air
quality, energy savings, and stormwater benefits) by the health and dimensions of each
tree canopy. Therefore each report shown below has been scrutinized to determine the
credibility for this study and found that the carbon sequestration, air quality, energy
savings, and stormwater benefit reports were unreliable based on the current Olympia
inventory attributes used in this study.
55
Table 10. Excerpt from iTree Eco results for street tree benefits (ex. carbon storage and
sequestration) by tree species.
Carbon storage and carbon sequestration values were calculated in iTree Eco derived
from the EPA Social Cost of Carbon value of $171 per ton (Forest Service, 2020).
The sequestering of carbon by trees involves the absorption of atmospheric
carbon dioxide by tree vegetation. Carbon storage is the amount of total carbon
stored in the above-soil and below-soil parts of trees over the life of the tree. To
determine the current carbon storage of Olympia’s street trees, the biomass for
each tree was calculated using embedded equations (based on iTree source
literature) and measured tree DBH from the tree inventory (Forest Service, 2020).
Of the street tree species in Olympia, Northern red oak stores and sequesters the
most carbon: approximately 16.1% of the total carbon stored and 13.9% of all
sequestered carbon. According to iTree Eco software models, Olympia street trees
sequester 13 tons of atmospheric carbon during the annual growth cycle, as seen
in Table 10.
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Figure 12. Water interception and evaporation provided by trees per month for the weather data year of 2016.
Water intercepted reflects the amount of rainfall that fell on plants and was intercepted
by the plant’s leaves. This water eventually evaporates into the atmosphere.
Evaporation is the amount of water that is released to the atmosphere from
vegetation. Figure 12 displays of the hourly evaporation by trees. These results
are based on what’s referred to as a conservative stormwater interception
model estimate of stormwater dynamics provided by iTree Eco. Conservative
stormwater calculations depend on crown health variables to determine the
amount of precipitation intercepted by the leaves only. This approach excludes the
ability of tree stems and branches to intercept water. Because Olympia’s tree
inventory did not have the canopy variables required and only data for species and
DBH were entered, the model relies on national averages to estimate canopy
conditions. Therefore I do not consider the stormwater volume results to be
credible in this study. It can, however, provide insight into seasonal tree behavior
in stormwater processing potential. For example, Figure 12 shows the amount of
57
evaporation trees provided annually as inches per hour, illustrating the vast
increase in evaporation during warmer months, helping to cool the urban
landscape as a consequence. As we can see, the estimated stormwater interception
is fairly low in leaf-off seasons for deciduous trees and high during leaf-on
seasons. We can also begin to see the repeating pattern of seasonal tree benefits as
seen in the air pollution trends model (Figure 13) which both utilize local weather
data embedded in ECO.
Figure 13. iTree Eco results for monthly pollutant removal trends by Olympia’s street trees.
Air quality benefit trends of trees are shown in Figure 13. Trees intercept air pollutants
(such as PM2.5) with their leaves. However, tree canopy variables and the
distance and direction of trees to buildings did not exist in the Olympia street tree
inventory data. According to the iTree Streets results, the total pounds of avoided
emissions of air pollutants exceeded the total pounds of air pollutants intercepted
58
by tree canopy. That indicates that the energy benefit model in iTree Eco is
needed to determine the missing air quality benefits. Therefore, the estimated air
quality results (in pounds and dollar values) are incomplete and inconclusive
using iTree Eco. Much like the stormwater results, however, the line graph in
Figure 13 can illustrate trends in providing air quality benefits according to local
weather data and leaf-off seasonality. For example, the ozone (O3) pollution
removal rates rise dramatically in the summer (around May) as a consequence of
new canopy growth, and trees continue to provide air quality services until
August when the ability of trees to intercept air pollutants drops down
considerably as the leaves begin to fall away.
Energy: To calculate tree energy benefits, iTree Eco requires the input of each tree’s
canopy fullness (as a percentage), canopy dimensions, distance in feet to the
nearest building, and the location of the tree to the building (NW, SE, etcetera).
Because the City of Olympia personnel did not collect these variables for their
street tree inventory, I could not run the energy benefits report in iTree Eco. In
contrast, the model used to determine the energy-saving benefits in iTree Streets
employs an average for all trees, regardless of their proximity to a structure.
Those results are included in this study. The iTree Eco guidelines for collecting
this information was, however, used in GIS to determine how many of Olympia
street trees are providing some form of energy benefits to buildings. In the
following section, I outline my GIS results on potential energy effects, cooling
benefits, and canopy health.
59
ArcGIS Results
In this section, I present GIS maps to see how they supplement the limitations of
both of the iTree programs and expand our understanding of the ecosystem services of
Olympia’s street trees.
Energy Maps: In order to expand on the estimated street tree energy-benefits from iTree
Streets and supplement the unavailable results for energy-benefits using the iTree
Eco software, I assessed the potential of street trees using aerial imagery and city
data of businesses and hard surfaces, as seen in Figure 14.
Figure 14. Tree locations are shown in green and buildings are shown in orange. The halo around each tree represents the
60-foot buffer which has been used as a measure of a tree’s effect on nearby structures for reducing energy demands.
Following the logic of buildings receiving energy benefits from nearby trees, I selected
all trees (normally green dots) within 60-feet from a building and color-coded
them in purple as seen in Figure 14. I then selected all buildings (normally
orange) within 60-feet from a tree and color-coded them in blue, representing all
the buildings being cooled by trees in warm summer months. These maps help us
60
better understand the relationship of street trees to buildings and hard surfaces.
Based on this geospatial analysis, I could determine that 72% of the street trees in
Olympia (1,692 out of 2,334 live trees) are within range to alter energy demands
in nearby buildings. I could also determine that 673 buildings in Olympia benefit
from the proximity of street trees. (Note: This analysis accounts for only the street
trees and does not account for any other surrounding trees.) I also used GIS to
supplement iTree results and analyzed Olympia’s street tree canopy health, which
is linked to air-quality and stormwater benefits, as outlined below.
Heat Response: In addition to the tree and building spatial assessment done in GIS, I
also performed a Heat Index analysis in GIS using aerial imagery of the City of
Olympia (Figure 15), showing the response of landcover surface types to heat.
This map illustrates the heat-response of building and hard surfaces to heat
(shown in orange and red), and the relationship of trees and the temperature
(shown in shades of blue) of their surrounding environment.
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Figure 15. Heat Index GIS map showing the Downtown Olympia area and the heat response of buildings in bright red
and orange, and the tree locations shown in the black circles. This image demonstrates the cooling potential of street
trees in dense urban areas like Olympia.
This cooling effect is one of the main arguments of the energy benefit phenomena, and
has the secondary benefit of mitigating the urban heat island effect. As seen in
Figure 15, the heat response of the landscape in areas with more canopy cover and
pervious landcover types show a lower temperature response.
62
Figure 16. GIS NDVI raster image of Legion Way showing the near-red infrared response (red color) from vegetation
during the Winter (leaf-off) season. The dark circles indicate where street trees are located.
Stormwater and air quality: The normalized difference in vegetation index (NDVI)
assessment results show the health of the vegetation in Olympia in both winter
and summer months. I used this analysis to visually assess the difference between
the canopy cover of deciduous to conifer trees in leaf-off seasons, and to
determine how these visual assessments compare to the iTree results for estimated
ecosystem benefits for stormwater and air quality. The red color (near-infrared
response) in the 2015 leaf-off winter season image shown in Figure 16 highlights
vegetation with active chlorophyll production in red; in this case conifer species
and ground-cover appear in red whereas the deciduous street tree locations
(indicated by the black circles) show no-to very little response.
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Figure 17. Planting sites (growspace) examples of street trees in Olympia. From left to right; 4’x8’ tree grate, 4x4’
tree grate, parking lot, unrestricted growspace.
Esri Insights for tree sizes and planter types: It is widely recognized in the literature
that larger healthier trees provide greater ecosystem benefits, including
stormwater infiltration abilities (Berland et al., 2017; Szota et al., 2019). I used
Esri Insight Workbooks to consider the population of Olympia’s urban forest and
the planting site types used to estimate infiltration. I looked at the planter types of
Olympia’s trees and found that large 8+ feet planting sites were the most
common. More large trees (based on DBH) grow in unrestricted and 8+ feet
planter sites than in the other planter types, as seen in Figure 18. Unrestricted and
8+ feet planters comprise 37% of the active tree sites. The second most common
planter type is the tree grate, which is 4’x4’ cement planter, and the DBH of trees
in these locations do not exceed 18”. Planter types that are 0-4’, 4’-8’ medium,
and 4’x4’ tree grate make up 44% of the active tree sites with DBH on average
measuring less than 20”.
64
Figure 18. ArcGIS Insights results for growspace trends and relative DBH distributions of street trees.
Figure 19. ArcGIS Insights results for DBH trends showing the location and sizes of the street trees.
65
The bar graph in Figure 19 shows that the mean DBH in Olympia’s street tree
population is 9 inches and most trees have small DBH, mirroring the forest structure
results from both iTree programs. The map to the left of the bar graph shows the
locations of trees by circle size based on associated DBH measurements. These results
could help the City of Olympia determine urban zones with either high or low potential
ecosystem services by trees, based on associated DBH trends for the area.
Figure 20. ArcGIS Insights results for Land Use and Tree Condition trends
The land use trends from Esri Insights show that street trees in Olympia have been
planted predominantly in business areas (as seen in Figure 20), and the trees recorded as
good condition are found predominantly in business land use areas. Therefore ecosystem
benefits in business land-use types such as energy savings, and air pollution would be
important to consider for both social and economic reasons.
66
Figure 21. Outreach and education storymap using Esri Storymaps:
https://storymaps.arcgis.com/stories/791052f6bbd84edd8e3991fe42a437fd
Educational Storymaps: I created a story map using Esri Online for educational
purposes and to share the study results with the City of Olympia’s urban forestry
department, Thurston County, and publically to Olympia residents. The story map
narrates the study using excerpts from this thesis, infographics, iTree reports, and
GIS maps to highlight how street trees in Olympia provide an array of ecosystem
benefits. This format allows the user to freely interact with the various GIS maps
made in this study, and to get a more simplified summary of the results of this
study.
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DISCUSSION AND RECOMMENDATIONS
When one tugs at a single thing in nature
he finds it attached to the rest of the world.
― John Muir
As indicated earlier in this thesis, urban forests are complex living systems within
our urban landscapes that interact with the environment by a process of equally complex
exchanges. It is therefore a challenge to measure the many benefits they provide in
quantitative and monetary terms for our purposes of urban planning and study. However,
by using city tree inventory data, regional satellite imagery, and urban forestry tools such
as iTree, we can better understand these systems and the contribution of street trees to
environmental and public health. In the pages that follow, I discuss the iTree Streets and
iTree Eco results, and GIS maps in more depth and how these findings shed light on the
social, environmental, and economic benefits to the City of Olympia and its residents.
The research results outlined in this chapter illustrate how Olympia street trees support
the goals outlined in the Thurston Climate Mitigation Plan and the City of Olympia’s
Stormwater Management Program (SWMP) Plan, and the importance of further research
on trees as a nature-based solution for climate mitigation planning. At the close of this
chapter I have included a discussion of my study limitation and recommendations for
future research on the ecosystem services of our urban forest.
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Street trees and carbon services
Carbon storage capabilities by trees are widely recognized, with trees
accumulating carbon in their woody material above and belowground over their course of
their lifetime (Mcpherson et al., 1997; Nowak et al., n.d.). In developed landscapes, an
urban forest can behave as a carbon sink, absorbing more atmospheric carbon than it
emits (TRPC, 2017). Olympia’s street trees have collectively stored roughly 5 million
pounds of carbon, according to iTree Streets results. When using the monetary values as
determined by the EPA for the Social Cost of Carbon, this equates to a value of $430,000
(Epa & Change Division, 2016; Vargas, 2018). Carbon stored by trees accumulates over
the years, and therefore our urban street trees will increase their storage of carbon with
time as long as we keep them healthy (Nowak et al., n.d.).
Based on Olympia’s average annual urban forestry program costs (2014-2019)
tree and stump removal accounts for approximately 25% of the annual budget ($120,000)
as seen in Appendix B. Additionally, tree removal activities result in the emission of
greenhouse gases as a result of the tools and equipment used by tree crews (Mcpherson et
al., 1997). Therefore, keeping the street trees maintained and healthy would provide the
environmental and social service by trees to store carbon, and the economic benefit of
reducing tree removal costs for the City of Olympia.
Of all the greenhouse gases associated with climate change emitted each year,
carbon dioxide is emitted in the largest quantity as a by-product of human activities (Epa
& Change Division, 2016). In 2016, emissions in the Olympia area reached
approximately 750,000 U.S. short tons of carbon dioxide (TRPC, 2018). Street tree
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carbon sequestration is an important ecosystem service when considering the global issue
of climate change and the adverse climate stressors on local urban environments like
Olympia, leading to increased summer temperatures and poor air quality (Nowak et al.,
n.d.; TRPC, 2017). Trees absorb atmospheric carbon dioxide through small openings on
their leaves called stomata as they grow new leaves and woody biomass (Kuehler,
Hathaway, & Tirpak, 2017). Using iTree Streets I determined that each year Olympia’s
street trees sequester 420,000 pounds of atmospheric carbon dioxide. This demonstrates
that trees are active participants in the carbon cycle and should be considered as a tactic
for regional climate mitigation (Glaeser & Kahn, 2010; Nowak et al., n.d.)
Figure 22. ArcGIS Insights results for growspace trends.
Olympia’s street tree population composition warrants a closer look when
discussing the sequestration of carbon. The actual rate of carbon sequestration will vary
with species, but in general, younger and faster growing trees have higher annual
sequestration rates (US DOE EIA, 1998). As we can see in Figure 22, the average DBH
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of Olympia’s street trees is 9 inches, with 46.5% of the population smaller than 6 inches
DBH. This younger, still maturing street tree population may have an increased carbon
sequestration rate as a consequence. I recommend further research on this is topic to
refine the annual sequestration estimates for Olympia’s street trees.
Street trees also help to mitigate carbon dioxide (CO2) emissions by regulating
ambient temperature and thus reducing energy use in nearby buildings, thereby reducing
energy demands from local energy providers, such as Puget Sound Energy for Olympia.
In this study I determined that this ecosystem service has resulted in a decrease of
150,000 pounds of carbon emissions from power plants due from reduced energy use,
with an associated annual net savings of approximately $45,000, incorporating Social
Cost of Carbon values (Epa & Change Division, 2016; Vargas, 2018). Note that these
results do account for CO2 released as trees die and decompose and CO2 released during
the care and maintenance of trees. In total, I determined in this study that Olympia street
trees sequester and mitigate 530,000 pounds or 265 tons of atmospheric carbon dioxide
each year.
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Street trees and energy reduction
Figure 23. iTree Streets graph results from iTree Streets showing the total amount of
ecosystem services provided by each street tree in an average year. Note that the Olympia
inventory was not broken down into zones, Zone 1 is the same as the Citywide Total.
For the past 50 years the primary sources of CO2 emissions for the City of
Olympia have been from energy supply, industry, and deforestation (Olympia & Lacey,
2010). Now however, the top two leading sources of emissions in Olympia are from
buildings and vehicles (City of Olympia, 2016). As we can see in the bar graph of the
iTree Streets report (Figure 23), the estimated energy benefits and avoided emissions
from the presence of nearby trees to buildings are one of the most important economic
contributions of Olympia’s street trees (shown in black). The energy and emissions
savings arise from the cooling benefits of trees during summer months and the wind-chill
reduction benefits in winter months (Manning, 2008). According to iTree Streets results,
Olympia’s street trees reduce building energy demands citywide at an associated cost of
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more than $785,000 each year. On average, that’s an annual energy savings benefit of
$330 per tree for nearby homes and businesses.
When looking at the landscape in GIS, we can use a Heat Index method of
detecting surface heat from aerial imagery to determine the relationship of trees to the
temperature of their surroundings (Figure 24). The cooler colors of green to blue show
the areas that have cooler temperature responses, and the warmer colors of orange to red
represent higher temperature responses from surfaces. In these images we can see how
the image on the top has more tree cover and a more general cooler response, while the
bottom image shows a predominance of warm surfaces, with street trees circles in black
in each image. We can determine that the trees in the downtown image on the right are in
a prime position to be performing these important services of reducing urban
temperatures, reducing A/C unit emissions, and thereby improving air quality in dense
urban environments.
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Figure 24. Heat Index GIS maps of Westside Olympia (top) and Downtown
Olympia (bottom) showing the relationship of trees (circled in black) and the
cooling effect they have on the landscape, thereby reducing building energy
demands.
Because of the predominantly urban nature of the region analyzed (especially
accounting for the roughly 680 trees in the downtown region) the potential energy
benefits from trees and the associated mitigation of emissions could be considerable, as
reflected by the average energy-saving values calculated by iTree Streets and as
illustrated Heat Index map. Because of missing tree measurements required for energy
analysis in iTree Eco, the energy benefit results from iTree Streets were used in this
study, and reflect estimated energy benefits across the whole city. Therefore, a geospatial
analysis of trees was of particular importance in order to illustrate the potential benefits
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of the street trees under study on building emissions and avoided emissions from energy
production by Puget Sound Energy.
The map detail of Downtown Olympia (Figure 25) includes tree locations (shown
as green points), impervious surfaces (shown in grey), and buildings (shown in orange).
The blue halos around each green point show the 60-foot radius around each tree
location.
Figure 25. All trees within 60-feet of buildings are here colored purple, and all trees 60-feet from a tree are colored
blue. Visual assessment of tree energy benefits to buildings may be possible from this GIS method of analysis.
Figure 25 depicts this snapshot of the downtown area and highlights the number
of street trees providing energy savings (purple points) to businesses and apartments
(highlighted in blue). Out of the entire living street tree population (2,334), 73% of
Olympia’s street trees (1,692) are within 60 feet of a building— the range for providing
some form of energy benefits for 673 buildings and homes.
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Figure 26. All trees within 60-feet of buildings are here colored purple, and all trees 60-feet from a tree are
colored blue. Visual assessment of tree energy benefits to buildings may be possible from this GIS method
of analysis.
Although green energy and building efficiency tactics are being employed to
address building emissions, these research results are exciting when considering the
Thurston County Action Team (TCAT) report showing commercial and residential
buildings as the primary contributors to greenhouse gas emissions in the county (Figure
26). The TCAT Climate Mitigation Plan also set an emission reduction target of 45% by
2030, a portion of which must come from those very buildings. Further research needs to
be done to improve these energy reducing ecosystem service estimates however, as the
tree height and canopy size affect the amount of shade they provide, and the distance and
cardinal direction to nearby buildings are important factors to take into account to best
determine the energy benefits.
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Street trees and air quality
Healthy leaf area links directly to many of the benefits of trees, including shade
and absorption of airborne pollutants (Grote et al., 2016). As Glanzberg states, “all
exchanges occur across boundaries, [therefore] the more surface area, the more exchange
is possible” (Glanzberg, 2020). According to iTree Eco, Olympia street trees canopy
covers approximately 75.1 acres of leaf area across the urban landscape. Using GIS we
can visualize the range of canopy cooling benefits across the city as seen in Figure 27
The blue areas in this snapshot depict the 60-foot range of cooling benefits provided by
the street tree population along the major travel corridors and downtown Olympia (Forest
Service, 2020).
Figure 27. Tree energy range in blue across the City of Olympia, showing the range of cooling, and air quality services
provided by street trees.
Based on the air pollution models of iTree Streets, I calculated Olympia’s street
tree population intercepts and prevents almost 2,500 pounds of airborne pollutants every
year, including nitrogen dioxide, sulfur dioxide, ozone, and particulate matter of 10
microns (PM2.5) (Table 11). Sulfur dioxide (SO2) is a byproduct of fuel combustion in
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vehicles and energy production (Grote et al., 2016). According to iTree Streets, street
trees intercept (through deposition) 33 pounds of sulfur dioxide from vehicle emissions,
and provide a secondary benefit of reducing the energy demands of nearby buildings
thereby avoiding the emissions of 327 pounds of sulfur dioxide from the process of
energy production by Puget Sound Energy. If we consider that vehicles are a major
source of sulfur dioxide emissions and are the second leading source of greenhouse gas
emissions in Thurston County (Figure 26) street trees in Olympia could play a role in
mitigating air pollution and carbon dioxide at the source of emissions.
Table 11. iTree Streets results for annual air quality shown in total pounds of deposition
(pollutants intercepted by the trees), total pounds avoided (energy emissions reduced
thereby reducing air pollutants), and total biogenic volatile compound (BVOC)
emissions (natural emissions from trees) here shown as a negative value.
As stated previously, city residents are exposed to an average of 200 different
types of air pollutants in a day (Sicard et al., 2018). Using the air pollution models of
iTree Streets, I determined that Olympia’s street trees intercept 794 pounds of air
pollutants annually, including 200 pounds of particulate matter (PM10) which have
been linked to increased rates of asthma (Epa & of Air, 2014). This socio-economic
benefit of the trees is further illustrated in the graph shown in Figure 28, which tracks the
air-pollution interception trends over the course of an average year in Olympia.
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Figure 28. iTree Eco results for monthly pollutant removal trends by Olympia’s street trees.
iTree Eco results for air quality (Figure 28) includes the rate of carbon dioxide,
nitrogen dioxide, ozone, particulate matter 2.5, and sulfur dioxide interception by
Olympia’s street trees. Although the exact numbers in the iTree Eco results cannot be
considered reliable, the behavior trends illustrated in this graph are a worthy inclusion in
this discussion. Because deciduous trees make up the bulk of Olympia’s street trees, the
amount of air pollution that they intercept drops off as they lose their leaves each autumn,
increasing again as they put on fresh growth in the spring (Figure 28). On average, trees
growing in Thurston County experience 280 leaf-off days, and 127 leaf-on days (Weather
Atlas, n.d.). This pattern of change in leaf canopy can also help to explain some
differences in other ecosystem services, such as their role in intercepting seasonal
rainfall.
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Street trees and stormwater
iTree Eco results depend on crown health variables to determine the amount of
precipitation intercepted by leaves; therefore, the volume of intercepted rainfall results,
and their relative dollar amounts are inconclusive. However, these results can illustrate
trends in tree behavior based on local weather data and leaf-off seasonality, explained
below.
Figure 29. iTree Eco result of water interception and evaporation provided by trees each month for the weather data
year of 2016, showing a dramatic rise in stormwater benefits in leaf-on seasons.
The iTree Eco result (Figure 29) displays the monthly evaporation provided by
street trees in inches per hour and cubic feet per hour, illustrating the vast increase of
evaporation during warmer months which helps to cool the urban landscape (Berland et
al., 2017). As shown in Figure 29, the trend of stormwater interception is fairly low in
leaf-off seasons and increases dramatically during leaf-on seasons. This could be
explained by the dominance of deciduous tree species in the Olympia street tree
population, and by the Mediterranean climate of the Northwest—wet winters and arid
summers. However, the stormwater interception potential during the leaf-on season could
still be substantial as seen in the summer months shown in Figure 29. According to the
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local weather station data, the total average rainfall is 15.4” for the leaf-on months (AprilOctober), which is equal to approximately 6 billion gallons of rainfall during leaf-on
months in Olympia (“Weather Atlas,” n.d.).
This stormwater service has a secondary benefit of reducing the flow of pollutants
into receiving bodies of water. The Stormwater Management Plan of 2019 and new state
discharge permit requirements mandate that the City of Olympia reduce stormwater flows
and the discharge of pollutants to protect regional water quality. The stormwater services
of street trees support the efforts of the City of Olympia to reduce the volume of
stormwater runoff by more than 1.5 million gallons of stormwater over the year.
Economically speaking, with stormwater costs valued at $0.0277 per gallon (based on
2016 weather data and City of Olympia utility costs) this provides a savings to Olympia’s
stormwater department of approximately $42,500 annually.
Figure 30. NDVI analysis using aerial imagery of Downtown Olympia in winter, showing red chloryphil
response of grass and conifer trees, but no response from street trees (circled in black).
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These stormwater results suggest that the predominantly deciduous tree
population of our street trees provides less in terms of stormwater services during the
rainy-season in Olympia. Based on studies in the Pacific Northwest, a conifer intercepts
and transpires an estimated 30% of the precipitation that falls on it, while a deciduous
tree intercepts and transpires 15% (Clapp, Ryan, Harper, & Bloniarz, 2014; Illgen, 2011).
Based on the NDVI maps (Figure 30) showing the canopy of evergreen and deciduous
trees during the winter leaf-off season, there is a chlorophyll response from conifer trees
on private property and green spaces throughout the city during winter months, showing
that the conifer canopy cover actively intercepts stormwater year-round. Considering
conifer species only make up 0.5% of the street tree population, these stormwater results
in this study should be considered preliminary research and recommendations for further
study are included at the close of this chapter.
Figure 31. ArcGIS Insights results for DBH
trends showing the location and sizes of the street
trees.
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Finally, rainfall infiltration into the soil and uptake by tree roots may be important
to consider in urban settings that have planted predominantly deciduous trees. It is widely
recognized in the literature that larger healthier trees provide greater ecosystem benefits
including stormwater interception and infiltration abilities; therefore, the logic follows
that we should consider surface types, local annual rainfall patterns, tree health, and the
growspace size of our deciduous urban trees to understand the potential for stormwater
soil infiltration (Mcpherson, Nowak, & Rowntree, 1994). As seen in Figure 30,
4’x4’planter spaces (0-4 feet small, tree grate, brick/paving) show street trees with a
smaller DBH sizes on average than 8+ feet large and unrestricted growspaces. This
suggests that larger growspaces for Olympia’s street trees would facilitate tree growth
and long-term tree health.
Limitations and recommendations
The inventory of street trees used in this research represents a relatively small
population of trees within Olympia and does not capture the ecosystem services provided
by the entire urban forest within the City of Olympia. Based on the limitations of this
inventory I recommend that an urban tree canopy cover assessment be completed for the
entire City of Olympia boundary. A complete assessment would determine the percentage
of total canopy cover, and evergreen and deciduous makeup of our urban forest,
providing a more comprehensive calculation of stormwater and other ecosystem services
provided by our urban forest. I also recommend that conservation of conifer tree species
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within our city boundaries be supported in our city’s tree policies, and preferential
planting of conifer species should be given when suitable open grow spaces are available
to support year-round stormwater processing benefits within city limits to reduce water
pollution and stormwater volume.
iTree software limitations exist as well. iTree Streets results rely on models used
to determine average annual tree benefits from the 2002 study of the City of Longview
Washington (Gregory et al., 2002). Although my research methods included current
regional utility rates and regional city information, results from this study should serve as
a preliminary study of the potential ecosystem benefits of Olympia’s street trees and
guideline for an improved method of ecosystem-benefits analysis. The iTree Eco results
serve to show the importance of collecting tree variables during routine street tree
maintenance that can improve the data needed for future analysis using the superior
ecosystem benefit models embedded in the iTree Eco software.
Based on the limitations I encountered using the existing tree inventory in this
study, I further recommend that the street tree inventory include additional tree
measurements so that the iTree Eco models could be run with the street tree inventory
data. This would enable future ecosystem service research, including the important
energy benefits of our street trees using superior models of calculation included in iTree
Eco or similar urban forestry software. I recommend to the urban forestry department that
the following variables be added to their inventory for future study and planning:
1. Tree health condition as a percentage (0-100%).
2. Crown condition as a percentage (0-100%).
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3. Percent of crown missing.
4. Top and bottom crown height in feet.
5. Location of tree to structure (North, South, East, West).
6. Distance in feet to nearest structure to determine energy benefits.
Additionally, the inventory used in this study included the qualitative
measurement of the tree’s condition (Good-Poor) that proved ill suited for the
quantitative parameters required for canopy and tree health by iTree Streets and iTree
Eco. Currently, the condition assessment of the street trees in the inventory by
maintenance crews “in practice... turn out to be fairly arbitrary” (B. Moulton, pers.
comm. Feb. 13, 2020). Therefore I recommend that the “iTree Eco Tree Assessment
Guide” or similar tree health guide be followed by tree maintenance staff in order to
establish a baseline and standard of tree health ratings.
Lastly, this research included ecosystem services and the relative monetary values
for air quality, stormwater, carbon, and energy, and did not include other equally
important benefits of trees such as the psychological benefits, community enhancement,
crime reduction, and property value increase benefits of urban trees. Further research on
the benefits of our urban forest is recommended. Most importantly, I recommend that
urban forest strategies for enhancing the ecosystem services provided by street trees be
adopted by the Olympia urban forestry department (Hastie, 2003). For example, by
supplying generous water to trees seasonally, tree health and canopy fullness increases,
improving ecosystem service yields such as air pollution removal (Nowak, 2000), and
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evaporation during summer months to cool the surrounding urban landscape (Berland et
al., 2017).
With the study limitations and recommendations for future research in mind, these
results highlight the possibility of improved future urban forest valuation assessments for
Olympia and other cities, to further advocate for the conservation and care of our urban
tree. As seen from the multiple results from the iTree programs and GIS geospatial
analyses in this section, the many services of trees can be quantified and understood in
greater detail using urban forestry software and existing tree inventory data. Furthermore,
these results illustrate the social, environmental, and economic benefits of a healthy urban
forest, and how these benefits can support our local climate mitigation goals.
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CONCLUSION
Nature’s economy shall be the base for our own, for it is
immutable, but ours is secondary. An economist without
knowledge of nature is therefore like a physicist without
knowledge of mathematics.
—Carl Linnaeus (as translated by Lisbet Koerner)
In this study I was able to determine the annual ecosystem services of Olympia’s
street trees using iTree urban forestry software and GIS mapping software. Using these
valuation methods can provide a deeper understanding of urban trees, and the benefits of
the complex living network of trees within the city environment. We have seen in this
study that Olympia street trees support the long-term health of our residents by improving
our local air-quality by removing 2,500 pounds of air pollutants, reduce asthma-related
medical costs by $45,000, and improve water quality by processing 1.5 million gallons of
stormwater each year.
Throughout this study we have also seen how urban trees can contribute to our
goals of urban sustainability and climate mitigation. Using the iTree methods of valuation
I determined that our street trees sequester 420,000 pounds of atmospheric carbon
dioxide each year, and reduce citywide energy spending by $785,000 annually. As part of
the cost-benefit analysis of Olympia’s street tree program I was found that the multitude
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of ecosystem services provided by our street trees outweigh the costs associated with tree
planting and maintenance. Annually, these ecosystem services culminate in an annual net
benefit of more than $600,000 for the City of Olympia and its residents, with an
associated average annual benefit value of $450 per tree. Essentially, for every $1 spent
annually by the City on street trees, $2.30 in ecosystem benefits are provided by street
trees in return.
The results of this study may be for a relatively small population of trees within
the City of Olympia, but it underscores the important role of trees in the long-term health
and resilience of our local communities and the natural environment. The collective
ecosystem services of our urban forest deserve to be included in our climate mitigation
plans; further research needs to aid our urban forestry departments in their efforts to
maintain our trees for the health and well being of residents and the environment.
In fact, this research has shown that the street trees within Olympia help to reduce
carbon emissions, improve local air quality, intercept stormwater, and reduce energy
demands, but as a final thought, let us imagine the City of Olympia as seen from above.
Imagine the entire urban forest that stretches across the 20 square miles of the city
landscape, and the magnitude of services they are silently performing. Pull back then
even farther and imagine the tree canopy that stretches across the 774 square miles of
Thurston County. As we continue to grow our cities and develop our climate mitigation
plans we should do so holistically and with awareness of the ways our trees support that
goal of sustainable development. By recognizing and valuing the ecosystem services of
trees we can better advocate for the conservation and health of our urban forests. These
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natural processes are happening all around us, cleaning our air and improving our
water— all we have to do is look.
Planning, building, and implementing a comprehensive climate mitigation plan is
founded on the principle that we are responsible for the present and future health of each
other and the environment; we seek a sustainable way to grow and exist in a world with
finite resources. Sustainable development of our cities and surrounding environments
must include a methodical approach to the economic management of our natural
resources, including our urban forests (Hastie, 2003; Munasinghe, 1993). If “value” is
defined as the belief that something is held to deserve; the importance, worth, or
usefulness of something, how then do we as a society, or a city, value our urban forest?
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APPENDIX A. EMISSION RATES
Appendix A1. Research methods for acquiring rates.
Tree Mitigation Benefit Rates
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Appendix A2. Olympia City Data
Population (2019)
51,609
Total land area (sq.miles)
20.09 square miles
Average sidewalk width
6.9 feet
Total linear miles of streets
19,024,260
Average street width
31’
Median home resale value
(2019)
$354,494.00
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APPENDIX B. REGIONAL DATA
Appendix B1. Olympia Tree Inventory
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Appendix B2. Olympia City sidewalk and street design standards
97
Appendix B3. Annual itemized list of expenses by Olympia’s Urban Forestry
program.*
Program Expenses
2014-2019
Average
Planting costs**
$28,407
Maintenance/pruning costs
$76,402
Watering costs
$3,500
Tree and stump removal costs
$119,610
Litter/Storm clean-up costs
$9,180
Annual inspection/answer service
requests
$22,790
Infrastructure repair due to trees
$60,186
Program administration costs
$165,648
*Annual costs rounded to the nearest dollar during import into software.
**Planting moratorium in effect since 2016. Planting efforts have steadily declined since 2015.
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APPENDIX C. ITREE METHODS
Appendix C1. Land use data entered into iTree programs
Code
Land use category
Code
Planter Type
1
Single family/private
residential
1
Tree grate
2
Multi-family residential
2
Brick/paving
3
Small
commercial/business/church/school
3
0-4 feet, small
4
Industrial/large
commercial/municipal
4
4-8 feet,
medium
5
8+ feet,
medium
5
Median/street
planting/island
6
Park tree
6
8+ feet, large
7
Park/vacant/parking lot
7
Unrestricted
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Appendix C2. Tree measurement variables and descriptions entered into iTree
programs.
Tree
Variables
DBH
Condition
Details
Diameter breast height of tree (in inches)
Health of the overall trees, including canopy health, for future
management recommendations (coded values e.g. 70=fair 80=good)
Land use
General land use type surrounding the tree location (e.g. commercial or
industrial)
Site type
Planting types such as a cutout or planting strip
Tree height
Height measured to the top of the crown in feet
Sidewalk
damage
Pest detection
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Represented in inches of sidewalk lift, e.g. 1” lift. Coded values 1: 0-¾
in. 2: ¾-1½ in. 3: >1 ½ in
Coded values (1=yes, 2=no)
Appendix C3. Eco Canopy Health Classes
Entered Olympia inventory crown condition percentages as relative to Eco default values.
Code 1: Excellent =
Code 5: Good =
Code 7: Fair =
Code 11: Poor =
Code 15: V. Poor =
Code 22: Stump =
Code 22: Vacant =
N/A
82
72
52
32
0
0
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APPENDIX D. ARCGIS WORKFLOW
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APPENDIX E. ITREE RESULTS COMPARISON
iTree Streets and iTree Eco results comparison
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