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BIRD COMMUNITY RESPONSE TO PASSIVE VERSUS ACTIVE
MANAGEMENT ON MOUNT ST. HELENS WASHINGTON
by
Heather May
A Thesis
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
June 2016
©2016 by Heather May. All rights reserved.
This Thesis for the Master of Environmental Studies Degree
by
Heather May
has been approved for
The Evergreen State College
by
________________________
Kevin Francis, PhD
Member of the Faculty
________________________
Date
Abstract
Bird community response to passive versus active
management on Mount St. Helens Washington
Heather May
Post disturbance management, specifically salvage logging, is used regularly in
Pacific Northwest forest ecosystems to off-set economic losses, reduce fire fuel loads and
restore industrial timberlands. This practice also removes habitat components such as
snags and downed wood which are important to many species of birds and other wildlife.
The study of birds post-disturbance can provide information to managers on habitat
components which have persisted and those which are lacking in the ecosystem, leading
to more informed management decision-making. The body of knowledge regarding bird
response to salvage logging continues to be sparse and studies lack the necessary
longevity and comparison to a control to be conclusive. This study provides these
necessary pieces by comparing study sites that are under long-term conservation for
research (passively managed) and those that were salvage logged and replanted (actively
managed) following the May 18, 1980 eruption of Mount St. Helens. We found a pattern
where passively managed sites had greater bird abundance and diversity than noble fir
plantations and significantly greater than Douglas-fir plantations. These results indicate a
need for more understanding of the habitat components which are lacking in actively
managed sites in order to develop best management practices for salvage logging.
Table of Contents
I.
Introduction……………………………………………………………..…..1-5
II.
Literature Review………………………………………………….............5-31
A. Impact of research at Mount St. Helens on successional theory…..............6-12
1. Conceptual foundations of disturbance and succession…..……….........6-8
2. Lessons from Mount St. Helens: refugia provide the seeds of
succession……………………………………………………………...8-11
3. Lessons from Mount St. Helens: succession is a series of chance events
leading to structural change across an ecosystem...…………...…......11-12
B. Plant and bird colonization post-disturbance……………………….…….13-20
1. Birds indicate response of ecosystem functions and processes to
disturbance events……………………………….……………………13-15
2. Studies of bird functional groups provide broad analysis and
generalization of result……………………….………………………15-16
3. Birds and plants exhibit highly specialized interactions which provide
mechanisms for seed dispersal and drive succession following
disturbance……………………………………………………………16-17
4. Disturbance patch size determines available mechanisms of
recolonization………………………………………………………...17-20
C. Mutualistic relationship between birds and plants increases biodiversity..20-24
1. Birds drive succession by aiding dispersal of plants…………………20-22
2. Avian role in plant dispersal enhanced during secondary succession
………………………………………………………………………..22-23
iv
3. Legacy structures attract bird dispersers to a disturbed area…………23-24
D. Disturbance enhances biodiversity in Pacific Northwest forests…………24-31
1. Birds respond to changes post-disturbance and species who colonize are
adapted to survive in new conditions…………………………..…….24-26
2. Fire suppression and salvage logging in Pacific Northwest forests may
have unintended and unknown ecological consequences…………….26-30
3. Increased knowledge of salvage logging impacts necessary for future
implementation……………………………………………………….30-31
Research manuscript……………………………………………………..32-60
III.
A.
Abstract……………………………………………………………….......32
B.
Research setting……………………………………………………….32-33
C.
Climate and ecology…………………………………………………..33-34
D.
Recent volcanic history……………………………………………….34-38
E.
Forest composition prior to May 18, 1980……………………………38-40
F.
Methods: study sites…………………………………………………..40-43
G.
Methods: bird data collection…………………………………………….43
H.
Methods: data trimming………………………………………………….44
I.
Bird guild structure analysis………………………………….……….44-47
J.
Methods: bird data analysis…………………………………….……..47-49
K.
Results: bird abundance and diversity among sites…………….……..49-51
L.
Results: bird guild analysis…………………………………….……...51-53
M.
Results: bird abundance and diversity by treatment…………….…….53-57
N.
Discussion……………………………………………………….…….57-60
v
List of Figures
Page 2
Figure 1: before and after images of Mount St. Helens, source: National Geographic
Page 3
Figure 2: blowdown zone of Mount St. Helens, source: Robert Krimmel (USGS)
Page 10
Figure 3: image explaining how canopy openings increase light availability for plants,
diversifying vegetation, source: W.H. Freeman
Page 25
Figure 4: diagram representing connections between fire ecology, habitat niches and bird
species abundance, source: Alexander et al. 2000
Page 27
Figure 5: example of postfire salvage logging, source: John Muir Project
Page 29
Figure 6: depictions of habitat created by down wood and how they change with decay,
source: USFS
Page 30
Figure 7: depiction of vertical forest niches and the diversity of species which inhabit
them, source: Texas Parks and Wildlife
Page 35
Figure 8: time lapse photo series of the initial events of May 18, 1980 on Mount St.
Helens, source: Gary Rosenquist
Page 36
Figure 9: map of Mount St. Helens blast zone, source: Tilling 1984
Page 37
Figure 10: map of Mount St. Helens ash fallout, source: USGS
Page 41
Figure 11: map of study site locations on Mount St. Helens. See table 1 for site names,
source: USFS
Page 44
Figure 12: example of data trimming exercise results performed for each transect
vi
Page 52
Figure 12: bar graph depicting mean species per guild between sites with standard
deviation (SD) error bars
Figure 13: bar graph depicting abundance of guilds represented in each treatment
Page 54
Figure 14: bar graph of mean avian abundance per treatment with standard deviation
error bars
vii
List of Tables
Page 42
Table 1: table representing site codes, names, treatment and coordinates of locations on
Mount St. Helens
Page 45-47
Table 2: table of bird species codes, common names, guilds and guild definitions
Page 50-51
Table 3: table with bird species abundance per site with summary statistics. Species
highlighted in green were detected on only one site, species highlighted in blue were
detected on all sites
Page 53
Table 4: indicators of bird abundance and diversity by treatment
Page 54
Table 5: p-values obtained with a One-way Analysis of Variance Test of bird abundance
compared between treatments, significant p-value indicated in red
Table 6: table of results of Shannon’s Index and related calculations which describe
species diversity
Page 57
Table 7: table comparing results of three different calculations comparing species
richness, Jaccard similarity coefficient, Sorenson index and Renkonen similarity index
viii
Acknowledgements
I wish to humbly thank some folks who have enriched my educational experience
and enabled me to successfully complete this work. First and foremost thank you to my
US Forest Service collaborator Charles M. Crisafulli for providing me with such an
interesting dataset and guiding my analysis. Thank you Kevin Francis, MES Director, for
guiding my thesis writing process and for providing me excellent feedback on my writing
throughout the program. Thank you to all of my fellow 2014-2016 MES cohort who
offered inspiration and commitment to learning over the past two years. I especially
want to thank Rhianna Hruska and Hannah Trageser for their excellent feedback and
motivation and Josh Carter for technical support. Thank you very much to my family
who has always supported my ongoing educational and professional ambition, especially
my parents Milo and Kay. Thank you Oliver for providing me with original images of
birds for use in my thesis presentation. Thank you Anthony Saucier and Cash the
labradoodle for unconditional love and support.
ix
I. INTRODUCTION
The eruption of Mount St. Helens created a large-scale natural experiment useful
for the study of volcanic ecosystems but also offers the opportunity to understand
ecosystem succession following cataclysmic disturbance. It is the most vigorously
studied eruption in history with long-term datasets monitoring plants, invertebrates, small
mammals, birds, and volcanic processes (Dale, Swanson, and Crisafulli 2005).
Disturbance is an important component of all ecosystems and must be understood and
studied in the context of the natural disturbance regime of the area in question.
The eruption of Mount St. Helens constituted a natural part of the local
disturbance regime, however a severe one which had not taken place for 123 years (Dale,
Swanson, and Crisafulli 2005). The events of May 18, 1980 began with a 5.1 magnitude
earthquake and the collapse of the entire northeast side of the mountain that became the
largest landslide in recorded history (Lipman and Mullineaux 1981). The landslide
released volcanic pressure in the form of magma and gases within the mountain
triggering a supersonic blast which leveled a 500 square kilometer forested area (Lipman
and Mullineaux 1981). The eruption also included a ‘hot, ash-charged gas shot’ which
rose rapidly from the volcano to elevations of at least 23 km and proceeded to distribute
ash throughout the northwest and Midwest United States (Lipman and Mullineaux 1981).
The ash continued to circle the globe multiple times and most likely remained in the
atmosphere for years (Lipman and Mullineaux 1981).
The culmination of the events of May 18, 1980 on Mount St. Helens was the total
disruption of the ecosystem within the blast zone. Topography was leveled, lakes were
relocated, and many large lifeforms such as deer, bear, and many birds were killed.
1
Smaller mammals such as mice as well as many plants, algae, and fungi were protected in
micro-climates and survived the eruption (Dale, Swanson, and Crisafulli 2005). All life
on Mount St. Helens, whether surviving or returning to the blast zone as colonizers, must
adapt to an ecosystem recovering from cataclysmic change (See figure 1).
Figure 1: before and after images of Mount St. Helens, source: National Geographic
In 1982 the United States Federal Government set aside 44,550 hectares of the
Mount St. Helens blast zone as a long-term study area. The Mount St. Helens National
Volcanic Monument is an outdoor laboratory for the study of volcanism and natural
succession following cataclysmic disturbance. The blast zone includes areas referred to as
2
the scorch zone, where magma and fires started from magma flows created conditions
similar to forest fires, and the blowdown zone where the majority of trees were toppled
by the blast (See figure 2). Other parts of the blast zone are outside of the Monument and
are undergoing another type of experiment, in reforestation. After the eruption,
Weyerhaeuser Corporation and the United States Forest Service collaborated to recover
merchantable timber from the blast zone. This process included salvage logging, piling
and burning of remaining material, scarification of soils, and planting of commercial tree
species. Salvage logging has been a large part of the management regime on United
States Forest Service lands for decades, however the ecological effects of this practice
have not been well determined (McIver and Starr 2000).
Figure 2: blowdown zone of Mount St. Helens, source: J. Devine (USGS)
3
Measurements of bird abundance and diversity provide important insight for
managers about the health of ecosystem processes and functions, especially following
disturbance. This research compares bird communities in areas of the blowdown zone
which were salvage logged and replanted (actively managed) with those in the Mount St.
Helens National Volcanic Monument which were left to regenerate naturally (passively
managed). Bird community response to these management regimes can help managers
understand the effects of salvage logging and inform appropriate management decisions
following disturbance. The protected portion of the blast zone within the Mount St.
Helens National Volcanic Monument is a control which may lead to greater
understanding of the long-term effects of management following natural disturbance. As
human footprints on ecosystems continue to increase in size, a natural experiment such as
this provides the opportunity to inform conservation biology and restoration ecology, two
fields concerned with protecting ecosystems from threats such as climate change.
As the nature tourism industry has grown in the Pacific Northwest, birds have
become cultural icons and important to local economies. Their story on Mount St.
Helens, like the human one, begins with massive devastation and is just now beginning to
make a comeback. From total annihilation in the blast zone, to slow colonizers, to drivers
of succession, their progress has been well-documented since May 18, 1980. It is certain
however, due to birds’ ease of identification and quantification and overall importance as
ecosystem architects, that there will be more focus on their role as their numbers increase
with continued succession on Mount St. Helens. Presence, or absence, of birds from a
habitat provides important insight on the effects of disturbance, how an ecosystem is
responding, and the ultimate path it will follow.
4
This document contains an introduction, a literature review, and a research
manuscript. The introduction explains the context and setting of this research as well as
the importance of understanding the effects of post-disturbance management on future
biological communities. The research manuscript outlines our research, methods, and
findings which seek to understand the differences between bird communities in areas that
were salvage logged versus those that were not following the eruption of Mount St.
Helens. It ends with a discussion of management implications of this research and
recommendations for future research.
II. LITERATURE REVIEW
The following literature review will summarize the key lessons learned about
succession of ecosystems after cataclysmic disturbance from 25 years of research on
Mount St. Helens (MSH) and how the theoretical history of succession has shaped
current ecological research on disturbance and management. It continues to define the
differences between large and small disturbances, how they are studied, and how they
shape the resulting landscape. It specifically reviews the processes of wind and fire
disturbance, their importance to temperate ecosystems, and the role of salvage logging as
a management tool in their wake. The literature review then explains colonization of
plants and birds following disturbance, the unique interactions which shape future plant
and animal communities, and why birds are appropriate indicators of habitat change. It
ends with an exploration of salvage logging as a management technique, what is currently
known about its effects, and questions about its impacts which still remain.
5
A. Impact of research at Mount St. Helens on successional theory
1.
Conceptual foundations of disturbance and succession
Disturbance is an important component of all ecosystems, with the ability to
increase heterogeneity over the landscape, providing opportunities for a greater diversity
of organisms to thrive in a given environment (Wiens 1989). According to Picket and
White (1985), “a disturbance is any relatively discrete event in time that disrupts
ecosystems, community, or population structure and changes resources, substrate
availability, or the physical environment”. This definition is the product of generations
of research studying disturbance and the ensuing effects on the ecosystem, known as
succession, which is one of the oldest and most central ecological theories. While Henry
David Thoreau talked about his understanding of succession as early as 1859, Henry
Cowles published formal works on the theory of succession beginning in 1899 describing
how dune vegetation advanced through more or less formal stages as it matured (Cowles
1899). The theory evolved when Frederic Clements published Plant Succession (1916),
introducing the idea of a climax community, a steady state which each ecosystem grows
toward as it matures. Beginning in 1920, Henry Gleason began to publish his more
complex and subtle version of succession which included chance events that could
change the entire course of succession, opposing the idea of a climax community
(Gleason 1939). More recently, research at MSH has offered deeper insight to some of
the fundamental aspects of this complex and dynamic ecological process.
The magnitude, severity, and spatial distribution of a disturbance determines its
effect on the surrounding ecosystem and ecological processes. Small-scale disturbances
of high or low magnitude and large-scale disturbances of low to moderate magnitude are
6
studied relative to “patch dynamics” (Thompson 1978; Pickett and Thompson 1978;
Pickett and White 1985). According to Pickett and White (1985),
1. “Patch” implies a relatively discrete spatial pattern, but does not establish any
constraint on patch size, internal homogeneity, or discreteness.
2. “Patch implies a relationship of one patch to another in space and to the
surrounding, unaffected or less affected matrix.
3. “Patch dynamics” emphasizes patch change (Pickett and White 1985).
Disturbance events create patches across a landscape introducing heterogeneity with
effects on the community of organisms it supports. The alteration of habitat structure and
resource availability has unique effects on birds which can be negative or positive
depending on a species’ specific adaptations.
Large-scale, high magnitude disturbances are studied in terms of succession
because they affect the ecological processes supporting communities of species and
require longer timeframes to recover (Pickett and White 1985). The study of ecosystem
response to natural disturbance at MSH provides insights on important ecological
questions about how succession proceeds on a large scale without human intervention.
Dale, Swanson, and Crisafulli published the results of 20 years of research on MSH in
Ecological Responses to the 1980 Eruption of Mount St. Helens (2005) and provided their
own definition of succession.
Succession: The process of gradual replacement of one species or population by
another over time and the concurrent change in ecosystem properties after a site
has been disturbed. The concept can be extended to the replacement of one kind
of community by another, the progressive change in vegetation and animal life
7
that may culminate in dominance by a community that is stable until the next
disturbance. Succession refers to changes that occur over 1 to 500 years and not
to seasonal changes in populations and communities (Dale, Swanson, and
Crisafulli 2005).
This definition of succession was directly informed by research at MSH where succession
did not proceed as a rapid march toward a climax community but instead in sporadic
bursts and lags. (del Moral 1999). This is emphasized in the definition used by Dale,
Swanson, and Crisafulli by use of the word “gradual” and clarification of long
timeframes versus seasonal changes. The idea of a climax community is included but not
guaranteed, while the inevitability of the next disturbance is emphasized. The following
chapter reviews lessons learned about succession directly from study at MSH and situates
them within the historical theoretical framework.
2.
Lessons from Mount St. Helens: refugia provide the seeds of succession
One of the first surprises for biologists following the eruption did not consist of
what was lost but what remained. Looking across the landscape after the events of May
18, 1980, it may have been hard to see anything more than total destruction. Not only did
timber companies and the United States Forest Service (USFS) lose major investments in
industrial timberlands but most wildlife within the blast zone were killed or driven away.
However, small plants, insects, and even mammals were able to survive in refugia,
locations where geographical features or natural structures (microsites) provided
protection and stability for life to persist after the eruption (Dale, Swanson, and Crisafulli
2005). Microsite conditions protected intact fragments of the former ecosystem from
which succession could build. Then, following the definition, succession proceeded with
8
species being added and changing ecosystem processes, creating niches to allow new
species to colonize. For instance in the blowdown zone of MSH, the first wave of plant
colonization included late-successional species which emerged from under as much as a
meter of ash, pioneer species, and wind-dispersed plants from areas outside of the
impacted area (Crisafulli and Hawkins 1998). The wind-dispersed plant colonization
follows traditional successional theory while the late-successional species were
unexpected (Crisafulli and Hawkins 1998).
Late-successional species survived in refugia created by an array of site-specific
conditions.
Four factors appeared to increase the probability that individual plants would
survive in these locations: 1) patches of late-lying snow shielded some plants
from the blast; 2) plants living on the lee sides of ridges were not exposed to the
main force of the blast; 3) some plants survived in soils on the exposed rootwads
of large blown-down trees; and 4) some plants were able to resprout from
perennial root stock on steep slopes where erosion quickly cut through ash
deposits (Mac et al. 1998).
Some understory conifers survived within the blowdown zone and they demonstrated
rapid growth after the loss of shade and began producing cones by 1993 (Mac et al.
1998). This phenomena greatly accelerated the overall process of succession in the
blowdown zone providing habitat for colonizing species (See figure 3). Other survivors
in refugia who were more vulnerable understory plants, such as forbs, could not tolerate
new conditions created in the blowdown zone and did not survive to reproduce (Mac et
al. 1998).
9
Figure 3: image explaining how canopy openings increase light availability for plants, diversifying vegetation, source:
W.H. Freeman
The survival of understory conifers provided areas of the blowdown zone with a
successional advantage due to the vertical structure gained by the trees’ rapid growth.
Ksudach, a volcano in Russia which erupted in 1907, was studied in the context of
succession by extrapolating a successional theory from the relationships between
vegetation and animal communities existing decades later. They noted that vertical
structures such as dead standing trees left after the eruption may have attracted animal
10
seed dispersers including birds, acting as key drivers of succession. This work became
the basis for further successional studies as well as ecological restoration work in the
tropics (Thompson and Wilson 1979; McDonnell and Stiles 1983; Yarranton and
Morrison 1974; McDonnell 1986). Vertical structure is now a known attractor of seeddispersing animals, especially birds, who use the structures to move between areas of
habitat and introduce colonizing plant species as they travel (McDonnell 1986).
Herbivores, especially arthropods initially, were as found to be as influential as
plants themselves at dispersing seeds in the first wave of succession on MSH (Dale,
Swanson, and Crisafulli 2005; Bishop 2002). Since insects are small they are able to take
advantage of refugia to a larger degree than mammals, allowing them to function as the
primary drivers of succession along with plants. As they emerged from volcanic soils
they acted as tillers, bringing fertile soil buried by the eruption to the surface and creating
a bed for dispersing seeds. These colonizing efforts are a mechanism of trophic
interactions between species which allow them to gain the necessary resources for
survival. As insects go about their business of decomposing items into soil they are
inadvertently moving seeds and sowing future plant communities. These minute details
of trophic interactions are often overlooked in healthy, intact climax ecosystems with
glorious megafauna to distract; however they become crucial mechanisms of change
when they are all that is left after cataclysmic disturbance.
11
3.
Lessons from Mount St. Helens: succession is a series of chance events leading to
structural change across an ecosystem
Another lesson learned from MSH is that succession does not proceed linearly
toward the ecosystem which existed prior to disturbance, but on a novel route to an
entirely different product (Dale, Swanson, and Crisafulli 2005). Not only are the flora
and fauna existing on MSH today a product of a relict ecosystem spread by survivors
after the eruption, but an ongoing series of smaller successions. Over time small trophic
interactions between new immigrants to a disturbed area combine to create niches for
other species to fill and provide opportunities for more trophic interactions. This process
slowly changes the structure and dynamics of the vegetation and ecosystem processes
eventually causing colonizing species to go locally extinct, a process MacArthur and
Wilson (1963) defined as turnover. Eventually an equilibrium may be reached where
immigration rates are equal to extinction rates and this is the theoretical ‘normal’ state of
populations in insular (island-like) regions (Diamond 1969). These theories are the
primary paradigm for conservation biologists who are engaged in answering the question
of ‘how much is enough’ in reference to creating reserves to protect biological diversity
(Quammen 1996).
While published in the late 1960s and early 1970s, MacArthur and Wilson’s
theories on colonization and extinction have remained the quintessential models for
ecologists, although some are beginning to build upon their work. Recent invasions of
species such as the Barred Owl and Eurasian Collared-Dove have motivated biologists to
think about colonization and extinction in less simplistic terms and take into account
species-specific interactions and environmental change (Yackulic et al. 2015).
12
Researchers studying succession on MSH have not found general patterns across the
disturbed landscape, but instead ‘chance colonization events’ which are the product of
local conditions (del Moral and Bliss 1993; del Moral and Wood 1993; Turner et al.
1998). While disturbance is an intrinsic part of every ecosystem, it is extremely variable
through space and time, imparting chance as a component of response (Wiens 1989).
B. Plant and bird colonization post-disturbance
1. Birds indicate response of ecosystem functions and processes to disturbance events
Birds have long been recognized as effective tools for ecological monitoring of
landscape change due to management or disturbance (Greenwood et al. 1993; Hutto
1998). There are three main reasons for their importance as indicators which are
emphasized by Hutto (1998): 1) ease of identification and cost-effective survey methods;
2) allow for rapid collection of large amounts of data; 3) birds can be generalist or
specialist, representing varied habitat needs and life-history traits. Wiens et al. (1986)
and others have found that small to moderate severity disturbance to a stable ecosystem
increases diversity by providing new opportunities for colonizers, while high severity
disturbance may alter the habitat too much to sustain some species (Denslow 1985;
Wiens 1985; Sousa 1979; Sousa 1984). While this phenomena has been well
documented among many wildlife species, “there are no quantitative treatments of this
relationship for bird communities” (Wiens 1989).
Because large infrequent disturbances (LIDs) produce a diversity of vegetative
types throughout the affected area, there may be many available niches for birds to recolonize as succession progresses (Turner et al. 1998). As vegetation changes however,
13
assemblages of bird species in the community change as well, creating local extinction
episodes (Kennedy et al. 2011). Most of the research concerning bird re-colonization of
disturbed habitats has been completed in patches created by land management such as
logging. One reason for this is that habitat destruction is the main threat to birds
worldwide; another is that opportunities for this research are widespread in reference to
management activities versus opportunities such as MSH to study large-scale disturbance
in a natural experiment, which happen rarely.
This research compares bird communities in 2010, 30 years following the
eruption, between sites which were passively managed and those which were actively
managed, including salvage logging. Bird abundance and diversity are expected to be
generally greater on the passively managed sites because debris was left onsite providing
habitat for prey species, adding nutrients to the soil to support developing vegetation
communities, and surviving plants on passively managed sites grew more quickly than
planted trees species on actively managed sites. Salvage logging has rarely been looked
at in an experimental study containing a control plot for comparison (USDA 2000).
MSH offers this opportunity and therefore the results of this study may provide
information which has remained elusive regarding long-term effects salvage logging has
on ecosystem functions and processes as well as how managers can augment the practice
to protect important wildlife habitat.
The recolonization of birds in the MSH blast zone has been studied with varying
intensity since the May 18, 1980 eruption. Several species were documented in the blast
zone during the summer of 1980, however observed species richness here was incredibly
variable until 1984 showing no observable trends (Andersen and MacMahon 1986).
14
Recent studies show a correlated increase in species abundance and diversity on MSH as
plant community complexity has increased (Crisafulli and Ronnenberg 2012). This
confirms previous, classic studies of succession by Karr (1986) studying temperate
forests in the Northeast United States and MacArthur et al. (1966) in the tropics relating
disturbance and bird re-colonization. Andersen and MacMahon (1986) studied groundnest predation on the pumice plain post-eruption and found that not only did birds die
directly but also suffered from increased nest predation due to vegetation loss, a factor
which may affect bird communities in the blowdown zone to a lesser degree. Factors
affecting recolonization of birds on MSH are complicated, varied, and require further
analysis to document how they are interacting to determine bird community structure.
2. Studies of bird functional groups provide broad analysis and generalization of results
For the study of re-colonization following a disturbance event, it is common to
group bird species with similar life-histories into guilds. Members of a guild display
similar patterns of foraging behavior as a functional group and will generally react in
similar ways to disturbance. Analysis of guilds creates a more general picture of the
effects of disturbance versus analyzing a species which produces more specific results.
An analysis of guilds therefore can be generalized to a greater degree which is useful for
management decision-making. A disturbed patch surrounded by extensive, intact habitat
tends to exhibit higher rates of bird immigration while turnover (or extinction) lowers
(Crooks et al. 2001; Boulinier et al. 2001; Hinsley et al. 1995; Schmiegelow et al. 1997;
Mason 2001; Kraus et al. 2003). Extinction also generally happens at a higher rate the
more isolated an intact patch is within a large disturbed area (Schiegelow et al. 1997;
15
Crooks et al. 2001; Kraus et al. 2003). Kraus et al. (2003) further tested these theories on
butterflies, having similar life histories to birds, and found differences between various
guilds of generalist species versus specialists. The study took place over two years and
species richness and abundance was measured before and after a disturbance event. The
total species richness and abundance was similar between years, but in larger patches of
intact habitat generalists increased while specialists decreased in smaller patches (Kraus
et al. 2003). These findings suggest that certain sizes of patches may provide a source of
individuals to feed population growth while others may be creating a sink. This
information is important for managers working with imperiled species whose populations
are closely monitored.
There are serious limitations in existing studies of bird re-colonization of patches.
The main limitation is that most bird studies in general are based on species abundance,
diversity, and occupancy patterns without taking into account processes like colonization
and extinction that may be affecting results (Fahrig 2003; Lampila et al. 2005; Kennedy
2011). In this case, long-term research on MSH could inform this gap by providing data
about bird re-colonization alongside simultaneous studies of the larger processes at work.
This study analyzes the effects of active management versus passive management in the
context of large-scale natural disturbance, a truly rare opportunity. While few studies
have examined bird communities in salvage logged stands compared to un-salvaged
areas, none have compared them across forests recovering from large-scale disturbance.
Within the context of natural succession on MSH, researchers may gain understanding of
the larger processes driving bird community organization post-disturbance. This
16
understanding could lead to better management of industrial forests to maintain structural
components that support greater biodiversity.
3.
Birds and plants exhibit highly specialized interactions which provide mechanisms
for seed dispersal and drive succession following disturbance
Birds and plants are two of the most dynamic mechanisms that drive succession
consisting of specific interactions between species which change landscape conditions
and provide opportunities for new species. There is no specific model for how birds and
plants will interact to shape future communities on the landscape. This process
represents elements of chance such as which species survive on a landscape following
disturbance or who conditions following disturbance favor for re-colonization. Another
level of complexity is found in the non-linear relationship which exists between birds and
plants where each asserts influences over the other. There are certain bird species which
are highly adapted to particular plant species or communities which are necessary for
their existence and persistence on the landscape. As well, plants have adapted to attract
avian seed dispersers and they depend on birds to spread their seeds and expand their
ranges. On MSH these specific bird and plant (and vice versa) interactions are still
developing and demand study for deeper understanding.
Colonization occurs after LIDs as species are introduced into habitats that they are
able to exploit. The rate at which this occurs depends upon the species available in
surrounding intact habitats to invade the disturbed area and to take advantage of
remaining habitat niches (Clements 1915; Pickett et al. 1987a; Picket et al. 1987b; Turner
et al. 1998). This process necessitates intricate relationships between various species of
17
plants and animals within and outside of the area (Jordano et al. 2007). In an area as
thoroughly devastated as the MSH blast zone, the number of potential colonizers adjacent
to the habitat to be colonized is limited. The extent of available niches is also limited by
the complete lack of vegetation, therefore colonization may be slow, especially toward
the interior of the blast zone (Turner et al. 1998). Multiple stages of succession may need
to proceed before species with diverse habitat needs, such as birds, are able to
successfully colonize. As birds are able to colonize however, they play important roles as
seed dispersers and habitat architects driving succession. The following chapter will
focus on the limited information available in the literature that refers to colonization of
LIDs and the role that birds may play in succession.
4. Disturbance patch size determines available mechanisms of recolonization
LIDs differ from small-scale disturbances in the rate of processes and
mechanisms of colonization. In smaller disturbances, colonizing plants recruited from
outside of the affected area quickly exploit the newly exposed soils, dispersed by various
mechanisms including wind and herbivores. In the wake of a LID such as MSH there
exists a large interior area of disturbance which is far from the source of colonizing plants
and animals, limiting the mechanisms of colonization. Multiple studies have shown that
the number of species available to colonize a disturbed area decreases with distance from
surrounding intact habitat (Aide and Cavalier 1994; da Silva et al. 1996; Nepstad et al.
1996; Turner et al. 1998). Events such as the eruption of MSH are important to the study
of succession, teasing apart mechanisms which differ between large and small-scale
disturbances and informing modern theory.
18
Patch size of disturbance also affects the colonizing herbivores, such as birds,
who play a major role in spreading seeds creating unique biotic interactions and rates of
colonization, many of which have yet to be determined on MSH (Lubchenco 1978; Mills
1986; Bowers 1993; Davidson 1993; Runkle 1985; Long et al. 1998; Turner et al. 1998).
Keeping this pattern in mind it is easy to imagine how varying stages of succession may
be taking place simultaneously across the landscape of MSH at a given time.
Turner et al. (1998) compiled research on various LIDs including MSH,
describing current understanding about how varying degrees of disturbance regulate
patch size and influence processes and mechanisms of succession:
Our analysis suggests that succession following LIDs will differ from smaller
disturbances if biological legacies are minimal and colonization from surrounding
undisturbed habitats is required; if new substrates are created, particularly if
unique species assemblages can develop; or if bio-physical conditions or biotic
interactions such as herbivory vary with patch size.
All of these limitations are a factor on MSH, although to varying degrees depending on
where you are in the blast zone. In a managed setting, Diane De Steven (1993) found
through studying abandoned agriculture fields that initial differences in seed dispersal
based on the presence or absence of adjacent intact vegetation and visits by dispersers has
a profound effect on succession. While the majority of studies on patch size have been
carried out in managed landscapes, opportunities such as MSH offer an alternative view
which adds to the complexity and relativity of successional theory.
Although study of patches does not directly correlate to large-scale, cataclysmic
disturbance, lessons from this research can be used to inform how birds may be
19
colonizing MSH. Spatial organization of patches has been found to be important in how
birds re-colonize habitats (Kennedy et al. 2011; Zamora et al. 2010). As with distance to
surrounding intact habitat, distance between patches is important for colonizing birds
because some birds refuse to fly in open areas to protect themselves from predation. The
results vary on which elements of habitat diversity enable bird species to colonize and
persist in a fragmented habitat. Kennedy et al. (2011), working in tropical rainforests,
found that “the effect of vegetation structure on extinction probabilities was greater than
the effect of patch area,” hinting at complex relationships existing in these habitats. This
study further revealed that the quality of the intact habitat matrix surrounding a disturbed
area may have the most effect on whether birds colonize an area and are eventually able
to persist there.
This phenomenon could result in lower extinction rates in smaller patches
embedded in a hospitable matrix than in larger patches embedded in an
inhospitable matrix (Sisk et al. 1997; Estades 2001; Kennedy et al. 2011).
On MSH the forests surrounding the long-term study area are mostly Douglas-fir
plantations managed intensely by Weyerhaeuser Corporation for wood production,
potentially degrading habitat quality.
C. Mutualistic relationship between birds and plants increases biodiversity
1. Birds drive succession by aiding dispersal of plants
Plant dispersal aided by birds depends on complicated interactions between seed
ecology, plant locations, bird behavior and conditions at the site of deposition. These
factors are all changed drastically following cataclysmic disturbance such as the eruption
20
of MSH. Some relationships between plants and their bird colonizers may be broken by a
disturbance event and take long periods of time to re-establish. Disturbance also provides
the opportunity for new and novel relationships to develop through chance interactions
between colonizing species and subsequent adaptation. The following chapter provides a
summary of the literature regarding how structural changes resulting from disturbance,
natural or anthropogenic, influences bird colonization and succession.
Dispersal methods of plants are dependent on various factors, the most important
of which is climate (Jordano et al. 2007). Dry and windy conditions favor plants that are
wind-dispersed while animal dispersal is more successful in wet conditions (Howe and
Smallwood 1982). Local ecology also plays a role in how plants disperse; for instance,
plants time fruiting to coincide with migration patterns of frugivores (fruit-eating birds).
These strategies have advantages for plants who want to spread their seeds long distances
and for birds needing highly nutritious food sources for migration, overwintering, or
breeding (Howe and Smallwood 1982). Competition may also play an important role in
deciding which dispersal mechanisms a plant develops, especially regarding animal
dispersal mechanisms. Researchers hypothesize that large fruit-bearing trees in the
tropics are competing for bird species such as Parrots and Toucans who spread their seeds
(Thompson and Wilson 1979). The abundance and preference of potential dispersers can
affect the ability of a plant to reproduce successfully (Howe and Smallwood 1982).
Different functional groups of birds disseminate seeds of plants in various ways
depending on how many seeds they consume, where and how far they travel before
depositing them (McAtee 1947; Jordano et al. 2007).
21
For some plant species the digestion process of birds may also contribute to the
ability of a plant to sprout and ultimately survive, as some plant seeds need to be
processed in order to germinate properly (McAtee 1947; Jordano et al. 2007). For other
species seeds may be damaged by digestion in a bird’s stomach rendering them unable to
reproduce successfully. Jordano et al. (2007) found in a thorough literature review of
seed disperser studies, that small birds contribute a disproportionately large amount to
seed dispersal than other frugivores, suggesting they are interacting with plant
communities to play an important role in succession. This finding reinforces the
importance of studying succession as an interaction between plant communities and
dispersers, because they affect each other’s survival and the ultimate results of succession
(Jordano et al. 2007).
Bird dispersal mechanisms utilized by plants have been well-documented in the
tropics where both large, frugivorous birds and fruit-bearing trees are common
(Thompson and Wilson 1979). McDonnell and Stiles (1983) studied recruitment of birddispersed plant species in abandoned agriculture fields in deciduous forests of eastern
North America. They were looking to fill the gap of knowledge about temperate forest
dispersal strategies by birds and the theory (also derived in the tropics) that patches of
intact vegetation could recruit dispersers to the area. They found that not only do such
patches function to recruit bird dispersers to potentially disperse seeds to adjacent areas
but also individual trees act as important vectors (McDonnell and Stiles 1983; Yarranton
and Morrison 1974). This effect has led to increased study on the effect of structure,
especially height of vegetation, as a determinant of the rate of bird colonization after
disturbance with the ability to affect vegetation communities.
22
2.
Avian role in plant dispersal enhanced during secondary succession
Just as birds play a critical role in dispersing seeds throughout a disturbed area,
the ability of birds to colonize a given area is highly dependent on the plant resources
available. Primary succession is the initiation of development of soils and plant
communities in conditions totally lacking organic matter (Grishin et al .1996). Secondary
succession takes place when soils are formed or were not completely destroyed. The
blowdown zone of MSH is undergoing secondary succession (Grishin et al. 1996). Bird
dispersers of plant seeds become more prominent during secondary succession when
resources, such as berries and perch sites, are available to attract them to disturbed areas
(McDonnell 1986).
Although birds generally contribute to later stages of succession to a greater
degree, studies of natural succession following large, infrequent disturbances (LIDs) such
as Ksudach and MSH have led to blurred lines between successional stages; reinforcing
the new school of thought that each succession following a LID is a unique path toward a
novel steady state (Grishin et al. 1996). Birds become architects of future plant
community structure by depositing seeds in areas where they are foraging, nesting, or
resting. As plant species diversify and more birds are attracted to a recovering habitat,
both plant community and bird species diversity rise exponentially, creating a positive
feedback loop of increasing biodiversity (Robinson and Handel 1993).
3. Legacy structures attract bird dispersers to a disturbed area
The geographic pattern and species diversity of seeds dispersed by birds during
secondary succession may be heavily influenced by existing vegetation structure and
23
adjacent intact habitats. The mechanisms of the relationship between plants and birds
that disperse them however are not well documented in the literature (McDonnell and
Stiles 1983; McDonnell 1986). Results from work by Robinson and Handel (1993) on
abandoned landfills produced strong evidence of the overwhelming effects animal
dispersers have on succession after disturbance. Of nineteen species of colonizing plants
on their study site only six were not dispersed by animals (Robinson and Handel 1993).
Expanding his work on vegetation structure as vectors for bird dispersal, McDonnell
(1986) created structures of various heights in abandoned agricultural fields to see which
attracted more bird-disseminated seeds. Results were conclusive that the higher the
structure, the more use it received from perching birds, and the more seeds were
disseminated at the site (McDonnell 1986). He extrapolated from these findings; “it
appears that saplings become more attractive perches and serve as recruitment foci for
bird dispersed seeds after they project above the existing matrix of herbaceous
vegetation.”
Differences between the heights of structures remaining post-disturbance in the
blowdown zone may have played an important role in initiating succession driven by bird
dispersal. The process of salvage logging removes dead standing and down wood which
would have provided height structure across a disturbed area. On MSH, salvage logging
removed surviving conifer saplings from actively managed sites which grew rapidly in
the passively managed sites post-disturbance (Crisafulli and Hawkins 1998; Mac et al.
1998). Studies have shown that even minimal amounts of height structures left in a site
post-disturbance attracts birds dispersers and can increase greatly the colonization of
plants from outside sources (Robinson and Handel 1993; McDonnell 1986).
24
D. Disturbance enhances biodiversity in Pacific Northwest forests
1. Birds respond to changes post-disturbance and species who colonize are adapted to
survive in the new conditions
The disturbance regime of the PNW is diverse, varied in scale and helps to shape
the available niches for bird species, increasing biodiversity in forest ecosystems (See
figure 4). Coastal storms and weather slam into the Cascade Mountain Range, impacting
forests on and surrounding MSH with wind and fire disturbance. These disturbances are
normally small-scale, creating patchiness across the landscape resulting in a large-scale
mosaic of different habitat niches with the ability to support complex communities of
plant and animal species (Franklin and Dyrness 1973; Pickett and White 1985). Some of
the effects of fire and wind disturbance include increased down and standing dead wood
which attracts insects and other prey items for many wildlife species (Alexander et al.
2004; Rumbiatis del Rio 2006; Blake 1982). Dead and dying wood in a forest also
creates cavities which provide many wildlife species with nesting habitat (Hutto and
Gallo 2006; Clark et al. 2013; Schwab et al. 2006; Abbot et al. 2003). The patchiness
created by small scale wind and fire disturbance opens up closed forest canopies,
invigorating vegetation, leading to a greater diversity of understory niches, and increased
bird abundance and diversity.
25
Figure 4: diagram representing connections between fire ecology, habitat niches and bird species abundance, source:
Alexander et al. 2000
In the blowdown zone of MSH, disturbance mimicked closely large-scale fire and
wind effects, with areas of total destruction interspersed with remnant vegetation and
structure. Large-scale wind and fire disturbance are increasing in the PNW forest as
climate change produces more extreme weather (IPCC 2014). Studies within MSH
National Volcanic Monument, may produce results revealing long-term effects of these
processes and providing insight for managers who are working to mimic local
disturbance regimes and protect ecosystem resiliency.
Changes in bird communities post-disturbance reflect changes in habitat
availability representing ecosystem structures and functions which remain or have
26
disappeared. While intense fire or wind removes opportunities for many bird species to
inhabit a disturbed area, other species have adaptations allowing them to thrive postdisturbance. These behaviors, for instance boring by woodpeckers in search of insects,
produce habitat structure for other species such as cavity nesters. Species such as
woodpeckers function as ecosystem architects post-disturbance, and their occurrence due
to structures remaining post-disturbance may greatly increase the rate of colonization of
wildlife species. Salvage logging by definition removes some of these important
resources and the effect that has on wildlife habitat must be understood to improve its use
a management tool.
2. Fire suppression and salvage logging in Pacific Northwest forests may have
unintended and unknown ecological consequences
While forest fires are an important component of PNW forest ecosystems, they
also produce great risk to forestry resources and human structures leading to fire
suppression and alteration of natural fire regimes (Agee 1993). Fire suppression is
roundly blamed for recent increases in fuel loads leading to larger forest fires (Agee
2002). One product of this phenomenon is salvage logging which was implemented to
produce economic gain post-disturbance on valuable industrial forest lands and decrease
fuel loads (See figure 5). Initially integrated as post-fire logging, the practice was
expanded to other lands impacted by wind and other disturbance after the passing of the
so-called ‘salvage rider’ by the U.S. Congress in 1995 (McIver and Starr 2000). While
green tree harvest on federal lands has declined recently to protect wildlife, salvage
logging has not (McIver and Starr 2000). Salvage logging was introduced to the industry
27
rapidly to protect the economic interests of logging communities without the research in
place to develop best management practices (McIver and Starr 2000; Foundation for
Deep Ecology 2006). It is important to study the short- and long-term effects of salvage
logging in order to improve the practice as its use continues to increase.
Figure 5: example of postfire salvage logging, source: John Muir Project
In 2000 the United States Department of Agriculture completed Environmental
Effects of Post-fire Logging: Literature Review and Annotated Bibliography to assess the
28
need for more research to inform management of post disturbance forests. They found
only 21 studies worldwide which consider the ecological effects of salvage logging, most
of which did not include un-salvaged controls (McIver and Starr 2000). All of the studies
documented took place on lands managed intensely for timber harvest pre and postfire
disturbance. Franklin et al. (2000) stated that the rarest conditions on forest lands in the
Pacific Northwest are those which have undergone natural disturbance where natural
succession proceeded without management. This is the condition which is represented in
this study, comparing post-eruption forests that were actively managed versus passively
managed. The results of this study therefore will add significant knowledge to this field
and can be expanded to gain further insight regarding long-term impacts of salvage
logging on PNW forests, a current gap in the literature.
There are many ways in which salvage logging can create negative consequences
for wildlife. Habitats is altered by removing dead and dying trees and other structural
legacies and logging practices disturb sensitive soils. Hutto and Gallo (2006) found sites
un-salvaged after forest fire had “significantly larger and taller trees, a higher density of
trees, trees with more bark, higher live-tree density, and a higher proportion of intact
snags than salvage logged plots.” Titus and Householder (2007) compared vegetation in
plots on MSH that were salvage logged and replanted versus those that were not. They
found an increased abundance of non-native species in salvage logged plots and limited
nitrate and phosphorus in soils, potentially limiting the ability of native plants to thrive
(Titus and Householder 2007). Un-salvaged plots contained more down woody debris,
more tip-ups, fewer stumps and overall greater diversity and abundance of understory
vegetation (Titus and Householder 2007). Studying wind disturbance in forests,
29
Rumbiatis del Rio (2006) found similar results and went further to say that from a
landscape perspective “disturbance helps maintain understory diversity, whereas salvage
logging does not.” When most or all of the remnant structures are removed from an area
post-disturbance, colonization is reduced, providing surviving wildlife populations with
limited to no resources (See figure 6).
Figure 6: depictions of habitat created by down wood and how they change with decay, source: USFS
Birds specifically occupy a variety of niches within an ecosystem and their
abundance and diversity increases as structural diversity increases (See figure 7). When
structure is removed, prey items diminish and birds may be unable to inhabit an area until
structural diversity develops. This in turn, affects colonization of plants as birds are not
contributing to seed dispersal in the disturbed area. In the blowdown zone of MSH,
structure was removed or maintained at varying levels depending on the severity of
impact and the post disturbance management regime in a given area.
30
Figure 7: depiction of vertical forest niches and the diversity of species which inhabit them, source: Texas Parks and
Wildlife
3. Increased knowledge of salvage logging impacts necessary for future implementation
Because birds and other wildlife are sensitive to changes in habitat structure and
ecosystem function across a landscape, greater knowledge must be gained to guide
managers as they work to balance habitat needs with harvest goals. Unlike the
conclusive evidence for effects on forest vegetation, literature reviewed for this document
found mixed impacts from salvage logging on bird species (Cahall and Hayes 2009; Rost
et al. 2012; Greenberg et al. 1995; Azeria et al. 2011; Lain et al. 2008; Hutto and Gallo
31
2006). For instance, Hutto and Gallo (2006) found salvage logging to be extremely
detrimental to cavity-nesting species, who depend on snags created from fire and wind
for nesting habitat. However, Cahall and Hayes (2009) found many species who prefer
open habitats for berry production and insect foraging do not seem to be affected by
salvage logging activities. Other species such as the Black-backed and American Threetoed woodpeckers are only found in recently, heavily burned forests where a large
amount of dead and dying wood attracts their prey making them severely threatened by
fire suppression and salvage logging (Hutto and Gallo 2006).
Other studies have questioned the ability of salvage logging as a post-disturbance
management technique to meet its overall goals, adding to the urgency of conclusive
research.
One of the most controversial studies on salvage logging found that the
practice did not meet some of its intended purposes: 1) to limit fuel loads lowering the
chances of another forest fire; 2) clear space for reforestation (Cahall and Hayes 2009).
Donato et al. (2006) found that salvage logging increased fine woody debris on the forest
floor adding to fuel loads, and that damage to sensitive soils by large logging equipment
reduced regeneration of conifers. Cahall and Hayes (2009) among others discuss the
need for analyses of the effects of salvage logging on a larger scale and with greater use
of un-salvaged controls.
This research offers the rare opportunity to study salvage logging within the
context of large-scale disturbance with a control are that is protected for long-term
research. Hutto (2006) and Beschta (2004) among others have attempted to recommend
best management practices for salvage logging based on the research available. They
32
also acknowledge the need for more research on this issue and importance of control sites
and long-term analysis.
III. RESEARCH MANUSCRIPT
A. Abstract
Post disturbance management, specifically salvage logging, is used regularly in
Pacific Northwest forest ecosystems to off-set economic losses, reduce fire fuel loads and
restore industrial timberlands. This practice also removes habitat components such as
snags and downed wood which are important to many species of birds and other wildlife.
The study of birds post-disturbance can provide information to managers on habitat
components which have persisted and those which are lacking in the ecosystem, leading
to more informed management decision-making. The body of knowledge regarding bird
response to salvage logging continues to be sparse and studies lack the necessary
longevity and comparison to a control to be conclusive. This study provides these
necessary pieces by comparing study sites that are under long-term conservation for
research (passively managed) and those that were salvage logged and replanted (actively
managed) following the May 18, 1980 eruption of Mount St. Helens. We found a pattern
where passively managed sites had greater bird abundance and diversity than noble fir
plantations and significantly greater than Douglas-fir plantations. These results indicate a
need for more understanding of the habitat components which are lacking in actively
managed sites in order to develop best management practices for salvage logging.
33
B. Research setting
Mount St. Helens is an active stratovolcano situated in the Cascade Mountain
Range of Washington State, USA. It is part of a larger string of volcanoes known as the
‘ring of fire’ which circle the Pacific Ocean and whose volcanism arises from the
convergence zone of the North American Plate and the Juan De Fuca Plate (Swanson,
Crisafulli, and Yamaguchi 2005). The Cascade Mountain Range runs north and south
approximately 50 miles inland from the Pacific Ocean where it creates a barrier for
moisture-laden coastal waters which shape the wet climate west of the mountains versus
the dry climate to the east. Mount St. Helens has erupted at least 20 times in the past
4000 years, producing variable vegetation communities surrounding the volcano
(Swanson, Crisafulli, and Yamaguchi 2005). Prior to the May 18, 1980 eruption specific
portions of the volcano had undergone the impacts of volcanic processes including tephra
fall, pyroclastic flow, lava flow, dome growth, mudflow, and lateral blasts at varying
spatial and temporal scales (Swanson, Crisafulli, and Yamaguchi 2005). Other nonvolcanic processes also at work on and around Mount St. Helens include glacial, river,
landslide, anthropogenic resource management, and local climate events.
C. Climate and ecology
The climate of Western Washington in the vicinity of Mount St. Helens is most
strongly affected by the Cascade Mountain Range interactions with marine air. Moistureladen marine air masses move east from the west coast of the Pacific Ocean where they
encounter the Cascades. As they move east and upward in elevation, they drop their
moisture in the form of snow and rain along the Cascade crest creating a rain shadow
34
effect. The effect produces a wet, mild climate on the west side of the mountains and a
drier side with more extreme temperature range, on the east. Wind is an important factor
in the creation of the rain shadow and also drives wildfires in the summer months when it
is north-northeast prevailing. During winter, winds mostly come from the ocean, are
southwest prevailing, and bring with them storms and other weather phenomena.
The west side of the Cascade Mountains experiences mild temperatures in the
summer (mean maximum=22.3oC, mean minimum=7.3oC) as well as winter months
(mean maximum=0.4oC, mean minimum=-4.4oC) (Dale, Swanson, and Crisafulli, 2005).
The climate is also very wet with mean annual precipitation at Spirit Lake, located at 988
m elevation on Mount St. Helens, totaling 2372 mm from 1932-1962 (Dale, Swanson,
and Crisafulli). The climate supports a diverse conifer-dominant temperate forest in
various stages of succession across the landscape due to management and local
disturbance regimes including wind and fire. While the local Pacific Northwest climate
sets the stage for the development of the ecosystem surrounding Mount St. Helens, the
large volcano also has a significant effect on the local climate and ecology.
D. Recent volcanic history
The most recent eruption of MSH consisted of multiple volcanic processes that
disturbed a large area with heat, gases, and various types of flows. Beginning on March
15, 1980 after 123 peaceful years, a series of small earthquakes began which indicated
the intrusion of magma into the volcano. This activity continued for 2 months and
included outward signs such as swelling of the north side of the mountain and steamdriven explosions from the top. After this initial awakening, the north side of the
35
mountain collapsed on the morning of May 18, 1980 resulting in the largest landslide in
recorded history (See figure 8). Almost simultaneously, a 5.1 magnitude earthquake took
place below the volcano (Dale, Swanson, and Crisafulli 2005). The 2.5 km3 debris
avalanche split into 3 parts; one deposited into Spirit Lake, creating a 260 meter seiche (a
large, contained wave), permanently raising the water level of the lake 60 meters; another
jumped Johnston Ridge 7 km north; while the majority of the debris traveled west
through the North Fork Toutle River Valley. All structures including glaciers, forest, and
plants were scoured in the wake of the debris avalanche, leaving hot (70-100o C) deposits
of rock and debris mostly devoid of organic material (Dale, Swanson, and Crisafulli
2005).
Figure 8: time lapse photo series of the initial events of May 18, 1980 on Mount St. Helens, source: Gary Rosenquist
36
Pressure was released as the north side of the mountain collapsed creating a
pyroclastic density current (blast surge) of hot gas, rock, and ash which shot north of
MSH. The blast was pushing a 0.2 km3 cloud of hot rock debris which caught up with the
landslide, proceeding to scorch and level vegetation ahead of it for a 570 km2 area
(Swanson and Major 2005). The area impacted by the eruption is known as the blast
zone and it is subdivided into three separate zones (blowdown, scorch, and mudflow)
based on severity of impact from the eruption due to distance from the blast origin
(Swanson and Major 2005) (See figure 9). Deposits resulting from the eruption
contained smaller particles, lower temperatures (100-300o C), and more organic matter as
distance from the blast origin increased (Swanson and Major 2005). Approximately 1.1
km3 of volcanic material was ejected and areas of the blast zone were buried under a layer
from 0.01 to 1.5 meters in depth. Areas closer to the volcano received deposits of hot
tephra while deposits beyond the blast zone had sufficiently cooled.
37
Figure 9: map of Mount St. Helens blast zone, source: Tilling 1984
A large, vertical tephra plume initiated minutes after the eruption and continued
for nine hours (See figure 10). This material was carried by prevailing winds to the eastnortheast, and eventually ultra-fine material circled the globe (Swanson and Major 2005).
38
Figure 10: map of Mount St. Helens ash fallout, source: USGS
Hot, pyroclastic flows from 300-850o C began next and continued for five hours,
leaving a trail of sterile rock in its wake (Swanson and Major 2005). The resulting area is
called the pumice plain because the majority of material deposited was pumice, leaving a
smooth, barren surface. Thickness of flows varied from 0.25 to 10 meters and covered 15
km2 north of the volcano (Swanson and Major 2005).
Lahars (mud flows) were created by various methods during the events of May
18, 1980 on MSH, carrying with them debris as they flowed down the major drainages of
the volcano. Mechanisms included liquefaction of soils from shaking, water-saturated
avalanche material, pumice melted glacier ice, and snow which combined as they moved
39
rapidly downhill (Swanson and Major 2005). Downstream riparian areas were stripped
of vegetation, streambeds were scoured, and infrastructure was carried away as lahars
moved through. Depth of lahars varied from about 0.1 to more than 10 meters and the
largest traveled from the North Fork Toutle River Valley and continued 120 km to the
Columbia River (Swanson and Major 2005).
The eruption left in its wake a diverse matrix of habitats varying from total
devastation of vegetation and soils to patches with legacy structures protecting some life,
useful for the study of natural succession following cataclysmic disturbance. All birds
and most wildlife present in the blast zone during the eruption were immediately killed
(Swanson, Crisafulli, and Yamaguchi 2005). Numerous studies were initiated by
researchers across the disturbance zone to document the short- and long-term ecological
impacts of the eruption and response of local ecosystem components and processes.
E. Forest composition prior to May 18, 1980
Prior to the May 18, 1980 eruption, vegetation on Mount St. Helens consisted of
mature temperate forest and managed industrial timberlands. The Western hemlock zone
(Tsuga heterophylla) dominates the lowlands and gives way to the pacific silver fir zone
(Abies amabilis) at higher elevations (Franklin and Dyrness 1973). Both zones contain
similar understories of hardwood trees, ferns and flowering shrubs, however their climax
forest conditions differ significantly. Dominant shrubs in both zones include vine maple
(Acer circinatum), salal (Gaultheria shallon), red huckleberry (Vaccinium parviflorum)
and trailing blackberry (Rubus ursinus) (Franklin and Dyrness 1973). These forest types
40
provide important habitat niches for the bird communities of the PNW throughout their
various successional stages.
The Western hemlock zone extends from sea level to approximately 1000 m and
its climax species thrive in the wet climate producing some of the greatest biomass in the
world, an important natural resource central to the economy of the Pacific Northwest
(Franklin and Dyrness 1973). While the climax conifer is Western hemlock, Douglas-fir
(Pseudotsuga menziessi) is its fast-growing counterpart and sub-climax conifer and
dominates many forest stands in this zone. Other sub-dominant tree species include a
third conifer, Western red-cedar (Thuja plicata), and hardwoods including red alder
(Alnus rubra), big-leaf maple (Acer macrophyllum) and Giant chinkapin (Castanopsis
chrysophylla) (Franklin and Dyrness 1973). Hardwoods are rare but make up a
significant portion of riparian vegetation where black cottonwood (Populus trichocarpa)
and Oregon ash (Fraxinus latifolia) accompany red alder and big-leaf maple (Franklin
and Dyrness 1973). The vast majority of this forest type had been converted from a
mosaic of climax forest to managed timberland before 1980, creating conservation issues
for bird species such as the Northern Spotted Owl.
The pacific silver fir zone exists above the Western hemlock zone up to 1,300 m
where precipitation increases and temperatures cool producing more snowfall. Winter
snowpack accumulates from 1-3 m while biomass and organic matter on the forest floor
are shallower than the Western hemlock zone (Franklin and Dyrness 1973). Some
conifer species span both zones including Western hemlock, Douglas-fir, Western redcedar with pacific silver fir (Abies amabilis) the climax conifer (Franklin and Dyrness
1973). Other co-dominant conifers include noble fir (Abies procera) and Western white
41
pine (Pinus monticola) with mountain hemlock (Tsuga mertensiana) and Alaska yellowcedar (Chamaecyparis nootkatensis) joining at higher elevations (Franklin and Dyrness
1973). Riparian areas contain a similar mix of hardwood and conifer species as the
Western Hemlock zone. This zone has remained more intact than lower elevations being
less accessible to logging operations, however some areas experience intense forest
management as well.
F. Methods: study sites
All six study sites used in this analysis are from the blowdown zone of MSH (See
figure 11, table 1). Two study sites representing passive management are located within
the 44,550 ha Mount St. Helens National Volcanic Monument which was set aside in
1982 for the long-term monitoring of successional processes post-eruption (Titus and
Householder 2007). These study sites are in long-term conservation status and received
no treatment; no salvage logging, no post-logging burning, no replanting.
42
Figure 11: map of study site locations on Mount St. Helens. See table 1 for site names, source: USFS
43
Site Code
Site Name
Management
Treatment
Coordinates
BDCC
Smith Creek
Blowdown
Natural
Succession
Passive
No salvage
logging, no replanting
Start: 557068,
5129262
Blowdown
Forest Natural
Succession
Passive
No salvage
logging, no replanting
Start: 0570456,
5129115
Blowdown
noble fir
Plantation
Active
Salvage
logging, replanting with
noble fir
Start: 0575300,
5121111
Coldwater
Ridge noble fir
Plantation
Active
Salvage
logging, replanting with
noble fir
Start: 0557068,
5129081
Clearwater
Douglas-fir
Plantation
South
Active
Salvage
logging, replanting with
Douglas-fir
Start: 555924,
5128198
Coldwater
Ridge
Douglas-fir
Plantation
Active
Salvage
logging, replanting with
Douglas-fir
Start: 0576153,
5124234
GRFOR
BDNF
CWRPLN
CWRDFP
CWPLNS
End: 556773,
5129081
End: 0570596,
5130088
End: 0575311,
5122057
End: 0556773,
5129081
End: 0555729,
5128824
End: 0575818,
5125156
Table 1: table representing site codes, names, treatment and coordinates of locations on Mount St. Helens
The other four study sites exist outside of the monument and have been actively
managed by Weyerhaeuser and the USFS to rehabilitate industrial timberlands (Swanson
and Major 2005). These four sites were all salvage logged and remaining debris post44
logging was burned on-site (Crisafulli 2016). On some actively managed sites, soils were
scarified with large machinery to reach more fertile soils less impacted by volcanic
activity to increase re-planting success (Crisafulli 2016). Actively managed sites were
then replanted with commercial species, in a uniform spatial pattern with eight foot
spacing (Crisafulli 2016). Most trees were planted using an auger or hoe on steep ground
to access mineral soils beneath the volcanic remains for fear that trees would not survive
in surface soils (Crisafulli 2016). Below 1000 m Douglas-fir was the primary species
planted and above 1000 m noble fir was the primary species planted, with lodgepole pine,
Engelmann spruce and western white pine representing a small minority of planting
across sites (Crisafulli 2016). The names of the two passively managed sites are Smith
Creek Blowdown Natural Succession (BDCC) and Blowdown Forest Natural Succession
(GRFOR). The names of the two noble fir plantation actively managed sites are
Blowdown noble fir plantation (BDNF) and Coldwater Ridge noble fir Plantation
(CWRPLN). The names of the two Douglas-fir plantation actively managed sites are
Clearwater Douglas-fir Plantation South (CWPLNS) and Coldwater Ridge Douglas-fir
Plantation (CWRDFP).
G. Methods: bird data collection
Long-term bird data was collected at six sites within the blowdown zone, four
representing active management and two representing passive management. These sites
represent two separate management regimes: 1) passively managed and 2) actively
managed. The actively managed sites are further subdivided based on the dominant tress
species planted, noble fir or Douglas-fir. The six sites are analyzed by three treatments:
45
1) passively managed 2) actively managed noble fir plantation; 3) actively managed
Douglas-fir plantation. Surveys sampling the local bird community were performed
using a modified line transect method (Emlen 1977). Distance-sampling method entails
walking transects and counting all birds observed within 100 meters of the transect line
by sight and sound (Emlen 1977). Surveys were performed by trained technicians who
rotated through sites to correct for observer inconsistency. Sites were surveyed in June,
July, and August 2010 with varying effort. BDNF, CWPLNS, CWRPLN and CWRDFP
were all surveyed four times while BDCC was surveyed five times and GRFOR was
surveyed three times.
H. Methods: data trimming
A data trimming exercise was completed to correct for differences in detectability
among individual bird species and differing site characteristics which may affect
detectability. The frequency distribution of bird observations were plotted for a set of
perpendicular distance categories from transects for each species and at each study site to
determine a threshold beyond which the frequency of detection declined markedly (See
figure 12). For this analysis, we decided not to trim any detections because of the focus
on descriptive and qualitative methods.
46
American Robin Detections on BDCC
Data Trimming
Number of Detections
8
7
6
5
4
3
Total
2
1
0
Perpendicular Distance from Transect (meters)
Figure 12: example of data trimming exercise results performed for each transect
I. Methods: bird guild structure analysis
Bird data was analyzed qualitatively by defining the foraging guild for each
species according to the protocol used by the USFS specifically for the Bird Master List
for MSH (USFS 2015). MSH researchers assigned guilds which include seasonal
foraging guilds for breeding, non-breeding and year-round resident bird species. Some
species who exhibit multiple foraging patterns depending on the season were assigned
multiple guilds in the Bird Master List for MSH. We condensed the multiple
assignments into one which represents a more general foraging guild or the most
commonly used by the species according to life history information from Cornell
Laboratory of Ornithology’s The Birds of North America (online) (Jackson, Ouellet, and
Jackson 2002). Foraging guilds consist of three letter codes which represent specific
47
attributes of a bird’s foraging behavior; 1) food type, 2) foraging substrate, and 3)
foraging technique (See appendix 1). Species are designated a code which pertains to the
three foraging descriptions and when combined create one 9-letter code indicating
foraging guild (See table 2).
Species
Code
Species
Guild
Definition
AMCR
American Crow
OMGROSCV
Omnivore, Ground,
Scavenger
AMDI
American Dipper
INRBOGLE
Insect, Riparian Bottom,
Gleaner
AMRO
American Robin
OMGROFOR
Omnivore, Ground,
Forager
BHGR
Black-headed Grosbeak
OMUCAFOR
Omnivore, Ground,
Forager
BTYW
Black-throated Grey Warbler
INLCAGLE
Insect, Lower
Canopy/Shrub, Gleaner
BUOR
Bullock’s Oriole
OMUCAFOR
Omnivore, Ground,
Forager
BUSH
Bushtit
OMLCAFOR
Omnivore, Ground,
Forager
CAFI
Cassin’s Finch
OMGROFOR
Omnivore, Ground,
Forager
CEDW
Cedar Waxwing
OMUCAFOR
Omnivore, Ground,
Forager
CBCH
Chestnut-backed Chickadee
OMLCAGLE
Omnivore, Lower
Canopy/Shrub, Gleaner
48
CHSP
Chipping Sparrow
OMGROFOR
Omnivore, Ground,
Forager
CONI
Common Nighthawk
INAIRSCR
Insect, Air, Screener
CORA
Common Raven
OMGROSCV
Omnivore, Ground,
Scavenger
DEJU
Dark-eyed Junco
OMGROFOR
Omnivore, Ground,
Forager
EVGR
Evening Grosbeak
OMGROFOR
Omnivore, Ground,
Forager
FOSP
Fox Sparrow
OMGROFOR
Omnivore, Ground,
Forager
GCKI
Golden-crowned Kinglet
INLCAGLE
Insect, Lower
Canopy/Shrub, Gleaner
GRAJ
Gray Jay
OMUCAFOR
Omnivore, Upper Canopy,
Forager
HAWO
Hairy Woodpecker
OMBARFOR
Omnivore, Bark, Forager
HEWA
Hermit Warbler
OMUCAFOR
Omnivore, Upper Canopy,
Forager
LABU
Lazuli Bunting
OMUCAFOR
Omnivore, Upper Canopy,
Forager
MGWA
McGillivray’s Warbler
INLCAGLE
Insect, Lower
Canopy/Shrub, Gleaner
NOFL
Northern Flicker
OMGROFOR
Omnivore, Ground,
Forager
NRWS
Northern Rough-winged
Swallow
INAIRSCR
Insect, Air, Screener
OCWA
Orange-crowned Warbler
OMLCAFOR
Omnivore, Lower
Canopy/Shrub, Forager
OSFL
Olive-sided Flycatcher
INAIRSAL
Insect, Air, Sallier
49
PAWR
Pacific Wren
INGROGLE
Insect, Ground, Gleaner
PISI
Pine Siskin
OMLCAFOR
Omnivore, Lower
Canopy/Shrub, Forager
PRFA
Prairie Falcon
CAGROHAW
Carnivore, Ground,
Hawker
PSFL
Pacific-slope Flycatcher
INAIRSAL
Insect, Air, Sallier
RBNU
Red-breasted Nuthatch
OMUCAFOR
Omnivore, Upper Canopy,
Forager
RBSA
Red-breasted Sapsucker
OMBAREXC
Omnivore, Bark,
Excavator
RECR
Red Crossbill
OMUCAFOR
Omnivore, Upper Canopy,
Forager
RUHU
Rufous Hummingbird
OMFLOHOG
Omnivore, Flower, HoverGleaner
SOGR
Sooty Grouse
OMUCAFOR
Omnivore, Upper Canopy,
Forager
SOSP
Song Sparrow
OMGROFOR
Omnivore, Ground,
Forager
SPTO
Spotted Towhee
OMGROFOR
Omnivore, Ground,
Forager
STJA
Steller’s Jay
OMGROFOR
Omnivore, Ground,
Forager
SWTH
Swainson’s Thrush
OMLCAFOR
Omnivore, Lower
Canopy/Shrub, Forager
TOWA
Townsend’s Warbler
INUCAGLE
Insect, Upper Canopy,
Gleaner
TRES
Tree Swallow
OMAIRFOR
Omnivore, Air, Forager
VATH
Varied Thrush
OMGROFOR
Omnivore, Ground,
Forager
50
VGSW
Violet-green Swallow
INAIRSCR
Insect, Air, Screener
WAVI
Warbling Vireo
INUCAGLE
Insect, Upper Canopy,
Gleaner
WCSP
White-crowned Sparrow
OMGROFOR
Omnivore, Ground,
Forager
WIFL
Willow Flycatcher
INAIRSAL
Insect, Air, Sallier
WETA
Western Tanager
OMUCAFOR
Omnivore, Upper Canopy,
Forager
WIWA
Wilson’s Warbler
INUCAFOR
Insect, Upper Canopy,
Forager
YRWA
Yellow-rumped Warbler
OMLCAGLE
Omnivore, Lower
Canopy/Shrub, Gleaner
YWAR
Yellow Warbler
INLCAGLE
Insect, Lower
Canopy/Shrub, Gleaner
Table 2: table of bird species codes, common names, guilds and guild definitions
J. Methods: bird data analysis
Bird data was summarized by treatments and analyzed to determine differences
between bird abundance and species richness. Bird abundance was corrected for
differences in survey effort by averaging detections per species before statistical analysis
was completed. In order to represent the total number of species for diversity and
richness analysis, detections per species were averaged by the days surveyed for the
abundance comparison. Data was organized and summary statistics were compiled as
indicators of bird abundance and species richness.
51
Relative abundance was analyzed with a one-way analysis of variance (ANOVA)
on the number of detections per species compared across treatments (Nur, Jones, and
Geupel 1999). The Shannon’s diversity index was applied to compare species richness
across treatments. Shannon’s diversity index is the most widely used diversity index
because it reflects both species richness and evenness of distribution among species. It
uses natural logarithms (ln) and is calculated using the equation below, where S=number
of species in the sample and pi=proportion of individuals belonging to the ith species:
i=S
H′ = Σ(pi)(lnp), i=1, 2,…S
i=1
(Nur, Jones, and Geupel 1999)
Hmax is also reported, a calculation related to Shannon’s diversity index, which is a
measurement of the diversity potential of the dataset given a fixed number of species and
is calculated using the formula:
–ln (1/S) = ln(S)
(Nur, Jones, and Geupel 1999)
The ratio of observed diversity to maximum diversity is reported as evenness according
to the following equation where H'=ln S:
E = H'/Hmax
(Nur, Jones, and Geupel 1999)
Species richness was also compared by applying two measurements of
community similarities which utilize presence/absence data. Jaccard similarity
coefficient and Sorenson index are simplistic calculations used to bolster more
sophisticated quantitative results. They are calculated using the following formulas,
52
where j=the number of species found at both site A and B, a=the number of species in
site A, and b=the number of species found in site B.
Jaccard similarity coefficient Cj = ____j___ (Nur, Jones, and Geupel 1999)
a+b–j
Sorenson index Cs = ____2j___
a+b
The indices both equal 1 when the species from the two compared sites are equal and 0 if
they have no species in common. The results from these indices can be used to show
how species richness varies with environmental indicators such as vegetation
measurements.
Jaccard similarity coefficient and Sorenson’s index do not account for species
abundance within the community so we also used the Renkonen similarity index or
percentage similarity index, where pAi = percentage of species i in sample A and pBi =
percentage of species i in sample B, and S = number of species found in either sample:
i=S
P = Σ minimum (pAi, pBi)
i=1
(Nur, Jones, and Geupel 1999)
The index delivers 0 when there is no overlap between species in the samples and 100
percent with complete overlap.
K. Results: bird abundance and diversity among sites
A total of 50 bird species were detected on 6 separate sites during 24 visits in
2010 for a total of 1,323 total detections (See table 3). Passively managed sites overall
had higher bird abundance and species diversity, while one actively managed site had
53
similarly high abundance and diversity. Smith Creek Blowdown Natural Succession Site
(BDCC) had the second highest bird abundance and the highest bird diversity.
Blowdown Forest Natural Succession Site (GRFOR) had the highest bird abundance and
the third highest bird diversity. The actively managed Blowdown Noble Fir Plantation
Site (BDNF) had a much lower bird abundance but the second highest bird diversity. The
remaining three sites were all actively managed and had lower bird abundance and
diversity. Coldwater Noble Fir Plantation Site (CWRPLN) had lower bird abundance
and lower number of bird diversity. Coldwater Ridge Douglas-fir Plantation Site
(CWRDFP) had a slightly higher number of bird abundance and lower bird diversity.
The site with the lowest bird abundance but the same bird diversity as other actively
managed sites was Clearwater Douglas-fir Plantation South (CWPLNS).
Bird Species
Code
AMCR
AMDI
AMRO
BHGR
BTYW
BUOR
BUSH
CAFI
CEDW
CBCH
CHSP
CONI
CORA
DEJU
EVGR
FOSP
GCKI
GRAJ
HAWO
BDCC BDNF CWPLNS CWRDFP CWRPLN GRFOR
0
0
0
0
1
0
0
0
0
0
0
1
20
15
9
6
8
11
14
1
1
11
0
0
1
0
0
0
0
0
1
0
0
0
0
0
1
0
0
0
0
0
3
7
0
0
0
0
0
1
0
0
0
1
10
11
22
17
3
5
1
0
0
0
10
1
0
2
0
0
0
0
1
5
0
4
0
0
92
34
19
29
49
58
0
2
0
0
0
0
11
2
0
0
0
3
0
4
19
27
0
0
0
0
0
0
4
0
1
4
1
0
1
2
54
HEWA
LABU
MGWA
NOFL
NRWS
OCWA
OSFL
PAWR
PISI
PRFA
PSFL
RBNU
RBSA
RECR
RUHU
SOGR
SOSP
SPTO
STJA
SWTH
TOWA
TRES
VATH
VGSW
WAVI
WCSP
WIFL
WETA
WIWA
YEWA
YRWA
Total Species
Total
Detection
Total Species
% Total
Species
0
0
20
0
1
1
2
0
5
0
8
1
3
0
15
4
15
5
11
8
0
1
1
0
5
13
15
8
0
6
13
50
0
0
1
0
0
0
29
1
3
0
11
19
0
2
3
10
0
0
5
1
0
1
3
0
7
0
1
7
0
0
30
0
0
0
5
0
0
0
4
0
0
7
11
0
0
4
1
0
0
7
9
1
0
6
1
0
1
0
0
0
0
1
0
0
0
0
0
0
0
1
0
1
19
11
0
0
0
4
0
1
5
2
0
1
1
0
5
3
0
0
0
0
17
1
0
0
5
0
0
0
0
3
0
1
13
0
0
0
8
0
0
3
1
0
2
4
0
2
10
2
0
0
2
30
0
1
14
8
1
2
12
0
0
0
10
0
0
0
34
2
32
18
10
21
0
5
0
0
8
0
34
1
4
41
1
317
34
222
29
129
19
165
19
162
21
340
28
0.68
0.58
0.38
0.38
0.42
0.56
Table 3: table with bird species abundance per site with summary statistics. Species highlighted in green were
detected on only one site, species highlighted in blue were detected on all sites
55
L. Results: bird guild analysis
Seventeen different foraging guilds of birds were identified in this study, with all
study sites representing at least half of them (See figure 12, 13). Both passively managed
sites (BDCC, GRFOR) represented the highest number of guilds (13). One of the
actively managed sites (BDNF) also represented 13 guilds of birds with the other three
slightly lower. CWPLNS and CWRDFP both represented 11 guilds and CWRPLN
represented ten. Four guilds were only represented on a single site; three of them were
detected on passively managed sites (OMBAREXC on BDCC, INUCAFOR on GRFOR
and INRBOGLE on GRFOR) and one was detected on an actively managed site
(CAGROHAW on CWRDFP). Actively managed sites planted with Douglas-fir
represented a lower number of guilds (10, 11) than actively managed sites planted with
noble fir (11, 13).
Mean Species per Guild by Site with SD
6
Species per Guild
5
4
3
2
1
0
BDCC
BDNF
CWPLNS
CWRDFP
CWRPLN
GRFOR
Site
Figure 12: bar graph depicting mean species per guild between sites with standard deviation (SD) error bars
56
Abundance of Guilds Represented by Treatment
15.5
15
15
Number of Guilds
14.5
14
14
13.5
13
13
12.5
12
Passive
Douglas-fir
Noble Fir
Treatment
Figure 13: bar graph depicting abundance of guilds represented in each treatment
Analysis of variance (One-way ANOVA) returned no significant difference
between the guild structures of the sites nor treatments (p-value<-0.05), however there
are some interesting and significant qualitative differences between bird guild structure
between treatments. Four separate guilds were only represented in one treatment. Prairie
Falcon, the only species found in this study representing the CAGROHAW guild
(carnivore, ground, hawker) was only detected in Douglas-fir plantation treatment. The
guild analysis clearly shows a lack of insect and bark foragers on actively managed sites.
The 3 guilds only detected on passively managed sites are INRBOGLE (insect, riparian
bottom, gleaner), INUCAFOR (insect, upper canopy, Forager) and OMBAREXC
(omnivore, bark, excavator). These guilds represent three different foraging areas and
three different foraging techniques, but they all have the target in common. Insects are
57
the common factor within these guilds and the fact that they are only found on passively
managed sights reinforces what other studies have found linking dead and dying wood to
larger insect populations (Kroll et al. 2010).
M. Results: bird abundance and diversity by treatment
Quantitative analysis of bird data was carried out with regard to treatment type;
passive, Douglas-fir plantation and noble fir plantation. The results of the bird analysis
by treatment type show an overall pattern with passive management treatment producing
the highest bird abundance, diversity and species richness followed by noble fir treatment
and finally Douglas-fir treatment (See table 4).
Treatment
Passive
Noble fir
Douglas-fir
Averaged Abundance
177
96
74
Cumulative Species
39
36
24
Cumulative Guilds
15
12
14
Table 4: indicators of bird abundance and diversity by treatment
Relative abundance, a measure of abundance of one species in relation to
community abundance, was measured with ANOVA. We found a significant difference
between passive and Douglas-fir plantation treatments (p-value=0.041). Comparison of
relative abundance between all three sites was nearly significant (p-value=0.066) There
was also no statistically significant difference between the passive treatment and noble fir
58
plantation treatments nor between Douglas-fir plantation and noble fir plantation
treatments, however the results followed the established trend (See figure 14, table 5).
Mean abundance per species
MEAN AVIAN ABUNDANCE
20
18
16
14
12
10
8
6
4
2
0
6.584
Passive
2.934
3.85
Douglas Fir
Noble Fir
Treatment
Figure 14: bar graph of mean avian abundance per treatment with standard deviation error bars
Comparison
All treatments
P-Value (ANOVA)
0.066
Passive vs noble fir
0.129
Passive vs Douglas-fir
0.041
noble fir vs Douglas-fir
0.506
Table 5: p-values obtained with a One-way Analysis of Variance Test of bird abundance compared between
treatments, significant p-value indicated in red
One method used to evaluate species richness in this study is Shannon’s diversity
index which increases as species richness and evenness increase in the community.
Values of the index normally range from 1.5-3.6 with 4 being among the highest possible
59
outputs (Magurran 2004). The passive treatment reported the highest among sites,
followed by noble fir plantation and finally Douglas-fir plantation (See table 6).
Evenness did not vary significantly between treatments, indicating more variation in
species richness between treatments (See table 6).
Species
Richness
24
Shannon’s
Index
2.64
Evenness
Hmax
Douglas-fir
Average
Detections
73.75
1.21
3.18
Noble fir
96.25
36
2.84
1.26
3.58
Passive
177.06
39
2.9
1.26
3.66
Treatment
Table 6: table of results of Shannon’s Index and related calculations which describe species diversity
Species richness was also evaluated using the Jaccard similarity coefficient
calculation (Cj=1 when species richness of communities is the same and Cj=0 when
communities have no species in common). Jaccard similarity coefficient calculated for
passive treatment versus noble fir plantation treatment produced the highest number,
followed by passive treatment versus Douglas-fir treatment and finally noble fir treatment
versus Douglas-fir treatment (See table 7).
The Sorenson index was calculated to
compare with Jaccard similarity coeffecient (Cs=1 when species richness of communities
is the same and Cs=0 when communities have no species in common). Sorenson index
calculated for passive treatment versus noble fir plantation produced the highest result,
followed by noble fir treatment versus Douglas-fir treatment and finally passive versus
Douglas-fir treatment (See table 7). Both indices show differences between all three
60
treatments with regard to species richness with the greatest difference between passive
and Douglas-fir plantation treatments.
The Renkonen similarity index differs from the Jaccard similarity coefficient and
Sorenson index because it also takes into account abundance of species within the
community. Results equal 0 when there are no species in common amongst the
compared communities and 100% when they are the same. The Renkonen similarity
index shows the greatest similarity between passive treatment versus noble fir plantation
treatment, followed by passive treatment versus Douglas-fir plantation treatment, and
finally noble fir plantation treatment versus Douglas-fir plantation treatment (See table
7). In other words the greatest similarity of species richness with respect to withincommunity species abundance is between noble fir plantation treatment versus Douglasfir plantation treatment and the least similarity between Douglas-fir plantation treatment
versus passive treatment.
Treatments Compared
Noble fir vs Passive
Douglas-fir vs Noble fir
Douglas-fir vs Passive
Jaccard
Sorenson
Renkonen
0.59
0.75
56%
0.5
0.67
57%
0.44
0.6
46%
Table 7: table comparing results of three different calculations comparing species richness, Jaccard similarity
coefficient, Sorenson index and Renkonen similarity index
61
N. Discussion
The results of this study produce a pattern where passively managed sites
produced higher abundance and diversity of bird species as well as foraging guilds than
actively managed sites which were salvage logged and replanted. Among actively
managed sites, noble fir plantations had higher bird abundance and diversity than
Douglas-fir plantations. This pattern was consistent throughout the study, however not
all differences were statistically significant (p-value<0.05). Based on the welldocumented ecology of PNW forests, two factors may explain these patterns: 1) Salvage
logging removes down and standing dead wood on the site that provide habitat for some
bird species and their prey; 2) Plant communities left to naturally regenerate may be more
diverse in species and age, providing varied structure and increased habitat niches. In
addition, the removal of wood in salvage logging might decrease nutrients available to
developing plant communities, thus limiting the diversity of both flora and fauna.
Salvage logging also removes dead standing trees (snags) which are very important for
many bird species. As they decay, snags attract a wide variety of insects and provide
foraging opportunities for many species. Snags also contain cavities which provide
nesting habitat for many bird species.
Open areas which support low shrubs, forbs, and grasses in the passive treatment
may also provide habitat components necessary to support a different set of bird species,
increasing abundance and species richness. Breaks in the forest canopy allow increased
light to reach the forest floor, producing a more varied vegetation community dominated
by flowering and fruiting shrubs which increase fruit and other prey items. Dense
planting of commercial tree species blocks light from the understory and may decrease
62
the diversity of the forest plant community. A forest representing a more complex
mosaic with this open component as well as closed-canopy conifer stands provides a
larger number of habitat components. With more niches to fill, bird diversity increases,
representing larger numbers of foraging guilds across the landscape. Passively managed
sites are more likely to contain these varied components because they are derived from a
mixture of surviving conifer saplings some of which grew rapidly after disturbance, and
colonizing plant species (Mac et al. 1998). While the lack of vegetation data to compare
with bird data limits the ability to answer questions about how habitat has responded,
results of this study reflect documented effects of anthropogenic forest management.
The species used in the plantation also appears to have a significant impact on
bird abundance and diversity. Noble fir plantations consistently produced higher bird
abundance and diversity than Douglas-fir plantations. The scope of this research limits
our ability to explain these intriguing results. One possible contributor to this result is
that higher elevation plantation (noble fir) were younger because they were salvage
logged and replanted at a later date than lower elevation (Douglas-fir) plantations
(Franklin 2016). Similarly, noble fir plantations do not grow as quickly or vigorously as
Douglas-fir, partly because they grow at higher elevations and have a shorter growing
season (Franklin 1981). This means that their canopy may have been more open at the
time, producing a more structurally complex understory with greater food resources to
support bird abundance and diversity (Crisafulli 2016).
Our results also follow what Mac et al. (1998) found on MSH where surviving
understory trees in the blowdown zone were invigorated by the loss of canopy and
demonstrated vigorous growth (Mac et al. 1998). According to their results surviving
63
understory trees in the passively managed sites started with an age and height advantage
over salvage logged sites. The rapid increase in height structure may have attracted a
greater number of bird seed dispersers, advancing plant community diversity in passively
managed sites faster than actively managed sites. This fact also helps to explain the
difference between noble fir and Douglas-fir stands because noble fir was planted at
higher elevations (~1300 meters) versus Douglas-fir (~750-950 meters) where snow
created microsites that protected further protected young trees, creating the same
advantage (Mac et al. 1998; Crisafulli 2016).
We researched the conservation status of all birds represented in this study with
reference to the United States Endangered Species List and the Washington State
Endangered Species List. The only species to occur on either list is the Prairie Falcon
which was listed by Washington as State Monitored. The lack of rare or listed species
may be indicative of the fact that the habitat is young for PNW temperate forest and has
not yet developed the structural complexity to support a more varied community.
Commonly, species of conservation concern are those that are specialized and appear
rarely across the ecosystem and are therefore more subject to the mechanisms of scarcity
(Quammen 1996). It is certain that many exciting changes will be occurring soon on
Mount St. Helens and studying bird communities is an important way to understand
them.
The results of this study point to significant differences in bird abundance and
diversity between areas that were salvage logged and planted in the blowdown zone of
MSH and those that were left to naturally regenerate. This analysis is limited however
due to the lack of data describing vegetation communities within the stands which are
64
responding to the post-disturbance treatments. This type of analysis should be carried out
with regard to this dataset in order to point to the specific habitat components which
differ between treatments that are responsible for the differences in bird abundance and
diversity between the treatments. Also we recommend continuing this research as it
provides the unique opportunity to compare lands heavily managed for timber production
versus those naturally recovering from cataclysmic disturbance. This research represents
a rare and meaningful opportunity to learn how post-disturbance management can be
augmented to protect important habitat for birds and other wildlife.
65
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Appendices
Appendix 1: List of guilds, guild codes and descriptions used for this analysis. Guilds
not represented in this study were removed from this list.
FOOD TYPE
CODE
DESCRIPTION
Carnivore
CA
Eats vertebrates
Insectivore
IN
Eats insects
Omnivore
OM
Eats a variety of foods including plants and
animals
SUBSTRATE
CODE
DESCRIPTION
Air
AIR
Catches food in air
Bark
BAR
Forages on, in or under bark of trees
Floral
FLO
Forages on or in flowers
Ground
GRO
Forages on the ground or on very low, weedy
vegetation
Lower Canopy/Shrub
LCA
Forages on leaves, twigs and branches of
shrubs, saplings and lower crowns of trees
Riparian Bottom
RBO
Forages on bottoms of rivers and streams
Upper Canopy
UCA
Forages on leaves, twigs and branches of tress
in main canopy
TECHNIQUE
CODE
DESCRIPTION
Excavator
EXC
Locates food in bark by drilling holes
Forager
FOR
Takes almost any food items encountered upon
the substrate
Gleaner
GLE
Feeds on grasses, sedges or grains in fields or
meadows
Hawker
HAW
Flies after prey and captures it either in air or
on ground
Sallier
SAL
Perches on exposed branch of twig, waits for
77
insect to fly by then pursues and catches in air
Scavenger
SCV
Takes a variety of items, including refuse or
carrion
Screener
SCR
Flies with bill open and screens prey from air
Appendix 2: Complete list of dataset used in this analysis including four-letter bird codes
and number of detections per study site and summary statistics.
Species
AMCR
AMDI
AMRO
BHGR
BTYW
BUOR
BUSH
CAFI
CEDW
CBCH
CHSP
CONI
CORA
DEJU
EVGR
FOSP
GCKI
GRAJ
HAWO
HEWA
LABU
MGWA
NOFL
NRWS
OCWA
OSFL
PAWR
PISI
PRFA
BDCC
BDNF
0
0
20
14
1
1
1
3
0
10
1
0
1
92
0
11
0
0
1
0
0
20
0
1
1
2
0
5
0
0
0
15
1
0
0
0
7
1
11
0
2
5
34
2
2
4
0
4
0
0
1
0
0
0
29
1
3
0
CWPLNS CWRDFP CWRPLN GRFOR
0
0
1
0
0
0
0
1
9
6
8
11
1
11
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
22
17
3
5
0
0
10
1
0
0
0
0
0
4
0
0
19
29
49
58
0
0
0
0
0
0
0
3
19
27
0
0
0
0
4
0
1
0
1
2
0
0
1
0
0
0
0
1
0
0
0
14
5
0
5
8
0
0
0
1
0
0
0
2
0
0
0
12
4
1
0
0
0
0
3
0
0
1
0
0
78
Species
RBNU
RBSA
RECR
RUHU
SOGR
SOSP
SPTO
STJA
SWTH
TOWA
TRES
VATH
VGSW
WAVI
WCSP
WIFL
WETA
WIWA
YEWA
YRWA
Total # Species
Total Detection
Total Species
% Total Species
BDCC
1
3
0
15
4
15
5
11
8
0
1
1
0
5
13
15
8
0
6
13
50
317
34
0.68
BDNF
CWPLNS
CWRDFP CWRPLN GRFOR
19
0
2
3
10
0
0
5
1
0
1
3
0
7
0
1
7
0
0
30
11
0
0
4
1
0
0
7
9
1
0
6
1
0
1
0
0
0
0
1
11
0
0
0
4
0
1
5
2
0
1
1
0
5
3
0
0
0
0
17
13
0
0
0
8
0
0
3
1
0
2
4
0
2
10
2
0
0
2
30
0
0
0
34
2
32
18
10
21
0
5
0
0
8
0
34
1
4
41
1
222
29
0.58
129
19
0.38
165
19
0.38
162
21
0.42
340
28
0.56
79