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Title
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Managing Young Stands in Western Washington to Expedite Complex Forest Structure and Biotic Diversity: Review, Rationale, and Recommendations
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Date
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2012
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Creator
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Wederspahn, Anna M
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Subject
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Environmental Studies
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MANAGING YOUNG STANDS IN WESTERN WASHINGTON TO EXPEDITE
COMPLEX FOREST STRUCTURE AND BIOTIC DIVERSITY: REVIEW,
RATIONALE, AND RECOMMENDATIONS.
by
Anna Wederspahn
A Thesis
Submitted in partial fulfillment
of the requirements of the degree
Master of Environmental Studies
The Evergreen State College
June 2012
© 2012 by Anna Wederspahn. All rights reserved.
This Thesis for the Master of Environmental Studies Degree
by
Anna Wederspahn
has been approved for
The Evergreen State College
by
________________________
Richard Bigley, PhD
Member of the Faculty
________________________
Date
ABSTRACT
Managing young stands in western Washington to expedite complex forest structure and
biotic diversity: review, rationale, and recommendations.
Anna Wederspahn
In the Pacific Northwest, second-growth forests are wide spread and this oversimplified
forest structure fails to meet the diverse needs for maintaining a wide species
composition. Maintaining and replacing older forest habitat to preserve species of
concern has become one of the greatest challenges to natural resources management in
western Washington. Ecosystem management emphasizing spatial heterogeneity and
environmental variability may increase successional development towards older forest
characteristics by providing niche differentiation that potentially can help meet the
diverse needs for maintaining high biological diversity.
Truffles, the fruiting bodies of mycorrhizal fungi, permit enhance tree nutrient
absorption, and serve as the primary food for mycophagous small mammals, and if
populations of small mammals are abundant, then there is an adequate prey base for
maintaining abundances and diversity of predators. Mycorrhizal fungi are also important
in promoting tree productivity and complex soil communities which are at the core of the
forest food web. There is an important cyclical relationship between trees, truffles and
small mammals which may be indicative of a keystone complex relationship that could
be used as an objective in managing second-growth forests to promote biological
diversity. By using variable-density thinning, snag creation, and coarse woody debris
maintenance along with forest models that can predict future forest succession, this vital
keystone complex relationship may be promoted along with increased biological
diversity.
The literature reviewed for this analysis points to the possibility that late-seral
communities could be restored with minimal intervention with a high probability of
success. Yet, while the management of forests to maintain or produce older-forest
conditions holds great promise, it remains a grand experiment, the results which may not
be clear for many decades.
Table of Contents
List of Tables and Figures…………………………………………………………….. vi
Acknowledgements……………………………………………………………………. vii
1. Introduction………………………………………………………………………… 1
Role of ecosystem management………………………………………………. 1
Understanding the challenges of ecosystem management……………………. 2
2. Background…………………………………………………………………………. 5
History of forest practices and current forest structure………………………… 5
What is biodiversity and why is it important? ..................................................... 6
Conventional forestry and its impact on biological diversity……………….…... 7
Changing management on public lands deals with the legacy of clear-cutting… 11
New forestry and its influence on biological diversity…………………………. 12
Implementing the new forestry paradigm………………………………………. 13
The beginnings: the northern spotted owl and old-growth controversy………. 15
3. Methods……………………………………………………………………………. 18
4. Results……………………………………………………………………………..
4.1 Literature Review………………………………………………………………
Pacific Northwest old-growth forest biotic diversity…………………………
Pacific Northwest truffles, trees and animals in symbiosis………………….
Rationale and treatments for increasing forest structural complexity……….
How overstory composition affects species diversity………………………..
Variable-density thinning…………………………………………………….
Fragmentation and Connectivity……………………………………………..
Forest soil……………………………………………………………………..
Understory herbs and shrubs…………………………………………………
Snags, dying and diseased trees………………………………………………
Coarse woody debris…………………………………………………………
Small mammal abundances………………………………………………...
Belowground fungi……………………………………………………………
4.2 Interview Responses……………………………………………………………
19
19
19
21
24
29
29
32
34
36
38
42
44
47
49
5. Discussion…………………………………………………………………………
Themes in literature…………………………………………………………..
Management considerations for snags and coarse woody debris…………….
Factors to consider when applying variable-density management…………..
Limitations of forestry models……………………………………………….
54
54
55
56
58
v
6. Conclusion………………………………………………………………………. 59
Recommendations for future forest management…….………………………... 61
References Cited………………………………………………………………….. 63
Appendices………………………………………………………………………… 73
Appendix A. Glossary of terms……………………………………………….. 73
Appendix B. Forest Vegetation Simulator model results……..…………........ 78
v
List of Tables
Table 1. Types of forest disturbance……………………………………………………. 8
Table 2. Resulting forest structures from disturbance…………………………………..14
Table 3. Response to resulting forest structures from disturbance……………………. 26
List of Figures
Figure 1. Examples of the structural heterogeneity found in old-growth forests…….. 20
Figure 2. The truffle life cycle………………………………………………………… 22
Figure 3. The tree, truffle, and small mammal symbiosis…………………………….. 23
Figure 4. Northern flying squirrel, Douglas’ squirrel, and Townsend’s chipmunk:
important spore dispersers for truffles………………………………………………….. 24
Figure 5. A densely stocked un-thinned forest and a variable-density thinned forest… 30
Figure 6. Examples of managing connectivity within the landscape matrix………….. 34
Figure 7. A snag to be maintained as a wildlife feature after thinning and
a snag with prevalent woodpecker cavities…………………………………………….. 39
Figure 8. A nest-web diagram showing which animals prefer which trees, and the
primary and weak excavators responsible for making the cavities…………………… 40
Figure 9. Decay stages of snags and logs in the Pacific Northwest…………………. 42
Figure 10. Examples of coarse woody debris on the forest floor……………………. 44
Figure 11. Truffles, the friuting bodies of belowground fungi imporant to tree
productivity and the diets of some small mammals…………………………………… 47
vi
Acknowledgements
I would like to thank my reader, Richard Bigley, for his immensely helpful feedback and
for his encouragement and optimism when times got tough. Eric Cummins for keeping
me well fed throughout this process and for listening to endless hours of frustrated
lamentations with a patient, kind, and open heart. To Melvyn Foster Jr. for expressing
how proud he is of me, to Sarah Barga for her helpful advice stemming from her own
recent master’s thesis completion. To my parents Keith and Eva Wederspahn for
believing in me and not letting me give up. And lastly, to my oma, Martha Humphries,
who passed away on March 1, 2012 at the age of 92, who would have loved to have seen
me graduate.
vii
1. Introduction
One of the greatest challenges to natural resources management in the Pacific
Northwest is maintaining and replacing older forest habitat to preserve species of concern
and watershed processes. Logging during the 20th century greatly reduced the area of
old-growth forests in the Pacific Northwest, reducing the habitat available for species
which depend on their habitat structure. Subsequently, the emergence of plantation
forestry simplified forest structure with low species diversity. Public lands have
considerable area of these simpler forests without active management, due to lower
intensity of management. Second-growth forests are widespread and there is much
debate about how to best manage for multiple uses, but in particular for high biological
diversity. Large tracks of forests on public lands can remain for decades in the
competitive-exclusion stage, which is poorly suited to older forest species, also known as
late-seral wildlife. Species recovery plans emphasize that suitable habitat needs to
increase if we want to ensure survival and long-term viability of older-forest dependent
species (U.S. Fish and Wildlife Service 2011).
Role of ecosystem management
It has been suggested that ecosystem management emphasizing spatial
heterogeneity and environmental variability can increase successional development
towards older forest characteristics (Franklin et al. 2006). This spatial heterogeneity and
variability provides niche differentiation that potentially can help meet the diverse needs
of old-growth dependent species, and is important for maintaining high biological
diversity (Carey and Curtis 1996). Additionally, because the presence of the northern
1
spotted owl (Strix occidentalis caurina) and other wide-ranging predator species cannot
be reliably used to evaluate stand levels at small scales, achieving prey species
abundances of small mammals (particularly mycophagist abundances) similar to those in
resilient old-growth forests, could better serve as an indication of forest ecosystem
management success in Pacific Northwest forests.
Truffles, the fruiting bodies of mycorrhizal fungi, permit enhance tree nutrient
absorption, and serve as the primary food for mycophagous small mammals, which in
turn are important prey for the northern spotted owl. Small mammals enhance truffle
abundances, through the dispersal of fungal spores; there is an important cyclical
relationship between these species. This relationship not only benefits tree growth, but
subsequently benefits a variety of birds and larger mammals which prey on these small
mammals. An example of this is the northern flying squirrel, which eats truffles and is
also the main food source of the northern spotted owl. This interaction between trees,
truffles and small mammals as well may be indicative of a keystone complex relationship
which could be a vital objective in managing second-growth forests to promote biological
diversity and structural heterogeneity characteristic of old-growth forests.
Understanding the challenges of ecosystem management
The challenge for forest managers today, is to encourage the development of
complex forest structures in support of biological diversity and habitat through new
forestry. It is a huge task because predicting behavior in ecosystems relies on multiple
variables and the interactions of those variables, of which causes and effects are difficult
to replicate. One tool being used to help guide forest managers in predicting forest
treatment outcomes is through the use of forest models. Predicting the consequences of
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management actions is one of the hallmarks of new forestry. Models can help predict
forest development and yields and have been instrumental in the maturing of commercial
forestry. The adaptability of non-spatially explicate stand models for the highly
developed science of plantation management to variable-density thinning used to create
habitat is a major challenge. Stand dynamics relay on the simulation of intercompetition. The assumption in all but a few models is that trees are evenly distributed
in the stand. New forestry is increasingly dismantling that assumption and the
predictions from existing models. Now forest models can summarize current stand
conditions and predict future stand conditions under various management alternatives
showing how management practices may affect stand structure and composition,
important to evaluating whether a stand will be appropriate as wildlife habitat (Dixon
2002).
My research hypothesis is: management or disturbance mimicking old-growth
structural complexity can increase second-growth forests habitat functionality by
promoting greater vegetation variety, snags, and coarse woody debris. This results in
increasing abundances of truffles and small mammals, which are part of a vital keystone
complex relationship indicating relative health in forests west of the Cascades. My null
hypothesis is: management or disturbance mimicking old-growth structural complexity
cannot increase second-growth forests natural succession because complexity is based on
synergistic interactions among plant and animal communities which can take centuries to
establish. In short structure alone cannot replace the time required to develop decadence
and large structure characteristic of old forests.
3
My research questions are:
Can late-successional/old-growth structure be accelerated in young stands with
new forestry techniques?
Will this enhanced structural diversity in young stands translate to latesuccessional functionality; i.e. ecosystem processes?
Is there evidence that small mammals and truffles are part of a vital keystone
complex relationship indicating relative health in Pacific Northwest forests which
could be used as an indicator of success in restoring forest complexity?
Do experts agree that modeling is a useful tool in predicting future forest dynamics?
4
2. Background
History of forest practices and current forest structure
Early European immigrants to North America viewed the vast forests they
encountered as limitless. They saw game, lumber and firewood to be taken and land to
be cleared. These forests were viewed as being hostile and in need of being tamed
(Worster 1979). Though it took two and a half centuries, from 1600 to 1850, to deforest
the first 40 % of the eventual total amount cut, it only took fifty years to deforest 150
million acres, which is far more land than was cut the previous 250 years (Berger 2008).
In the beginning, timber was cut for fuel and building materials or cleared for farming. In
the Pacific Northwest, early settlers largely avoided the dense forests because of scarce
labor for logging operations, difficulty in accessing timber more than two miles from
streams or river, and the poor quality soil for agriculture (Goble and Hirt 1999).
At the turn of the century, with the technological advances of the steam donkey
and later with railroads, areas of forests which were formerly inaccessible began to open
up. Even though it took commercial foresters until the 1920s to reach the Pacific
Northwest, by the 1860s, forests were beginning to be cleared at a much faster pace.
Then, after WW II when logging roads and logging trucks appeared, the region’s
economy became tied to lumber production. Limits on cutting began to be put in place
when land was set aside into national park and national forest status.
In the Pacific Northwest, in the early to mid-20th century selective logging of the
largest and most vigorous tress was the norm in Douglas-fir old-growth stands. By the
1950s through the 1980s, clear-cutting replaced selective logging in most areas. Clearcut systems dominated by timber production of which gave rise to even aged landscapes.
5
It left behind interspersed clear-cuts of various sizes, followed by slash burning and
planting. Planting and species discriminating thinning encouraged reduced tree species
forests. Within less than a century, forest structures resulting from natural disturbances
were replaced by forests originating from human activities. Wild-fire, windstorms, and
insect outbreaks of varying size, frequency, and intensity have been replaced by shortrotation timber harvest and prescribed burning; both are disturbances that are more
frequent and less variable in size and intensity.
On federal lands, management plans have caused rapid declines of old-growth
between the 1950s and 1980s, calling for the virtual elimination of all late-successional
forests on federal lands outside of parks and wildness areas (Franklin 1997). If the
logging rates of the mid to late 1990s were to be sustained, all remaining Pacific Coast
old-growth would be gone by 2025 (Berger 2008). Today, since the time of European
settlement about 72 percent of the original Pacific Northwest old-growth forest is gone
and only approximately 28 percent of it remains today (Strittholt et al. 2005), and to get it
back we have to wait a very long time.
What is biodiversity and why is it important?
Biodiversity is an extremely broad and complex subject that is usually defined as
the diversity of life on earth. Yet included in this definition should also be the
interconnections and processes supporting these numerous forms of life. Biodiversity
includes genetic, species, and ecosystem diversity (Wilson 1988). It can be seen has
having three parts to it; (a) compositional diversity which refers to the number of
elements in a system; (b) structural diversity which refers to the physical organization;
and (c) functional diversity which refers to the different processes (Zavala and Oria
6
1995).
The maintenance of the earth’s biological diversity is necessary for ecosystem
health as well as being aesthetically desirable (Hunter 1999) and there also is evidence
that multiple ecosystem services are enhanced by high local and regional diversity (Duffy
2009). The ability to draw from native biological diversity is an importance aspect of
adaptation to environmental climate change (Washington State Department of Ecology
2012). In ecosystems such as forests, which provide a range of resources, the theory and
practice of maintaining biodiversity are now seen as fundamental to successful
management. In the old-growth forests of the Pacific Northwest, the biodiversity
attributes include not only the diversity of conifers, mammals, birds, and understory
plants but also the myriad of mostly poorly understood fungi, lichens, insects, and soil
invertebrates.
Conventional forestry and its impact on biological diversity
Forestry during the mid-20th century accelerated disturbance in the forest at a rate
which was largely unseen naturally. The disturbance regime imposed by humans in this
region is typically based in intensive forest management with relatively short rotations
(40-80 years), clear-cut logging and preference for conifers, especially Douglas-fir (Table
1). On federal lands, which occupy about half of the forested area, the rate of cutting has
been substantially reduced since the early 1990s (FEMAT 1993). Despite these recent
changes, 30-40 percent of public forest landscapes contain a legacy of a patchwork of
forest plantations that were established in the 1950s through the 1980s (FEMAT 1993).
Relative to natural disturbance regimes, logging disturbances have typically been more
frequent, more severe, left fewer biological legacies (i.e., structure and species that
7
survive disturbances) and created more edge and fragmented landscapes (Spies et al.
1994). Clear-cutting, heavy soil disturbances, slash burning, and the removal of snags
and dying or diseased trees left a heavy impact on a forest’s biological diversity. The
resulting structures that remained depleted the forest of features which are essential to
many necessary functions in maintaining some types of habitat and wildlife species.
Table 1. Types of forest disturbance
Old forestry/lack of management
Clear-cut
Single entry
Slash fires
Removal of snags dying or diseased trees
New forestry/managing for complexity
Variable-density thinning
Repeated entries
Westside forests evolved w/infrequent
fires
Maintains snags, dying and diseased trees
Soil disturbance from ground-based clear-cutting can leave a lasting legacy of
compaction and impacts soil hydrology. As soil bulk density increases and soil becomes
resistant to penetration, tree roots cannot as easily absorb water and nutrients, leaving
behind a less productive site (Curran et al. 2008, Han et al. 2009). Forest soils can also
become less productive due to the removal of the trees and vegetation which uptake
nutrients, leaving these nutrients to leach out and be lost (Allen 1997). Clear-cutting has
been associated with decreased soil stability and therefore increases the occurrence of
landslides after the stabilizing effect of tree roots and the ability of the canopy to trap rain
is removed (Kimmins 2004). Logging roads also play a significant role in soil instability,
increasing slides significantly where the primary soil mantle is less stable (Kimmins
2004, Pgs. 324-325). Soil disturbance may have a direct impact on mycorrhizal fungi or
an indirect affect through changes to soil properties. Mycorrhizal fungi can be severely
8
influenced by damage to vegetation and soils resulting from conventional forestry
practices such as clear-cutting, intense fires, and exposure of subsurface soil to erosion
(Brundrett 1991). Disturbance impacts on fungi may include; (a) a reduction in viable
spores, (b) loss of the hyphal network in the soil, and (c) the prevention of hyphal growth
from root inoculum to new roots (Brundrett 1991). All of these factors decrease the
productivity of a site and the probability of future plant growth.
After live trees are removed in a clear-cut, without host trees, some mycorrhizal
fungi including many truffles cannot survive. Many fungi are also susceptible to the low
moisture conditions after the coarse woody debris (CWD) are removed during a clear-cut
(Amaranthus et al. 1994). The once common practice of slash-burning after a clear-cut
ended in western Washington in the mid 1980’s. However this once practiced slashburning also removed nutrient accumulations along with the slash. Slash-burning
negatively impacted fungi by removing the thick protective organic layers which they
colonize, and this lack of thick organic layer can impede their re-colonization after
young-stands have grown back (North and Greenberg 1998); in addition, the fungi in the
wet and mild climate of western Washington may not be adapted to frequent fires due to
their low historical occurrence (North and Greenberg 1998).
The practice of clear-cutting in forestry removes snags primarily because of the
United States Department of Labor Occupational Health and Safety Administration’s
requirement for worker safety (Carey and Wilson 2001). Snags are a vital feature for
many species to forage, den and nest. Primary cavity excavators such as the pileated
woodpecker create cavities for many small birds and mammals to den in, which could not
otherwise create themselves. Large mammals use hollowed out trees for denning which
9
would otherwise be removed in a clear-cut system (McComb 2003). Lack of snags,
defective or dying trees always means lower plant and animal diversity (Ohmann and
Waddell 2002).
In clear-cut forestry, there will be a shift in small mammals due to the lack of
understory development and CWD. Small mammals will have to travel farther to meet
their dietary needs and will not have adequate cover for dispersal and hiding areas. Clearcuts change the small mammal landscape, relative truffle abundance and loss of legacy
structures. When planted trees grow back into dense young stands, there will not be a
diversity of understory plants for these small mammals to forage. As a result, unthinned
clear-cuts that develop into dense young stands have lower densities of flying squirrels,
the primary diet of the endangered northern spotted owl (Carey 1995). In general, the
fewer number of small mammals, the less the area will be able to sustain larger predator
mammals and avian species which rely on an abundance of small mammals.
Our current decrease in biodiversity is from human conversion of land and
subsequent loss of habitat without connectivity (Anderson and Jenkins 2006). Clear-cuts
likewise leave behind a highly fragmentation landscape. What late-successional/oldgrowth forests that remain, are in patches fragmented by dense second-growth that is
inhospitable to many late-seral dependent species (Carey et al. 1992). Habitat loss not
only negatively affects species richness, population abundance, distribution, and genetic
diversity, but also indirectly effects species interactions reducing trophic levels, and the
number of large-bodied specialist species, as well as the general success rate in breeding,
dispersal, predation, and foraging (Fahrig 2003). Although populations of declining
animals still persist in some areas, their long-term viability is questionable as these
10
populations become more isolated from each other. Since the turn of the century, most of
Washington’s forested areas have been converted to other uses, resulting in the loss of
most of the state’s old-growth forests and the subsequent decline in biological diversity
and habitat for old-growth dependent species (Fisch 2000).
Changing management on public lands deals with the legacy of clear-cutting
The management objectives of public lands underwent a rapid evolution with
emergence of the concept of ecosystem management. The clear-cutting legacy reflected
in the even-aged age class distribution and landscape patterns on a large scale in forests
will take a decade’s if not a century to reshape. Commercial forestry plantations have
impressive homogeneity. Planting of a single species and controlling density and species
composition are the principles that make a tree farm productive and economically
attractive to investors. These stands, left unthinned, can have a high diameter to height
ratio leaving them susceptible to wind throw. Due to lack of management funding after
the emergence of ecosystem management, which occurred on much of our public lands,
forests can stay in the competitive exclusion stage decades. The competitive exclusion
stage does not support high species diversity due to the lack of understory, large trees,
and CDW. Commercial forestry removes less valuable trees, favors one tree species, and
maintains stocking levels and even spacing, all of which reduce heterogeneity important
for abundant wildlife (Carey and Wilson 2001). Though, pre-commercial thinning can
delay the onset of the stem-exclusion stage, it can only delay the loss of understory
development is understory plants are already well established (Zarborske et al. 2002).
Traditional timber production creates forests that lack spatial heterogeneity found
in late-successional/old-growth forests necessary for species diversity (Suzuki and Hayes
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2003). Conventional forestry traditionally views understory as competition to crop trees
and saw no needed to maintain various trophic levels in a forest and tends to see
understory development only as a by-product to timber (Thysell and Carey 2000). Yet,
understory is very important to small mammals and birds for forage, hiding and nesting
(Carey and Harrington 2001). Because a site has been held in clear-cut mode, favoring a
single species of tree, there will often not be a seed bank left of shade tolerant tree species
or some understory plants to be able to recolonize (Halpern and Spies 1995). Therefore
even if a site is left after a clear-cut and not entered again, it can remain very simplistic in
its diversity. Perpetual clear-cutting may also maintain persistent weedy plant species or
exotics that have evolved with disturbance and in the less shady conditions clear-cutting
offers (Halpern and Spies 1995).
By centuries end, our knowledge about forest ecology has increased as well as a
diminishing social acceptance for the continued harvest of old-growth pushing forestry in
a new direction. Now, in the 21st century, we are seeing different management objectives
and a change of focus, one which not only includes timber production, but also
management for biological diversity. Yet, due to loss of the regions old-growth forests,
many species that rely on old-growth for at least some part of their life cycle are at risk of
extinction.
New forestry and its influence on biological diversity
Ecosystem management was a tectonic shift of management objectives away from
entirely being about timber production and instead to balancing managing for biological
diversity with production. The results from these changes have demonstrated the
importance of maintaining the natural structure either currently or formerly present. By
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maintaining or restoring the complex structure of the forest, it is assumed that the
complex ecological functions in the forest, which so many plant and animals rely upon,
can be restored.
Implementing the new forestry paradigm
The new forestry paradigm is based on the assumptions that: (a) ecosystems and
landscapes are dynamic; (b) disturbance is a critical component of systems; (c)
ecosystems are controlled by biotic and physical processes that occur at different spatial
scales and levels; (d) succession does not necessarily follow the same path and end at the
same equilibrium point; (e) spatial pattern is important to biological diversity; (f) patternprocess interactions are organism specific; and (g) human activity of the recent and
distant past have had strong influences on many ecosystems that we may perceive as
‘natural’ today (Pickett and Ostfelt 1995). These new metaphors of ecology may help us
to sustain biological diversity but they also make management more complex and
difficult.
One of the foundations for conserving biological diversity in forested landscapes
is first to understand and then manage by mimicking disturbance regimes of a landscape
under past natural or semi-natural conditions (Table 2). The most important way for new
forestry to do this is through the practice of variable-density thinning (VDT). This is the
primary treatment in forest management which creates canopy variation through irregular
thinning, which helps to create a more uneven aged multi-canopy layer seen in older
forests.
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Table 2. Resulting structures from forest disturbance
Old forestry/lack of management
Heavy one time soil disturbance
Depleted below ground food webs
Lack of CWD
Homogeneity
No snags
Fragmented forest ecosystems
Even-aged single species
High diameter to height ratio susceptible to
wind fall
New forestry/managing for
complexity
Minimal soil disturbance if certain types
of equipment, entry time of year and low
impact techniques are used
Minimal/short-term negative effect on
below ground biotic community
Maintains CWD
Heterogeneity
Snags present
Maintains a matrix of types of suitable
forest ecosystems
Multiple species, uneven age
Promotes larger diameter trees less
susceptible to wind fall
New forestry practices look to maintain connectivity between forests of similar
characteristics. In order to benefit species which need larger areas of land or which are
reliant on a more heterogenetic canopy, both of which new forestry tries to emulate. Soil
disturbance tries to be minimized in new forestry by using less intensive logging
techniques. This is important also for the development of CWD, which is vital to
ecosystem functioning in the fungal community. New forestry looks to leave skip areas
where important decadent features of snags, dying, and diseased trees are located in order
to protect them since they are a major wildlife habitat feature. These structural legacies if
maintained, which sustain species and processes, can provide managed stands with
characteristics of a more successionally advanced forest (Franklin et al. 2002). VDT also
promotes development of understory herbs and shrubs by creating areas where more light
can penetrate to the forest floor and where there is less competition for water resources.
14
By increasing understory vegetation and CWD, it also helps to increase habitat for small
mammals which are a major prey base for many predator species. These variables in new
forestry which help promote biological diversity will be reviewed in detail in the results
section of this essay.
The beginnings: the northern spotted owl and old-growth controversy
The northern spotted owl remains at the heart of a controversy that still continues
today (U.S. Fish and Wildlife Service 2011). The core of the argument lies in the fact
that a pair of spotted owls requires very large tracts of old-growth conifer forest, and the
timber in these stands is extremely valuable. By the mid-1980s, most old-growth on
private lands in the Pacific Northwest had been logged, and pressure shifted to federal
lands. Also at this time, public concern about the spotted owl and the old-growth forests
it depends upon grew. By the late 1980s, evidence of the dependence of spotted owls on
old-growth was mounting, and their numbers and viability were declining (Thomas et al.
1990). Lawsuits began to mount from environmental groups against the United States
Department of Agriculture Forest Service and United States Department of the Interior
Bureau of Land Management. In 1990, the northern spotted owl, which ranges from
Washington to northwestern California, was listed as threatened under the U.S.
Endangered Species ACT (ESA), bringing into full force the requirement of the ESA to
provide the means to protect the ecosystems of which endangered and threatened species
depend upon. With the listing of the northern spotted owl, the question shifted from
whether the cost of saving the owls was worth the lost revenue from timber harvest, to
how the owl itself was going to be sustained.
In 1993, President Clinton commissioned the Forest Ecosystem Management
15
Assessment Team (FEMAT) along with other pivotal groups such as the Interagency
Scientific Team and the Scientific Analysis Team to create a management plan. This
plan came to be known as the Northwest Forest Plan (NWFP) and was heralded by some
as ushering in a new era of forest ecosystem management on federal public lands (Aubry
et al. 1999). Core elements of the NWFP, which covers a total area of about 9,896,900
ha and 90% of the northern spotted owls range on federal lands, included the
establishment of late-successional reserves (LSRs) and adaptive management areas
(AMAs) (Marcot 1997). LSRs, which comprised just over 30 % of the planning area,
would maintain varying but significant amounts of existing old-growth forests, and were
to be used for aggressive management of younger stands to attain old-growth
characteristics (Thomas et al. 1993). AMAs were designed in various forest types to
allow tests of alternative approaches to maintaining threatened species and comprised just
over 6 % of the planning area (Marcot 1997). Additional lands totaling 17 % (designed
matrix and managed late successional areas) were to be available for timber harvest if
environmental regulations were met (Marcot 1997).
While some might applaud the NWFP as a huge conservation success story, the
plan was not followed. Active management within LSRs was not aggressively pursued,
and experiments were not conducted within AMAs (Thomas 2003). Only modest
amounts of experimentation have occurred on AMAs. Restrictions on AMA
management in the NWFP, lack of flexibility or reluctance to approve habitat
modifications or departures from the plan, and lack of financial support have been the
limiting factors (Thomas 2003). The NWFP has been better at halting actions which
would diminish conservation of LSRs than it has been in promoting restoration or AMAs
16
(Thomas et al. 2005). As a result, the overall strategy became one of essentially static
reserve management. To many people, managing old forests may seem straightforward
and this may seem like a good idea -you protect as much as feasible, then lock it up.
Unfortunately, it is not quite that easy. This is not going to work if we want to increase
old-growth characteristics faster than they will be created on their own to provide habitat
for various species which require older forest conditions. Furthermore, old-growth
forests are dynamic and it will not be possible to provide cathedral groves of ancient
forest for future generations simply by preserving existing old-growth stands. Creating
reserves as should not be the only form of forest management. Given the large areas
required for managing spotted owls and other older forest species, active landscape
management is necessary, which requires an understanding of an ecosystem’s dynamics.
17
3. Methods
I conducted a systematic literature review of research being done to better
understand functional relationships within late seral/old-growth forest ecosystems and
research projects underway looking to increase structural complexity in forests. I
employed habitat analysis through the review of current literature concerning western
Washington and Oregon forest health, analysis of current forest restoration treatments
and ecological goals.
I consulted with three experts in the field of forest restoration, forest ecology, and
wildlife biology; Todd Wilson from the Pacific Northwest Research Station, Constance
Harrington from the Pacific Northwest Research Station, and Joe Buchanan from
Washington Department of Fish and Wildlife. Interview questions were: Is creating
complexity in forests working to restore biodiversity? What are, if any, indications of
success? How useful are models in planning future structural diversity?
18
4. Results
4.1 Literature review
Pacific Northwest old-growth forest biotic diversity
.
The old-growth forests of the Pacific Northwest are ecosystems dominated by
large conifers at least 250 years old and ranging beyond one thousand years. Twenty-five
species of conifer are found in these forests, but Douglas-fir tends to be the dominant tree
in Oregon and Washington. These forests historically covered millions of acres of land
before wide spread logging took hold (Marcot 1997). Old-growth forests are known to
have high biological diversity within plant, vertebrate, invertebrates and aquatic organism
communities (Maser et al. 2008). Many of these species are highly specialized to oldgrowth conditions with some exclusively preferring these types of forests (Franklin and
Spies 1991).
One of the most undeniable features of old-growth forests are their structural
diversity. This structure is what sets them apart from managed forests and is what many
wildlife species seek for their survival. It is believed that at least 118 vertebrate species
rely primarily on old-growth for habitat; of these, 41 rely exclusively on old-growth for
their nesting, breeding and forage habitat (Norse 1990), and over 1,000 species of plants
and animals are suspected to be closely associated with late-successional forest
conditions (Thomas et al. 1993). Often this is because they require some feature or
features, such as large standing snags or high density of large down timber that only oldgrowth forests can provide. Most forest-dwelling species are not associated with a
particular tree species, but with the structure of those species within the forest.
Availability of prey species is also crucial, because most of the vertebrate species found
19
in old-growth are predators.
Figure 1. Examples of the structural heterogeneity found in old-growth forests
(wilderness.org and bcfederationist.com).
Old-growth forests have many characteristics present at the same time. There will
be various large living tree species at differing age-classes giving the forest a multilayered canopy. The largest trees, two hundred feet tall or more, will have winddamaged tops with relatively few branches with the exception of a few large epicormic
branches, with a thick growth of mosses and lichen harboring many insects, birds and
small mammals. Because these forests have been there a very long time, each tree takes
on various forms which are important structural features. Some of these features are deep
fissures in the bark and very large irregular crowns. Overtime, these trees have
outcompeted others so they are spaced well apart from other similar dominate trees. The
huge thick trunks of these trees often show evidence of charcoal from a previous fire of
several centuries past which they have survived. Gaps form where some trees fell, being
replaced by understory vegetation giving a heterogenic patchy appearance (Figure 1).
This understory development is most complex in old-growth forests where more plant
species diversity is found (Spies 1991).
Large snags are another feature which is characteristic of old-growth forests.
20
Large standing snags may stay erect for over two hundred years. As their branches
slough off, sunlight is able to reach the forest floor, allowing species that require light,
such as Douglas-fir, to germinate. Insects and woodpeckers open up the dead wood,
providing habitat for many other species. These are of vital importance to many
vertebrate den and cavity nesters. In turn, these become food for larger predators such as
the northern spotted owl, marten and black bear.
Large down trees and accumulated downed wood in old-growth forests serves
multiple purposes and can remain for up to several centuries for some species (McComb
2003). Large logs crisscross the forest floor and as they decay over several hundred
years, dozens of species of insects, birds and mammals use them for shelter, as well as for
foraging and dispersal habitat. All this activity helps raise the concentration of nutrients
in the rooting wood, and the rootlets of nearby live trees tap them for food (Harmon
2009). Like live trees, down logs can hold extraordinary amounts of water. In this way,
they also serve as a substrate for microbial and fungal species which are a food source for
many small mammals and invertebrates. These fungi, called mycorrhizae, also form a
symbiotic relationship with the surrounding conifers and are responsible for adding them
in the uptake of nutrients and water from the soil. The downed trees themselves later also
become nurse logs for other trees to grow on.
The general structure of an old-growth forest will be varied with gaps and
multiple canopy layers, which is precisely what makes them so unique. Any of these
features may occasionally occur in younger forests, yet only old-growth has them all.
Pacific Northwest truffles, trees and animals in symbiosis
Soil organisms play an essential role in an ecosystems health, with truffles
21
playing a critical role in these dynamics, yet are often overlooked in forest management.
Truffles are the below-ground fruiting bodies of mycorrhizal fungi, often called
hypogeous fungi. These fungi assist trees and other plants in the uptake of water and
nutrients from the soil by forming thread-like hyphae mates which penetrate the root tips
(Figure 2). In return, carbohydrates are absorbed back to the fungi from the trees and this
carbon then supports a wide range of soil organisms (Carey 2004, Carey 2003).
Figure 2. The truffle life cycle (tartufiinlanga.it).
Truffles have evolved to depend on being eaten by animals for their spore
dispersal. When they reproduce, they emit a very aromatic odor that attracts animals.
When the truffles are eaten, the spores, bacteria and yeast pass through the digestive tract
unharmed (Carey et al. 2002). The spores of the fungi are then spread throughout the
forest in the feces of the animal and can relocate to other trees and plants to begin the
cycle again (Figure 3).
22
Figure 3. The tree, truffle, and small mammal symbiosis (natruffling.org).
Many animals will eat truffles as a food source, but the major mycophagists in the
Pacific Northwest are the northern flying squirrel (Glaucomys sabrinus), Douglas’
squirrel (Tamiasciurus douglasii), and Townsend’s chipmunk (Tamias townsendii)
(Carey et al. 2002) (Figure 4). These small mammals then become an important food
source for many avian and mammalian predators, like the northern spotted owl.
Diversity of truffles is important to maintaining abundant prey bases (Carey 2004). It has
been shown that some species of squirrel will decline in body mass corresponding to
diminishing availability of fungi (Smith et al. 2003). Fungi are also an excellent source
of water for small mammals. When the host plant of the mycorrhizae are removed, such
as in clear-cutting, the energy source for the fungi is lost and therefore will not fruit.
Without the mycorrhizal fungi and the truffles they produce, multiple plant and animal
species are negatively impacted.
23
Figure 4. Northern flying squirrel, Douglas’ squirrel, and Townsend’s chipmunk:
important spore dispersers for truffles (moonshineink.com, nationalgeographic.com, and
en.wikipedia.org).
The fungi are important to the growth and health of many Northwest tree species
such as Douglas-fir, spruce, and hemlock. It is believed that most plants in the forest
have developed mycorrhizal associations, with more than 100 Pacific Northwest plant
species having been identified that associate in mycorrhizal symbiosis with truffle fungi
(Molina and Smith 2009). In North America as a whole, there are over 2,000 species of
ectomycorrhizal fungi that have a symbiotic relationship with trees (Fisher and Binkley
2000).
Rationale and treatments for increasing forest structural complexity
One way to conserve biodiversity on managed lands is to create complexity by
using silvicultural practices patterned on natural disturbance. In this way, we can mimic
some of the physical and biological characteristics typical of an older forest, and
therefore influence a young stand onto the path of forest succession faster. In the early
1990s, several private research institutions and public interest groups in Washington and
Oregon sought federal funds for research aimed at addressing whether we can increase
24
the extent of forest stands with old-growth characteristics in the hopes of finding new
methods for management (Berger 2008). All these large-scale silvicultural experiments
(LSSEs), which are scattered throughout Washington and Oregon, use variations of VDT
to increase biological diversity in forests by providing spatial heterogeneity in
composition and structure that mimics conditions found in old-growth forests. Desired
structures include: 1) a multi-layered canopy consisting of a large range of tree species,
ages, and sizes; 2) large-diameter standing snags and fallen logs; and 3) a diverse
understory of many species and a variety of available habitats (Table 3). LSSEs in young
forests are an important part of adaptive management of natural resources on public land.
Two examples of the treatments being applied within LSSEs are the Forest
Ecosystem Study (FES) implemented in 1991-1993 located on Fort Lewis Military Base
and the Olympic Habitat Development Study (OHDS) with initial installation in 19972006 with several locations on the Olympic Peninsula. Both if these studies have
response variables looking at over, mid, and understory development, arboreal and small
mammals, fungi, woody debris, and snags among others. Both pretreatment sites were
dominated by even-aged Douglas-fir between the ages of 35-65 years (Poage and
Anderson 2007).
Within the FES, root rot thinning was applied to 15 percent of the stand with low
vigor trees removed and healthy ones being retained producing approximately 16 trees
per acre (TPA) of > 7.9 diameter at breast height (dbh). A light thinning was applied to
50-60 percent of the stand taking co-dominate trees of >7.9 dbh reducing the densities to
about 125 TPA with an average spacing of 19 feet between trees. A heavy thinning was
applied to 25 to 30 percent of the stand of trees > 7.9 dbh to create spacing of
25
approximating 24 feet between trees. They also retained any standing dead tree and all
deciduous trees. No underplanting was done and no coarse woody debris (CWD) was
supplemented. Den augmentation (nest boxes installed and/or cavities created) was done
at one control site (Poage and Anderson 2007).
At the OHDS sites they thinned 75 percent of the stands to 75 percent basal area,
cut 15 percent of the stand to create small gaps, left 10 percent of the stand as uncut skip
area. At the first site they did no underplanting. CWD was clumped and slash dispersed.
At the second site they did an identical treatment except that the CWD and slash were
both dispersed. The third site was identical except both CWD and slash were clumped;
native understory species were planted or seeded in newly created openings. The fourth
site treatments were identical except there was no underplanting and the CWD was
removed and the slash dispersed (Poage and Anderson 2007).
Table 3. Response to resulting forest structures from disturbance
Old forestry/lack of management
Soil
compaction
Downed wood
New forestry/managing for
complexity
Impacts soil hydrology and
productivity. Increases bulk
density (BD) and soil
resistance to penetration
(SRP) i.e. porosity and soil
strength. Although disturbs
less area for a given amount
of timber volume then VDT.
Repeated entries for thinning will have a
higher impact on soil than a one-time
clear-cut. Can have lower impact though,
depending on type of equipment used and
time of year for entry. Cut-to-length
harvesting uses and leaves slash to
decrease soil compaction over whole-tree
removal.
Curran et al. 2007, Han et al.
2009, Zarborske et al. 2002
Slash and burn debris on site,
does not leave behind CWD.
Han et al. 2009, Kimmins 2004, Zarborske
et al. 2002
Keeps CWD on the ground, keeping
shelter, foraging, and dispersal areas small
mammals. CDW creates a moist substrate
for many plants and fungi to occupy.
Bunnell and Houde 2010
26
Snags
Lack of snags, defective or
dying trees means lower plant
and animal diversity, OHSA
requires most snags to be
removed for worker safety.
Carey and Wilson 2001,
Ohmann and Waddell 2002
Belowground
fungi
Truffles can be completely
absent from clear-cut areas.
Once trees and CWD are
removed, fungi are unable to
survive (no trees for
mycorrhizae, high heat and
low moisture areas). High
soil disturbance and slashburning has negative impact
on fungi. Young-stands lack
thick organic layer needed for
mycorrhizae to live.
Keeps snags intact by arranging skip areas
around them with no treatment. Promotes
primary cavity excavators
(woodpeckers) which create cavities
important to some small mammals and
birds for den/nest use. The larger, the
snag the better (snags or limbs less than
10cm in diameter have little to no value to
wildlife).
Harrington 2009, McComb and
Lindenmayer 1999
Maintains canopy cover and CWD for
mycorrhizae. Some species decrease after
thinning, yet some understory plants (i.e.
salal) may have a symbiotic mycorrhizae
relationship, which can help fungi recover.
Thinning seems to change truffle species
dominance.
Luoma et al. 2003
North and Greenberg 1998
Small
mammals
Less number of small
mammals due to lack of
understory development and
CWD. Have to travel farther
to meet dietary needs. Clearcuts that develop into dense
young stands have lower
densities of flying squirrels.
Increases food supply and cover available
for small mammals, though depending on
spacing between thinning’s, can impede
travel for flying squirrels. Maintains
snags which are very important den sites
for flying squirrels that are known to
change dens every two weeks. Increase in
rodent population also benefits
mammalian and avian predators.
Carey 1995
Carey et al.1999, Suzuki and Hayes 2003,
Wilson and Carey 1996
Fragmentation Leaves LS/OG in patches
fragmented by dense secondgrowth.
Helps to improve habitat between residual
LS/OG forest fragments. Incorporates the
landscape perspective of the matrix into
management.
Carey and Spies 2002
Franklin 1993
27
Forest canopy
Understory
Leaves crowded, uniform and
dense canopies after clearcutting. High diameter to
height ratio. Due to lack of
management, stands can stay
in competitive exclusion stage
for long periods.
Conventional thinning
removes less valuable trees,
favors one tree species,
maintains stocking levels and
even spacing. Lacks spatial
heterogeneity found in
LS/OG forests.
Carey and Wilson 2001,
Suzuki and Hayes 2003
Gives minimal attention to
understory needs in
maintaining various trophic
levels and looks at understory
development only as a byproduct to timber. Some
plants are capable of
recolonizing after clearcutting & slash burning due to
deep tubers, roots or
rhizomes. Clear-cutting may
maintain persistent weedy
plant species or exotics.
Carey and Harrington 2001,
Halpern and Spies 1995,
Thysell and Carey 2000
VDT increases individual tree diameter
and crown growth by reducing crowding.
Creates spatial heterogeneity (vertical and
horizontal) similar to older forests. Can
eliminate long interval between stem
exclusion stage and understory
reinitiation.
Helps to create niche differentiation by
providing a more complex habitat which
contains more kinds of microclimates and
microhabitats for more species to occupy.
Helps to release understory shade-tolerant
tree species when present.
Brokaw and Lent 1999, Carey and
Johnson 1995, Carey and Harrington
2001, Thysell and Carey 2000
Promotes development of understory
herbs and shrubs by increasing light levels
and decreasing water competition, thus
contributing to forest vertical structure.
Increases plant diversity. Increases habitat
complexity and food sources (seeds &
berries) to increase faunal biocomplexity,
with large shrubs providing cover for
small mammals. Some shrubs may
decline after thinning depending on pretreatment conditions, intensity of thinning
and amount of ground disturbance, but
most commonly increases. Minimizes
opportunities for invasion and
establishment of exotic plant species,
though without residual understory shade
tolerant tree conifers and hardwoods,
exotics may be able to spread more easily.
May not have residual soil seed banks of
shade-tolerant tree species or woodland
herbs (the later possess limited dispersal
capability). Canopy gaps, created with
VDT, may promote large shrub growth.
VDT increases abundance and probability
of flowering/fruiting in some shrub
species.
Halpern et al. 1999, Lezberg et al. 1999,
Lindh 2008, Sullivan et al. 2001, Thysell
and Carey 2000, Wender et al. 2004,
Wilson et al. 2009
28
How overstory composition affects species diversity
Overstory trees influence the structural and functional characteristics of
ecosystems and, consequently, shape the biota of a forest. Overstory trees regulate
structure and function by virtue of their physical dominance and in doing so control the
distribution and abundance of other taxa in the forest. Specifically, trees affect forest
biota through provision of resources, such as nutrients, water, and substrates. Further,
they alter light environments and microclimate in the forest understory through their
crown characteristics. Trees also affect ecosystem process like nutrient cycling and
disturbances, which, in turn, can affect the type, number, and abundances of other
species. When thinking about these functional links, it is important to understand that
when overstory composition changes, the nature of the links that trees provide to other
taxa also changes.
Variable-density thinning
Variable-density thinning (VDT), used in new forestry, helps to create spatial
heterogeneity (vertical & horizontal) similar to older forests, while maintaining canopy
cover. Vertical structure is the bottom to top configuration of above-ground vegetation
within a forest stand. One can think of vertical structure as vegetation complexity, and
horizontal variation among stands as vegetation heterogeneity. In general, the more
vertically diverse a forest is the more diverse will be its biota, for two main reasons.
First, a more complex habitat contains more kinds of microclimates and microhabitats for
more species. Second, it follows that a more complex vertical structure, supporting more
29
kinds of plants and animals, provides more diverse food resources for more diverse
consumers (Hunter and Schmiegelow 2011).
Variable-density thinning alters the density of managed stands through the partial
removal of trees to enhance the growth of those that remain (Puettmann et al. 2009)
(Figure 5). By regulating the stand density through thinning, foresters can accelerate
volume and diameter of trees, create trees with deeper crowns, and promote development
of understory herbs and shrubs. The variability of the thinning also encourages the
growth multiple tree species at uneven ages. It also promotes larger diameter trees and
crown growth by reducing crowding with providing space for them to respond to in new
growth. Overtime, these trees will be less susceptible to wind fall.
Figure 5. A densely stocked un-thinned forest and a variable-density thinned forest
(cityofseattle.net and fs.fed.us).
Variable-density thinning can also eliminate the long interval between stem
exclusion stage and understory reinitiation (Brokaw and Lent 1999). It helps to create
niche differentiation by providing a more complex habitat which contains more kinds of
microclimates & microhabitats for more species to occupy (Thysell and Carey 2000). As
the overstory layer matures, it becomes more horizontally heterogeneous and vertically
30
complex due to the thinning and to differential height growth and shaping of crowns.
This permits more light to penetrate lower levels, and in response an understory layer
develops, further complicating vertical structure. It also helps to release understory
shade-tolerant tree species when they are present. To avoid negative impacts in multicohort management, tree felling should not greatly exceed natural tree fall rates and
loggers should practice reduced impact methods of logging and skidding (Brokaw and
Lent 1999).
Forest management practices that replace mature, multi-layered coniferous forests
with young, structurally homogenous, single-species stands may adversely affect arboreal
rodent populations. An important positive outcome of VDT in promoting various tree
species at varying ages is in its effect on the habitat requirements of pine squirrels. These
squirrels need diverse conifers providing a supply of reliable seed sources, as well, these
squirrels need closed-canopy multi-layered forests with large trees that have some
interlocking branches to best navigate through, all which VDT can provide (Smith et al.
2003). Older forests between 80-100 years old probably support more Douglas squirrels
since cone production increases as a tree ages (Smith et al. 2003). Yet, in old-growth
stands greater than 250 years, cone bearing may taper off with age and squirrels may
need to look for additional food sources (Smith et al. 2003). Incidentally, a negative
outcome of VDT can occur if the spacing is too wide between thinning’s, impeding travel
for flying squirrels (Carey and Wilson 2001). Most studies report an increase of flying
squirrels in older forests or in young forests with old-growth components, than in
younger, managed stands. Flying squirrels also consistently chose dens sites with higher
amounts of downed logs on the forest floor. Carey (2000) found flying squirrels to be
31
twice as abundant in Douglas-fir forests managed for retention of standing live trees,
snags, and fallen trees than in stands intensively managed for timber production.
Fragmentation and Connectivity
Fragmentation is the process of creating a scattered network of land patches as a
result of disturbances, particularly from human activity (Anderson and Jenkins 2006).
Land-use changes fall into three main categories: reduction in total forest acreage;
conversion of naturally regenerated forests to even-aged monoculture plantations; and
fragmentation of remaining natural forests into progressively smaller patches (Bennett
2003). Fragmentation sets up a process of species loss in response to three types of
changes: overall loss of habitat, reduction in size of fragments, and increased isolation of
fragments.
Spatial arrangement of habitat becomes critical as availability of habitat declines,
and therefore simply managing for total amount of habitat will not necessarily be
sufficient for assuring the persistence of species. The reduced ability of animals to move
through the landscape has some major consequences; it limits their capacity to
supplement declining populations, to re-colonize habitats where extinctions have
occurred, or to colonize newly suitable habitats (Anderson and Jenkins 2006).
Fragmentation may have more of an impact on species declines and population
abundances than from habitat loss alone (Tyler and Peterson 2003).
Connectivity is a measure of the ability of organisms to move among separated
patches of suitable habitat and is used to describe the arrangement and quality of
elements in the landscape which may affect the movement of organisms among these
patches (Hilty et al. 2006). The level of connectivity can be described as the degree at
32
which the landscape facilitates or impedes movements among these habitat patches.
Connectivity for dispersal is a problem felt most strongly by late-successional forest
species (Noss 1993), which need blocks of habitat which are close together.
New forestry practices look to lesson fragmentation by maintaining connectivity
between suitable forest ecosystems by incorporating the matrix into management from a
landscape perspective (Franklin 1993). It strives to create improved habitat between
residual late-successional/old-growth forest fragments by creating buffers and corridors
of other habitat types (Figure 6), in the hope to ultimately help a diverse array of species
move more freely between habitats increasing their survival. By creating connectivity
between forest habitats, land managers are hoping forest biodiversity will be increased.
An example of this concept being implemented to restore spotted owl populations is with
the Interagency Scientific Committee (ISC), which in 1990 recognized the importance of
lands surrounding reserves and recommended managing for a matrix design between
these sites (Marcot and Thomas 1997). By doing so, it would provide for the movement
between habitats and for conservation of organisms and processes within the matrix
(Thomas et al. 1990).
33
Figure 6. Examples of managing connectivity within the landscape matrix.
(greateasternranges.org.au).
The ultimate goal in creating a landscape matrix is to maintain habitat at smaller
scales, increase the effectiveness of buffers around these habitats, and provide adequate
connectivity between habitats (Franklin 1993). This view looks at the interactions among
smaller habitats and incorporates them into a greater landscape design, ultimately getting
more out of less.
Forest soil
Much of the terrestrial biosphere resides in the soil and contains more species
diversity than any other terrestrial habitat (Kuyper and Giller 2011), although most of
these soil organism have not been identified (Hawksworth 1991). The foundation of a
forest ecosystem is in the soil and is dominated by import functions of fungal and
bacterial activity (Carey et al. 1995). It is no surprise that an increase in species richness
belowground equates to species richness aboveground. Soil organisms are not just
inhabitants of the soil; they are part of the soil and are responsible for primary production
34
and decomposition, resources which then move on to sustain aboveground plants and
animals (Kuyper and Giller 2011). In particular, mycorrhizal fungi are important
assimilators of net primary productivity in Douglas-fir forests and enhance the
productivity of Douglas-firs (Carey et al. 1995). These ectomycorrhizal fungi comprise
as much as 50-80 percent of the fungal community in forest soils and are fundamental to
the many fungal-based trophic pathways (including those which support mycophagous
protozoans, arthropods, nematodes, and mammals) found in these forest communities
(Allen and Allen 1992). The fungi are also known to form mats which have been found
to cover 25-40 percent of some Douglas-fir forest floors (Kluber et al. 2011). Yet, it is
estimated that only 5 percent of living fungi have been described (Hawksworth 1991).
Any entry into a forest for cutting or thinning can damage these sensitive soil
communities. Even though VDT requires repeated entries, soil disturbance can be
minimized if certain types of equipment, entry time of year, and low impact techniques
are taken into account and used. Cut-to-length harvesting uses and leaves slash behind to
move equipment in order to decrease soil compaction over clear-cutting’s whole-tree
removal (Han et al. 2009). As well, protecting soils through the use of low-pressure tires,
reduced skidding, and less use of scarification will lessen the impact on functioning soil
communities (Thompson and Angelstam 1999). Ground disturbance can also be
minimized through the use of high-lead cable yarding, which also leaves understory
shrubs and tree seedlings intact (Lindh 2008). In these ways, minimal short-term
negative effects on the below ground biotic community are seen. One way to measure
the impacts of logging on forest soils is by looking at arthropod abundances, due to their
fine-scale associations and quick responses to disruption, they are important indicators of
35
soil disturbance (Brokaw and Lent 1999).
Understory herbs and shrubs
Variable-density thinning promotes development of understory herbs and shrubs
and therefore increases plant diversity by increasing light levels and decreasing water
competition, thus contributing to forest vertical structure. In this way, it increases habitat
complexity, food sources (seeds and berries), and the canopy gaps created with VDT
promote large shrub growth, all increasing faunal biocomplexity (Lindh 2008, Wender et
al. 2004). Wender et al. (2004) found that the determining factor for shrubs to flower
was their size, and that larger shrubs are also more common in canopy gaps, an important
component to VDT. They also saw that shrub flowering production was consistently
lower in unthinned stands than in thinned stands. An example of small mammals
response to thinning was shown by Hayes et al. (1995), they observed that abundances of
Townsend’s chipmunks was related to the percentage cover of salal (Gaultheria spp.) in
the Oregon Coast Range. Also several other small mammals are strongly associated with
shrubby habitats such as shrews, voles, and mice (Bunnell et al. 1999). Plant and small
mammal diversity will be preserved if understory vegetation and avoidance of the stemexclusion stage are goals in management (Carey et al. 1995). Carey and Thysell (2000)
found that VDT generally increased the diversity of native shrubs and trees and suggested
that maintaining some minimally disturbed areas could help to conserve any native
species that may be negatively impacted by thinning.
Some shrubs may decline after thinning depending on pre-treatment conditions,
intensity of thinning and amount of ground disturbance, but commonly understory cover
will increase within 3-5 years (Thysell and Carey 2000). Although a study Oregon found
36
that mechanical damage by thinning may be less disruptive to understory species than the
restriction from light resources that the stem-exclusion stage maintains for long periods
of time (Bailey 1996). The plants most likely to recover are the ones with deeper
regenerative structures like tubers, roots and rhizomes (Halpern and Spies 1995). Salal
rhizomes expanded 23.7 percent annually in thinned stands compared to 0 percent in
unthinned stands (Bailey 1996).
Variable-density thinning minimizes opportunities for invasion and establishment
of exotic plant species by lack of large disturbances, though if residual understory shade
tolerant tree conifers and hardwoods are missing, exotics may be able to spread more
easily by over-competing with native vegetation (Thysell and Carey 2000). If other
habitat elements are also not present like CWD, fungi, and mosses for instance, the
potential for VDT to decrease the spread of exotics is even more weak (Thysell and
Carey 2000). In stands that have been conventionally thinned vegetation control of
exotics may be necessary to restore ecological function.
Lindh (2008) found that five years after low-intensity thinning, old-growth
associated herbs and quick release species, both of which have evolved to respond to
canopy gaps, showed increase in flowering. The level of thinning was the most important
source of variation in flowering responses. Ares et al. (2010) studied three sites in
western Oregon eleven years after thinning which showed an increase in diversity of
plant species from varying levels of successional stages without decreasing tree
regeneration or triggering exotic plant invasion. Lindh and Muir (2004) speculated that
thinning probably affected understory composition most by slowing the occurrence of
species that would have increased following canopy closure. Because colonizers are
37
often weedy species of plants that do well in open areas, the heavier the thinning the
more likely you will have widespread persistent weedy species. Yet, if the thinning’s are
small scale, extensive or long-term weed colonization does not appear to be a problem
(Carey et al. 1995). Because some forests may not have residual soil seed banks of
shade-tolerant tree species or woodland herbs, since the later possess limited dispersal
capability, planting or seeding may be needed (Halpern et al. 1999, Lezberg et al. 1999).
In a study on the Olympic Peninsula, Halpern et al. (1999) found that young, closed
canopy forests supported a well-developed and diverse community of buried seeds,
although 30 percent of all species were exotics. They also found that the seeds of many
conifer species were absent. They concluded that when native vegetation and disturbance
regimes have been vastly altered by human activity, native seed banks will be greatly
decreased. Underplanting thinned stands with shade-tolerant tree species may be a
necessary component to developing an understory cohort (Peterson and Anderson 2009).
Snags, dying and diseased trees
New forestry maintains snags, dying and diseased trees because these are all
important to wildlife. VDT keeps snags intact by arranging skip areas around them with
no treatment (Figure 7). Snags can also be created from live trees using a number of
techniques, including topping with a chainsaw, girdling, injection with herbicide, and
inoculation with fungus.
38
Figure 7. A snag to be maintained as a wildlife feature after thinning and
a snag with prevalent woodpecker cavities (fs.fed.us/pnw/olympia/silv/ohds/virtualtrail/tour2/station3 and en.wikipedia.org).
By maintaining or creating snags, new forestry promotes primary cavity
excavators (woodpeckers) which create cavities important for small mammals and birds
as den and nest sites (McComb and Lindenmayer 1999) (Figure 8). This is important,
because if the habitat requirements are not meet for these primary cavities excavators,
secondary bird and mammal cavity nesters will also be eliminated from the site (Bunnell
et al. 1999). In this way, some woodpecker species are considered keystone modifiers
because they excavate cavities which are essential for other species to successfully
produce (Thompson and Angelstam 1999). In most forest types, about 25 to 30 percent
of vertebrate species use cavities for either preproduction or roosting (Bunnell et al.
1999).
39
Figure 8. A nest-web diagram showing which animals prefer which trees, and the
primary and weak excavators responsible for making the cavities (Hunter and
Schmiegelow 2011).
While a range of snag sizes is important, lack of larger sizes appears to be the
limiting factor or wildlife. The larger, the snag the better, snags or limbs less than 10cm
in diameter have little to no value to wildlife (Harrington 2009). In cedar-hemlock
forests of British Columbia, only 14 percent of 18-to 32-cm-diameter trees were
considered “wildlife trees”, compared to 64 percent of trees with diameters greater than
100 cm (Stevenson et al. 2006). The same study also found that “wildlife logs” were
much larger than logs that did not have features characteristic of wildlife use. The
average difference in size was about 20 cm with wildlife logs consistently greater than 50
40
cm in diameter. Bats in Douglas-fir forests also selected larger trees for roosting- the
odds of a snag being used increased by about 20 percent for every 10 cm increase in
diameter (Arnett and Hayes 2009). The main reason for this may be that larger trees
provide a more stable thermal environment due to the thicker insulation of the wood and
bark.
Diameter is probably the main consideration, but height is important too. Some
animals will use a cavity that is practically on the ground, but most prefer to be fairly
high, probably because they are more secure from ground predators. Taller trees may
also provide better access for species that forage some distance from where they nest or
roost; for example, raptors and bats. Carey et al. (1995) found higher abundances of
flying squirrels in areas where there were higher potential dens sites of large trees and
snags. Virtually all studies agree that large snags and cavity trees and larger logs are more
important to keep than small ones. Not only can a greater variety of wildlife use a tall,
girthy snag or a big, long log, but larger snags and logs are likely to last longer than
smaller ones.
Woodpeckers are known for their chisel-like bill, thick skull, and tough neck, but
not all primary excavators are as well equipped to excavate a cavity in a hard snag, and
even many woodpeckers prefer to nest in a well-rotted tree (Bunnell et al. 1999).
Consequently, it is important to have both soft and hard snags. It is known that up to 40
percent of birds in North America are cavity nesters (McComb and Lindenmayer 1999).
In southwest Oregon, an abundance of snags was the primary determinant of a diverse
bird community (Carey et al. 1999).
Having snags in various stages of decay, as well as living trees that can be
41
recruited into the dead wood pool, is necessary to ensure a continuous supply of snags
over time (Figure 9). Although most forest models estimate that when a tree dies it will
remain as a snag, it has actually been found that half of all trees that die will fall within
the first ten years (McComb and Lindenmayer 1999). The most important features that
will determine when and how large a tree will be when it falls are the site productivity
and stocking levels. High quality sites with low stocking levels will produce larger trees
which will remain as snags for longer periods (McComb and Lindenmayer 1999).
Figure 9. Decay stages of snags and logs in the Pacific Northwest (Maser et al. 1979).
Coarse woody debris
The accumulation of dead wood and its subsequent decomposition are essential
forest ecosystem processes. Dead wood is an important element of productive and
diverse forests and provides a mechanism for energy flow into detrital-based ecosystems
(Harmon et al. 1986). Dead wood is also an important structural feature that has many
42
ecological functions, including habitat for organisms, energy flow and nutrient cycling.
The density of a stand plays a major influence on coarse woody debris (CWD)
input to a site and therefore the availability of the site as wildlife habitat. High-density
stands produce many small dead stems early on in stand development, but they are of
small size and are not useful to wildlife promoting a much less array of biotic diversity
(McComb 2003). And because these stands can remain in this stage for decades, it can
take a very long time to become available has habitat. The longer a forest in on the
successional pathway the greater in size the snags it will produce, which will be
beneficial as wildlife habitat.
Clear-cutting and the reduction in downed wood are thought to be the cause of
local reductions in seven salamander species in the Pacific Northwest (Bunnell and
Houde 2010). Birds will be more abundant and have a greater variety of species with
higher amounts of downed wood, presumably due to better foraging and nesting sites
(Bunnell and Houde 2010).
In this regard, new forestry strives to either maintain or promote CWD on the
ground (Figure 10). This is done by either leaving slash or logs after harvest, or by
retaining trees which will fall to the ground and become downed wood. These structures
are vital to the biotic community as hiding shelter, food sources, and dispersal pathways.
CWD also creates a moist substrate for many plants and fungi to occupy (Bunnell and
Houde 2010). It is important to retain all levels of decay classes of downed wood if we
are to sustain a wide range of biodiversity.
43
Figure 10. Examples of coarse woody debris on the forest floor (tru.ca and gov.ns.ca).
In many forests logs are a major site for tree regeneration, and a line of small trees
growing on a well-rotted log called a “nurse log”, is common (Harmon and Franklin
1989). One of the reasons seeds find logs a favorable place to germinate is because these
are good sites for forming a symbiotic relationship with mycorrhizae fungi.
This could be particularly important to forest regeneration on a cleared site. For example,
Townsend’s chipmunks regularly venture into forest openings from the surrounding
cover and undoubtedly leave behind sport-bearing feces from which new mycorrhizal
partnerships can develop.
Small mammal abundances
VDT increases food supply buy increasing understory fruiting and cover available
for small mammals and therefore increases rodent populations. It is assumed that the
increase in pray base also benefits a wide range of mammalian and avian predators.
Suzuki and Hayes (2003) reported that thinning appears to increase the abundances of
small mammals in both the short-term and long-term. It is claimed that squirrels are good
indicators of ecological productivity in the Pacific Northwest because they specialize in
eating the fruiting bodies of plants and fungi and that their numbers correlate to the
44
carrying capacity of their predators, primarily raptor, owls, and mustelids (Carey et al.
1999). Wilson and Carey (1996) captured more short-tailed weasels in thinned areas than
unthinned areas of the Olympic Peninsula. This was hypothesized to be because their
primary small mammal prey was more abundant in thinned areas with high understory
development. Subsequently, they found that long-tailed weasels preferred more closedcanopy sites with little understory and greater amounts of CWD which would be have a
greater abundance of northern flying squirrels.
Although timber harvest profoundly influences the composition of small mammal
communities, it has been shown that once the forest develops certain structural features
adequate for certain species to survive, any additional age does not significantly improve
habitat conditions (Hallett et al. 2003). Carey (1995) reported that a prevalence of certain
flowering plants was highly correlated to northern flying squirrels and Townsend’s
chipmunks on the Olympic Peninsula. These plants most likely have a high value as
food, both in fruits and in mycorrhizal linkages promoting truffle production, and as
protection from predators. An earlier study in the southern coast ranges of Washington
found that densities were greater, diets more diverse and movements less for northern
flying squirrels in older forests than in young forests (Carey 1995), all of these being
important factors to their survival.
Leaving slash-piles can also be beneficial to small mammals, where an increase
was seen of small mammals around slash piles compared to open clear-cuts (Bunnell and
Houde 2010). In southwest Oregon, northern flying squirrels were highly correlated
with CWD and this abundance may be due to the increase in truffles from the retention of
the CWD (Carey et al. 2002). Yet, in the Olympic Peninsula, flying squirrels were more
45
highly correlated with understory development (Carey 1995). Snags are also a very
important habitat feature for flying squirrels as den sites, since they are know the change
dens every two weeks, with about 25 percent changing nests any given week (Carey et al.
1995). Even though, Carey et al. (1999) found that of all the habitat elements, CWD
proved to be the best indicator for the carrying capacity of northern flying squirrels and
Townsend’s chipmunk. They also found that canopy stratification was the single best
descriptor of chipmunk habitat. To solidify this finding, Carey and Harrington (2001)
found local extirpations of northern flying squirrels and Townsend’s chipmunk in youngmanaged stands, which may result in small mammal communities being non-supportive
of predator populations.
Recent studies have shown a reduction in northern flying squirrel abundances
following thinning and hypothesized that this may be driven by increased susceptibility to
predation created by removal of critical mid-story cover. One study confirmed that
abundances are lower in second-growth or partially harvested stands and revealed that
there may be a snag density threshold below which managed forest are unable to sustain
these squirrels’ populations (Holloway and Smith 2011). Because northern flying
squirrels are an important prey for the northern spotted owl, maintaining their abundance
levels is a high priority. It is important then to vary the levels of thinning and create
areas of skips to ensure that north flying squirrels have adequate hiding refuge. The longterm benefits of some thinning treatments may be positive for northern flying squirrels,
but may not be realized for several decades or more.
46
Belowground fungi
Because of the important role mycorrhizal fungi play in belowground food webs
and in the productivity of a forest, any alteration of the fungal community can negatively
impact the succession of the forest (Figure 11). Therefore, it is wise to manage by
looking at the needs of this community first.
Figure 11. Truffles, the friuting bodies of belowground fungi imporant to tree
productivity and the diets of some small mammals (source unknown and
chrismaser.com).
Since the Pacific Northwest is believed to have one of the largest assemblages of
mycorrhizal host tree species in the world (Trappe et al. 2009), and mycorrhizal fungi
need the energy from their tree host to survive, using new forestry techniques of VDT,
and which also retains CWD, allows this relationship to continue and therefore truffles to
be produced. Luoma et al. (2004) found that while all harvested treatments in their study
showed some reduction initially in sporocarp production compared to the control, the 75
and 40 percent basal area retention generally maintain higher levels of ectomycorrhizal
sporocarp biomass over the 15 percent basal area retention plot. They inferred that an
important factor in maintaining mycorrhizal fungi is in retaining a certain level of trees
while thinning enough to increase moisture to the forest floor. They found that certain
47
fungi species respond to disturbance at varying levels, reinforcing the benefit VDT can
have on diversity of belowground fungi.
There is a thread commonly see in multiple studies showing that truffles increase
with the increase in downed wood and also increase in natural-mature and old-growth
stands compared to young-managed stands. Maintaining mycorrhizal development
through CWD is vital because it is an important food source for 45 identified animals
(Molina and Smith 2009). One study found that 60 percent of the truffle biomass was
consumed by mycophagous small mammals (Colgan et al. 1999). Cazares et al. (1999)
observed that truffle sporocarps were significantly more abundant in the diets of the
northern flying squirrel, Douglas’ squirrel, and Townsend’s chipmunk over mushroom
sporocarps which were equality readily available. They made the suggestion that truffles
are the preferred food for these three small mammals in Douglas-fir forests. Jacobs and
Luoma (2008) observed high diversity in fungal genera in the diets of five small
mammals they studied, indicating the importance of mycophagy as a dispersal agent for
truffle fungi. Additionally, North and Greenberg (1998) saw that truffles made up 60
percent of the dietary needs of voles, pocket gophers, chipmunks and squirrels in their
study. Yet, because truffles are relatively low in nutrients, it is therefore important for
mammalian mycophagists to have a diet of multiple truffle species (Carey et al. 1999,
Carey et al. 2002).
If we are to restore diversity to forests, thinning is a priority. Unfortunately, as a
result from thinning, below ground fungal abundances may temporarily decline due to
soil microbial community alterations. Even though, if there are understory plants (i.e.
salal) present which have a symbiotic relationship with mycorrhizae, it is believed that
48
they can help fungi recover (Carey et al. 1995, Carey et al. 1999). Carey et al. (2002)
found that if some legacies from the previous stand (i.e. snags, logs, or soil communities
particularly from a previous old-growth stand) are retained after thinning, management
may actually increase truffle species diversity. Maintaining belowground fungal
communities could be a key component of an ecosystem’s resilience by being important
pathways for seedlings and other plants in the uptake of nutrients vital to their growth
(Luoma et al. 2006).
Thinning seems to initially change truffle species dominance, since different
species have adapted to different microclimate (Luoma et al. 2003). Yet, Colgan et al.
(1999) found that fungal diversity seemed to increase in areas of forest which had been
lightly thinned. Gomez et al. (2005) concluded that the density of belowground fungi
was positively associated with the proximity to large trees, whose growth is encouraged
through thinning. Belowground fungi also benefit from new forestry, by not having the
common conventional forestry practice of slash-burning being applied. These fires not
only remove the necessary organic layer on the forest floor for fungi to thrive, but the
high heat conditions harm the fungi which are not adapted to frequent fires in the wet
climate west of the Cascades (North and Greenberg 1998).
4.2 Interview responses
I consulted with three professionals in the field of forest restoration, forest
ecology, and wildlife biology and asked them each the same questions: 1) Is creating
complexity in forests working to restore biodiversity? 2) What are, if any, indications of
success? 3) How useful are models in planning future structural diversity?
49
In response to whether creating complexity in forests is working to restore
biodiversity and are there indications of success, Todd Wilson, from the Pacific
Northwest Research Station in Olympia Washington, answered that for the most part at
least all of the biotic measures that they have studied, including plants, small mammals,
birds, and amphibians have increased after complexity was increased. The exception is
arboreal rodents, including flying squirrels and red tree voles, which are primary prey for
northern spotted owls, which decline after thinning. He thinks that eventually, conditions
will likely favor arboreal rodents, but it could take several decades or more. Increases in
species richness and/or abundance (or neutral response) in the first decade or so after
thinning by mice, voles, shrews, terrestrial salamanders, understory plants, and resident
and neotropical songbirds is seen and a development of the start of a shade-tolerant midstory tree layer on some sites is seen.
Constance Harrington, also from the Pacific Northwest Research Station,
responded to whether creating complexity in forests is working to restore biodiversity and
are there indications of success by saying that she believes adding complexity to young
stands with simple stand structures will help accelerate the development of stand
structures and plant and animal communities associated with late-successional
forests. She thinks using the term "restore" can be tricky as it implies restoring
something to a previous condition when we don't necessarily know what conditions
(species assemblages and stand structures) were previously present. For example, much
of the Puget Sound lowlands were in prairies and oak savannas when the European
settlers arrived. Are we interested in restoring those conditions? For her, biodiversity in
the simplest sense means the number of species present. But more species isn't
50
necessarily good if the species are not the ones you are interested in. Most projects
designed to add complexity are fairly recent in scope so it is premature to judge them.
Joe Buchanan, from the Washington Department of Fish and Wildlife in Olympia,
when asked whether creating complexity in forests is working to restore biodiversity and
are there indications of success answered that with the exception of sedentary species,
most forest wildlife respond to successional changes, and most species are more abundant
in one forest age class than in others. This is an indication that many species recognize
differences in forest structure. It is logical that various species would then respond to
forests recruited, modified or restored by humans. Joe explained to me that the
Washington Department of Natural Resources began a snag creation program about 1012 years ago on one of their state forests and they invited him to participate in a field trip
to some of their study sites 4-5 years ago. These snags were created by a variety of
methods applied to living trees (topping, girdling, etc.), and this work occurred in
managed forest patches that otherwise lacked snags. The response by pileated
woodpeckers to these created snags was remarkable. He also said that A.J. Kroll
(Weyerhaeuser) has been working on a project where bird response to created snags in
clear-cuts was monitored. In addition, he has seen several examples where northern
spotted owls have subsequently occupied and nested in forests “sloppily” harvested in the
1930s (retaining legacy trees and large logs in the harvest unit because it was
inconvenient to remove back then) in western Washington. Responses likely differ from
one species to the next and also as a function of subtle differences in site
conditions. Indications of success might include measures such as increases in
abundance, nest success, survival rates, etc. Changes in abundance can sometimes be
51
misleading, especially if the individuals involved are not breeding. Increases in species
richness might be an appropriate indicator, but that depends on the composition of change
in richness (e.g. an increase in generalist or exotic species might not be a very meaningful
indicator); an increase in the richness of the cavity-user guild in a conifer forest might be
an excellent early indicator of a successful snag creation project. The best measures of
success will be those that involve estimates of productivity or survival, because this
information would indicate that not only are the species present, but they are also
breeding (and measures of this will range from low to high reproductive output) or
experiencing increased survival rates.
When asked how useful are models in planning future structural diversity, Todd
Wilson responded that in theory, they could be very useful, but much of what went into
any such model would probably still rely heavily on expert opinion rather than data. It
would be critical to include any site-specific conditions that could affect development of
structural diversity (e.g., presence of an aggressive understory like salal that could
impede tree regeneration, slope and aspect, presence of a shade-tolerant seed source for
regeneration, type and pattern of thinning, etc.), saying that it gets complex pretty
fast. Also, natural stochastic events at a variety of spatial and temporal scales could
greatly alter stand trajectories and structural complexity (everything from a localized
pocket of disease to a catastrophic, stand-replacing windstorm) making long-term
predictability difficult.
Connie stated when ask about the usefulness of forest models in predicting
structural diversity that models predicting tree growth and mortality have traditionally
52
done fairly well in stands with simple structure and not so well in more complex
stands. She has found that the models generally under-predict growth and over-predicts
survival of trees in the understory. She says that several people are working to improve
the models. For example, in her office they have been working this year on better
predicting growth of small trees (regenerations) in the USDA Forest Service Forest
Vegetation Simulator (FVS) so the models better reflect what they see in their field plots
in terms of growth and mortality. Lastly, she mentioned that models predicting aspects of
stand structure such as branch diameter and crown length probably also need revision for
use in complex stand conditions.
Joe thinks that models are very important tools that can help guide planning for
future forest complexity and diversity. Though he feels that some models are better than
others; the complexity of the community or ecosystem and the level of uncertainty
associated with species habitat associations or vegetation growth have the potential to
influence the model. However, he said that even those limitations can be addressed in
modeling efforts if there is a desire to understand the potential effects of various
information gaps or hypothesized interactions. He also feels that modeling sophistication
is increasing and thinks models are becoming increasingly important in wildlife work.
53
5. Discussion
Themes in literature
One of the themes that emerges from the literature is that old-growth likely
develops from a variety of initial conditions along a variety of pathways toward a variety
of old-growth endpoints. It is also clear that today’s dense young plantations exhibit
unprecedented uniformity of initial conditions, which could end up limiting both the
diversity of pathways and endpoints. While recognizing that disturbances will continue to
play a role in diversifying and resetting stands, there seems to be a consensus emerging
that some form of variable-density thinning can help diversify some young forests in
order to reintroduce more diversity. Another important area of agreement seems to be
that we do not fully understand how to create old-growth, nor is there one right way to
achieve restoration in dense young plantations. There are a variety of tools that should be
applied in a variety of ways at a variety of scales, and possibly, some areas, even dense
young plantations, should be left unthinned and undisturbed.
The available information indicates that thinning causes positive, negative,
neutral, and unknown consequences. It will be important to consider the costs of both
action and inaction (i.e., thinning and not thinning). Active management will realize
some ecological benefits while causing some unavoidable short-term adverse
consequences. Passive management will certainly avoid some negative consequences
that may be caused by thinning, but it will also cause some of its own negative
consequences (e.g., extended periods of competitive exclusion, unstable height/diameter
ratios) and forgo other benefits.
54
There is a growing body of evidence that using VDT in young plantations can
enhance development of many features associated with late-successional forests such as
large trees, well developed tree crowns and canopies, patchy mosaics of a variety of
habitat types, tree size diversity, tree species diversity, understory vegetation
development, wildlife habitat development, and large woody debris. There are also
numerous potential adverse impacts associated with thinning young forests on the
Westside of the Cascades, including adverse effects on soil, water quality, aesthetics,
invasive species, some wildlife species, and an increase in the risk of damage from wind.
Yet, the adverse consequences of thinning will be less intense than the effects of
traditional clear-cutting and the effects will usually be short-lived.
Management considerations for snags and coarse woody debris
Obviously with species using dead, dying or diseases trees representing a range of
organism sizes from microbes, mites, salamanders, fishers, and bears, managing the
spatial distribution of logs must consider a wide range of home range sizes. Ideally, the
habitat requirements of each species must be considered when deciding where logs
should be retained and what log characteristics are sufficient to meet their needs. If this
is not possible, than the desired size class distribution for the suite of species being
managed in the stand or landscape should be determined by the species requiring the
largest piece size. Alternately, a manager can assess functional relationships between
animals and CWD and manage for these conditions as part of a desired future condition
(McComb and Lindenmayer 1999). Natural and created snags are continually being lost
and degraded through disturbance and decay. Therefore, snag recruitment is an on-going
process requiring forethought and planning for the retention of green trees for future
55
snags.
Factors to consider when applying variable-density management
Most variable treatments are inherently more expensive to apply than uniform
ones, primarily because they require covering more ground to get the same amount of
work done. Therefore when starting a new type of treatment, managers with a limited
information base to work with may be tempted to use the same approach and treatment
each time, yet prescriptions will be most successful if they are varied. Additionally,
although many of the same general types of variables are sampled at multiple LSSEs,
very few specific values are sampled identically at multiple sites. This may hinder the
extraction of general patterns, making results difficult to compare or separate from
treatments applied at each site, therefore giving inconsistent results. This could be
improved if all studies were done with more uniformity between variables measured.
Also another reoccurring problem is in the time scale of these experiments; we have to
wait a long time to see results and maintaining their relevance could be a challenge with
changing societal values.
Variable-density thinning will be an important tool for forest manager charged
with promoting or creating old-growth structure within all or part of a landscape. But
like any tool, VDT will not be appropriate for all stands at all times. Dense stands of
young trees are a natural part of the landscape and should be maintained at appropriate
levels for those plants and animal species dependent of that type of habitat. Stands with
open canopies and existing vigorous shrub growth may not respond to thinning in terms
of confer regeneration without additional management (e.g. vegetation control or
planting).
56
Overstory trees with high height to diameter ratios may be susceptible to
extensive windthrow shortly after treatment with thinning and increasing gaps in forest
stands. Top breakage may also be greater in widely-spaced trees. By not thinning near
roads or on ridges where trees may be more susceptible to high winds, wind throw can be
minimized. In addition, creating too large of gaps may encourage colonization, growth,
or retention of exotic plants. These sized gaps or overly thinned forests also may
negatively impact the belowground fungi and northern flying squirrels relationships
causing populations declines in the short-term. It is important to maintain truffles and
flying squirrels in the landscape for the necessity of mycorrhizal fungi for tree growth
and northern flying squirrel as prey for the northern spotted owl, an endangered species.
It is unknown whether northern flying squirrel populations decline in thinned stands due
to their greater predation or if they are negatively influenced by the initial decrease in
truffles. However, by varying the intensity and arrangement of thinning’s, and how they
are interspersed with other treatments, for instance, northern flying squirrel and truffle
abundances may be less impacted. A forest may need a re-thinning treatment if the
original thinning was minimal due to concerns to belowground fungi and flying squirrel
populations, as a way to decrease these negative impacts. A single thinning may also not
be enough for a very young stand to achieve old-growth characteristics, here again a rethinning may be advisable (Tappeiner 2009).
An existing landscape can be manipulated at the stand-level to provide more oldgrowth-type habitat over time and across a landscape. This type of active management is
a preferred alternative to strict preservation given: 1) reduced time to create old-growthtype structures in young forested landscapes lacking such habitat; 2) an ability to
57
emphasize the creation of specific features lacking in a landscape (e.g. large snags); and
3) a lack of evidence showing negative long-term impacts on plant and animal species
composition and productivity following thinning.
Limitations of forestry models
Most growth and yield models were based on uniform stands with one or a few
species and a limited number and range of treatments. Most of these models assume the
treatments are applied uniformly and are not spatially explicit. Thus, existing models
may only provide general trends for spatially complex treatments like VDT. Existing
models may not reflect the wide range of growing conditions in a more complex stand by
under-predicting the potential growth influence resulting from internal edge effects
(Roberts and Harrington 2008). Gould and Harrington (2011) found that some models
under-predict growth and over-predict survival of understory trees. In response, they are
working toward making models better reflect growth rates they are seeing in their own
research plots in the hope that models will show greater variability than seen in previous
models. Forest models have been very good at predicting growth of uniform stands, but
with new forestry’s goal of complex un-even aged stands, this will be more challenging
to model.
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6. Conclusion
Given that millions of acres of our federal forest lands are covered with uniform
dense plantations with relatively low habitat value, the challenge we face is how to
prioritize restoration actions and continue to learn so that we can (a) increase the benefits
of biodiversity from thinning, (b) find ways to avoid, minimize, and mitigate the adverse
impacts of thinning, and (c) acknowledge and manage the risks and inherent uncertainty
involved in the choices about how to manage young stands.
Maintaining truffle productivity and diversity is critical to the maintenance of
small mammal populations because many small mammals use truffles as a food source,
with some using them as a major food source. If populations of small mammals are
abundant, then there is an adequate prey base for maintaining abundances and diversity of
predators. This is particularly important in maintaining the northern flying squirrel and
northern spotted owl species relationship. Mycorrhizal fungi are also important in
promoting tree productivity and complex soil communities which are at the core of the
forest food web. By using the techniques of VDT while retaining CWD, we can promote
this vital keystone complex relationship, which is at the center for maintaining biological
diversity in forested ecosystems.
The literature reviewed for this analysis points to the possibility that late-seral
communities could be restored with minimal intervention with a high probability of
success. One potential factor that stands out for success is the high degree of resiliency
in soil food webs, fungi, and vascular plant diversity in forest ecosystems. This
resiliency in the forest community may be due to the functional connections and
pathways among plant, microbial, and small mammals communities which we are only
59
beginning to shed a light on. By providing an adequate prey base for predators and
creating a complex forest structure we can go a long way in improving habitat for
multiple species. Continued research is needed to better understand how these
interactions are taking place and the role that other species relationships may be playing
in the ecosystem which we may have overlooked. More research is also needed to
improve our understanding of the functional relationships of soil biota, which is at the
core of ecosystem health, in order to better address forestry impacts on soil biodiversity.
In the meantime, forest managers should make the least possible entrances into a forest to
minimize soil disturbance.
Models are limited in how they can help plan for future complex forest structure
and should not be relied upon exclusively since they do not predict complex forest
structure on a landscape scale. More time for a forest to develop that has had VDT
treatments applied and the subsequent research from them should be our guide. In time,
models will become more accurate indicators of forest successional changes and forest
managers will more easily be able to meet a wide range of objectives through the design and
implementation of increased biological diversity on public lands.
Managing forests as dynamic ecosystems and landscapes will help meet the
multiple goals currently desired such as; reserves for biological diversity, connectivity on
a landscape scale for wildlife, human needs for wood products, a clean environment, and
recreational experiences in nature. Creating complex structure and composition leads to
complexity in forest function that translates to a high carrying capacity for diverse
animals, high productivity for plants, effective regulation of nutrients and water cycling,
and healthy, resilient forests.
60
While the management of forests to maintain or produce older-forest conditions
holds great promise, it remains a grand experiment, or more accurately, a series of grand
experiments, the result which will not be clear for many decades or, in some cases
centuries. Yet, it is my hope that as we move forward in forest management we use the
functions, relationships and linkages within a forest as our guides in maintaining and
restoring forest ecosystem health.
Recommendations for future forest management
1) Retain diversity of trees sizes starting with the largest trees, and then implementing
some smaller trees in all age-size classes.
2) Reduce tree densities to increase rates of diameter growth to produce large-diameter
trees and encourage development of large and deep crowns. Such trees are more
resistant to windthrow and remain standing longer after they are dead, providing
habitat to snag-associated species.
3) Retain and protect under-represented conifer and non-conifer trees and shrubs that
would have been eliminated under intensive Douglas-fir timber production. Plant
shade-tolerant tree species when seed sources of desired species are lacking.
4) Vary densities and frequency of entries by planning a variety of approaches within a
given area so that treatments will mimic natural disturbances seen in forests and
better meet the needs of wildlife.
5) Be creative in the use of moderate sized skips and small gaps to establish diversity
and complexity both within and between stands.
61
6) Retain or create abundant snags and course woody debris both distributed and in
clumps so that thinning mimics natural disturbance. Retain important features of
wildlife trees such as hollows, forked tops, broken tops, leaning trees, etc.
7) Thin heavy enough to stimulate development and diversity of understory vegetation,
but don’t thin too heavy, which may deplete belowground fungi and northern flying
squirrel populations.
8) Leave some cut trees and a portion of the tops in the forest to retain nutrients on site.
9) Avoid the damaging effects of soil compaction by making the least possible entries
into a stand and by using less intensity timber harvest practices.
10) Take proactive steps to avoid the spread of weeds. Wash weed seeds off of
equipment before it enters the forest.
11) Avoid road construction. Where road building is necessary, ensure that restoration
benefits far outweigh the adverse impacts of the road.
12) Management should ultimately focus on the ecological processes that lead to the
development of beneficial forest structures, rather than solely on the structures
themselves.
62
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Appendices
Appendix A.
Glossary of terms
Adaptive management: A formal process for a) evaluating the current resource status, b)
evaluating the effectiveness of rules and guidance necessary to meet the goals and
objectives for the protection, maintenance, and enhancement of resources, c) making any
necessary adjustments to management practices, and d) requiring mitigation, where
necessary to achieve resource objectives.
Basal area (BA): The area of a cross-section of a tree, including bark, at breast height.
Basal area of a forest stand is the sum of the basal areas of all individual trees in the
stand, usually reported as square feet per acre.
Biodiversity: The variety, distribution and abundance of living organisms in an
ecosystem. Maintaining biodiversity is believed to promote stability, sustainability and
resilience of ecosystems.
Canopy: The continuous cover of branches and foliage formed collectively by the
crowns of adjacent trees and other woody growth.
Clear-cut: A harvest method in which all or almost all of the trees are removed in one
cutting.
Coarse woody debris: Large pieces of wood on the ground include logs, pieces of logs,
and large chunks of wood; provides habitat complexity.
Competitive exclusion stage: Also called stem exclusion stage. This predominantly
developmental stage lacks the very large trees and multiple canopy layers found in the
later stages of stand development, and are usually deficient of large snags and significant
amounts of down wood. Within competitive exclusion developmental stages, understory
vegetation is generally severely depressed.
Crown ratio or live-crown ratio: The length of a tree's crown divided by the total
height of the tree.
Diameter and breast height (dbh): The diameter of a tree measured 4.5 feet above the
ground on the uphill side of the tree.
Ecosystem: An ecosystem is a biological system consisting of all the living organisms or
biotic components in a particular area and the nonliving or abiotic components with
which the organisms interact, such as air, mineral soil, water, and sunlight.
73
Ecosystem function: Ecosystem functions are a subset of ecological processes and
ecosystem structures. Each function is the result of the natural processes of the total
ecological sub-system of which it is a part. Natural processes, in turn, are the result of
complex interactions between biotic (living organisms) and abiotic (chemical and
physical) components of ecosystems through the universal driving forces of matter and
energy.
Endangered species: A formal classification by federal and/or state agencies defining a
population of organisms which is at risk of becoming extinct because it is either few in
numbers, or threatened by changing environmental or predation parameters.
Endangered Species Act (ESA): The federal Endangered Species Act of 1973 sets up
processes by which plant and animal species can be designated as threatened or
endangered. Once species are listed the act provides for the development of recovery
plans for these species, including conserving the ecosystems on which listed species
depend.
Even-aged: A system of forest management in which stands are produced or maintained
with relatively minor differences in age.
Forest: A biological community of plants and animals that is dominated by trees and
other woody plants.
Forest fragmentation: The splitting of forestlands into smaller, detached areas as a
result of road building, farming, suburban development, and other activities. This can
isolate wildlife populations, and may result in forested areas too small to meet the habitat
requirements of some species. Wildlife corridors help remedy this problem.
Forest Vegetation Simulator (FVS): Is an individual-tree, distance-independent,
growth and yield model which has been calibrated for specific geographic areas of the
United States. FVS can simulate a wide range of silvicultural treatments for most major
forest tree species, forest types, and stand conditions.
Forestry: The science of establishing, cultivating, and managing forests and their
attendant resources.
Habitat diversity: The variety of wildlife habitat features and types in a specific area.
Habitat diversity takes many forms: the variety of plants and animals on a site; structural
diversity or the vertical arrangement of vegetation from canopy to forest floor; horizontal
diversity or the distribution of habitat types across the landscape; and temporal diversity
or habitat changes over time. Generally, areas with substantial habitat diversity will
support more wildlife species than areas with less habitat diversity.
74
Hypha (plural hyphae): Any of the threadlike filaments forming the mycelium of a
fungus.
Indicator species: An organism that occurs only in areas with specific environmental
conditions. Because of their narrow ecological tolerance, the presence or absence of
these species on a site is a good indicator of environmental conditions. Foresters often
use the distribution of indicator understory plants to get a quick estimate of site
conditions, for example, drainage and fertility. Biologists may use indicator species to
evaluate the health of an ecosystem.
Keystone species: An organism that has a greater role in maintaining ecosystem
function than would be predicted based on its abundance. Concept named after the
wedge-shaped keystone that holds together the parts of an arch. If the keystone is
removed, the arch collapses.
Keystone complex: A more complicated idea that recognizes a number of components
that are building blocks of an ecosystem and supporters of it processes.
Large-scale silviculture experiments: Silviculture experiments conducted at operational
scales.
Late-successional forest: A mature and or old-growth forest stand, also called late seralstage forest. Typical characteristics are moderate to high canopy closure, a multi-layer,
multi-species canopy dominated by large overstory trees, many large snags, and abundant
large woody debris (such as fallen trees) on the ground. Typically, stands 80-120 years
old are entering this stage.
Late-successional reserve (LSR): An area of forest where the management objective is
to protect and enhance conditions of late successional and old-growth forest ecosystems.
Mycelium (plural mycelia): The vegetative part of a fungus, consisting of a mass of
branching, thread-like hyphae.
Mycorrhizae: The symbiotic association of beneficial fungi with the small roots of some
plants, including pines. Mycorrhizae may improve the water and nutrient uptake of trees.
Niche: The unique environment or set of ecological conditions in which a specific plant
or animal species occurs, and the function the organism serves within that ecosystem.
Niche differentiation: Refers to the process by which natural selection drives competing
species into different patterns of resource use or different niches. This process allows
two species to partition certain resources so that one species does not out-compete the
75
other; thus, coexistence is obtained through the differentiation of their realized ecological
niches.
Old-growth forest: A forest that is the successional stage after maturity, which may or
may not include climax old-growth species; the final seral stage. Typically, it contains
trees older than 200 years. Stands containing Douglas-fir older than 160 years which are
past full maturity and starting to deteriorate may be classified as old-growth.
Overstory: The trees that form the upper canopy layer in a forest that has more than one
story.
Pre-commercial thinning: Cutting trees from a young stand so that the remaining trees
will have more room to grow to marketable size. Trees cut in a pre-commercial thinning
have no commercial value and normally none of the felled trees are removed for
utilization. The primary intent is to improve growth potential for the trees left after
thinning.
Sensitive species: A state designation. A state sensitive species are species native to
Washington that are vulnerable of declining, and are likely to become endangered or
threatened in a significant portion of their ranges within the state without cooperative
management or the removal of threats.
Seral stage: One of the developmental stage that succeed each other as an ecosystem
changes over time; specifically the stage of ecological succession as a forest develops.
Shade tolerance: The ability of a tree species to survive in relatively low light
conditions.
Silviculture: The theory and practice of controlling the establishment, composition,
growth, and quality of forest stands in order to achieve management objectives.
Site Density Index (SDI): A measure of the stocking of a stand of trees based on the
number of trees per unit area and diameter at breast height of the tree of average basal
area. It may also be defined as the degree of crowding within stocked areas, using
various growing space ratios based on crown length or diameter, tree height or diameter,
and spacing. A SDI of 600 is the stand density limit. This is equal to a basal area factor
(BAF) of 400 or a relative density (RD) of 100.
Soil compaction: Compression of the soil resulting in: reduced soil pore space (the
spaces between soil particles); decreased movement of water and air into and within the
soil; decreased soil water storage; and increased surface runoff and erosion. The use of
heavy machinery during forest operations contributes to soil compaction.
Stand: A group of trees that possess sufficient uniformity in composition, structure, age,
spatial arrangement, or condition to distinguish them from adjacent groups.
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Structure: The presence, size, and physical arrangement of vegetation in a stand.
Vertical structure refers to the variety of plant heights, from the canopy to the forest
floor. Horizontal structure refers to the types, sizes, and distribution of trees and other
plants across the land surface. Forestlands with substantial structural diversity provide a
variety of niches for different wildlife species.
Symbiosis: The intimate association of two kinds of organisms.
Threatened species: A formal classification by federal and/or state agencies defining
any species (including animals, plants, fungi, etc.) which are vulnerable to endangerment
in the near future.
Understory: Forest undergrowth; the lowest canopy layer of trees and woody species.
Uneven-aged: Forests composed of trees that differ markedly in age; may be a result of
partial cutting practices or natural disturbance.
Wildlife habitat: The arrangement of food, water, cover, and space required to meet the
biological needs of an animal. Different wildlife species have different requirements, and
these requirements vary over the course of a year. Also, different plants provide fruit and
food in different seasons. Maintaining a variety of habitats generally benefits wildlife.
Wildlife tree: Includes large live trees, snags, cavities, and downed logs that provide
forest-habitat structures for wildlife.
Windthrow: A tree pushed over by wind. Most common among shallow-rooted species
on sites with shallow soils, and in areas where cutting has reduced the density of a stand,
exposing residual trees to the wind and depriving them of the accustomed support of
neighboring trees.
77
Appendix B.
Forest Vegetation Simulator model results
I analyzed two simulations from the USDA Forest Service Forest Vegetation
Simulator. One was an unmanaged stand and one a managed stand. Each had 300 TPA
planted on bare ground in 2010 with the model extending 180 years out. The thinning’s
were determined when the RD approached 40 to 60. The managed site was thinned once
in 2060 to 217 TPA, then again in 2130 to 58 TPA. At the end of the 180 year period, the
managed site had 43 TPA, a BA of 282, and a SDI of 316. The unmanaged stand with no
treatments after 180 years had 72 TPA, 349 BA, and SDI of 415.
The managed stand had fewer trees per acre, but the trees were overall larger in
diameter and had greater crowns. These trees, which grew larger, later provided larger
snags and downed wood than the unmanaged stand, although the managed stand was still
limited in the size of the downed wood. Snags were also larger overall in the managed
stand compared to the unmanaged stand. The unmanaged stand had a significantly higher
number of small snags in the >= 0-12” category then the managed stand.
The merchantable board feet generated from the managed stand with the first
thinning was $7800 per acre and $12,000 per acre in the second thinning. Not only is
there an economic benefit to thinning, but an ecological one, as some trees could be left
on site as downed wood. In addition, the income generated from the timber thinning
could also go towards other habitat enhancements or species presence monitoring.
Models are helpful, but are designed to grow trees, not habitat. The benefit to
managed stands seen in the models is that we can purposefully create snags by having the
flexibility to decide which trees would best serve as snags to enhance wildlife habitat,
and get to them early.
By using the Forest Vegetation Simulator, we can see that active management has
a role to play in shaping forests. Thinning allows us to create habitat as we go. Forest
models can give us an insight into what the possible outcomes may be in a forest when it
is managed or unmanaged, therefore making better management decision beforehand.
78
Summary statistics for managed stand
79
Summary statistics for unmanaged stand
80
Beginning of cyle. Forest after 20 years of growth from bare ground planting.
81
Forest after first thinning, 50 years after planting on bare ground. 92 trees were removed. BA
started at 253, was redued to 182. TPA went from 217 to 116. SDI went from 399 to 275.
82
Forest without management after 50 years since planting on bareground. 217 TPA, BA of 253,
and a SDI of 399.
83
Height to crown ratio in managed stand 140 years after bare ground planting and two thinning’s.
This diagram demonstrates a low height to crown ratio, which tells us that the trees have adequate
space and light resources to grow, increasing their crowns and diameters.
84
Height to crown ratio in unmanaged stand 130 years after planting on bare ground. This graph
demontrates a high height to crown ratio, which is reflected in the small crown volume compared
to the height of the trees. This tells us that the trees are overcrowded and competing for light
resources.
85
Hard Snags in Managed Stand
70
60
Snags/Acre
50
>=0"
40
>=12"
30
>=18"
>=24"
20
>=30"
10
>=36"
0
Year
Total number and size of hard snags in managed stand after 180 years.
Soft Snags in Managed Stand
70
60
Snags/Acre
50
>=0"
40
>=12"
30
>=18"
>=24"
20
>=30"
10
>=36"
0
Year
Total number and size of soft snags in managed stand after 180 years.
86
Hard Snags in Unmanaged Stand
70
60
Snags/Acre
50
>=0"
40
>=12"
30
>=18"
20
>=24"
>=30"
10
>=36"
0
Year
Total number and size of hard snags in unmanaged stand after 180 years.
Soft Snags in Unmanaged Stand
70
60
Snags/Acre
50
>=0"
40
>=12"
30
>=18'
20
>=24"
>=30"
10
>=36"
0
Year
Total number and size of soft snags in unmanaged stand after 180 years.
87
Snags in Managed Stand
70
60
Snags/Acre
50
>=0"
40
>=12"
>=18"
30
>=24"
20
>=30"
>=36"
10
0
Year
Total number and size of all snags in managed stand after 180 years.
Snags in Unmanaged Stand
70
60
Snags/Acre
50
>=0"
40
>=12"
>=18"
30
>=24"
20
>=30"
>=36"
10
0
Year
Total number and size of all snags in unmanaged stand after 180 years.
88
