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ASSEMBLAGES OF BIRD SPECIES IN WESTERN CONIFEROUS OLD-GROWTH FORESTSl/
R. William Mannan
Cooperative Wildlife Research Unit
Oregon State University
Corvallis, Oregon 97331
ABSTRACT
A review of available literature revealed that bird species richness and total bird density varied considerably among assemblages
of bird species in various forest types of old-growth timber.
However, proportions of species and individuals in foraging and
nesting guilds were similar. Among foraging guilds, the number of
species and individuals in the "tree-foliage-searching" and
"ground-brush-foraging" categories were most abundant. Among
nesting guilds, coniferous-tree-nesting birds and hole-nesting
birds comprised the greatest proportions of species and individuals. Changes in vegetation structure caused by timber management have a tremendous potential impact on assemblages of bird
species. One change that may have a particularly strong impact
is the elimination of older forest age classes. Several bird
species appear to be negatively impacted by the reduction of oldgrowth forests. Effects of altering natural assemblages of bird
species upon forest systems are unknown, but it is conjectured
that a reduction in the number of insectivorous birds may result
in reduced stability. Management for high species richness or
diversity, with little regard for natural species composition is
questioned. Intensive efforts directed at gaining information on
wildlife in old-growth forests are advocated.
KEY WORDS: birds, western coniferous forests, old-growth forests,
timber management, hole-nesting birds.
l/This paper is a contribution of the Oregon Cooperative Wildlife Research Unit:
Oregon State University, Oregon Department of Fish and Wildlife, U.S. Fish and
Wildlife Service, and the Wildlife Management Institute cooperating. The paper is an
outgrowth of a problem analysis conducted on "The habitat requirements of wildlife in
old-growth forests." The U.S.D.A. Forest Service, Pacific Northwest Forest and Range
Experiment Station, LaGrande, Oregon funded the work (Range and Wildlife Habitat
Research Project USDA-FS-PNW-214).
357
The primary goal of forest management on much of the commercial forest land in
the western United States, particularly in the Pacific Northwest, is to maintain a
high yield of timber. Although rotation schedules and harvest techniques vary among
areas, current silvicultural practices employed to attain this goal often include (1)
seeding or planting of one or a few preferred timber species following harvest, (2)
precommercial and commercial thinnings to maintain high growth rates of trees and
eliminate undesirable trees, and (3) harvesting at optimum tree size. These practices produce trees that are evenly spaced and approximately equal in size and age.
Managed forests that will result from a full implementation of these practices will be
structurally different from natural forests in that they generally will have fewer
tree species, reduced structural complexity, and a lower mean age of trees. Timber
management thus tends to simplify natural forest systems.
Changes in vegetation structure caused by timber management have a tremendous
potential impact on the assemblages of bird species that inhabit forest systems. One
change that may have a particularly strong impact is the reduction or elimination of
older forest age classes. Predicted age of trees at harvest in the Pacific Northwest
now varies from 45 to 140 years depending primarily on productivity of the site. When
one considers that coniferous trees often live for well over 200 years, it becomes
apparent that predicted harvest schedules will truncate the potential life span of
natural forest stands. Old stands of timber now being rapidly liquidated from forest
systems (e.g. in Oregon, Beuter et al. 1976) will not be allowed to redevelop under
intensive management regimes. Any bird species or group of species closely associated
with these older forest age classes may be negatively impacted if such forests are
greatly reduced or eliminated (Meslow and Wight 1975).
In the Pacific Northwest, the U.S. Forest Service and Bureau of Land Management
are currently developing plans to retain a small portion of commercial lands in oldgrowth timber. Whether these plans will be successful in the face of increasing demands for timber products remains to be seen.
The purposes of this paper are to examine available information on assemblages of
bird species in older forest age classes, and discuss how timber management may affect
these species assemblages.
CHARACTERISTICS OF BIRD SPECIES ASSEMBLAGES IN OLD-GROHTH FORESTS
The following examination of birds in old-growth forests is based on the results
of 11 breeding bird censuses in various forest types in western North America; most
are from the Pacific Northwest. Selection of the censuses was difficult because the
term "old-grmvth" has not been clearly defined. A forest is often considered "oldgrowth" when it acquires a set of characteristic structural components. Franklin
et al. (in press) suggested that 4 primary structural elements characterize old-growth
stands of Douglas-fir (Pseudotsuga menziesii) in western Oregon. These elements are
(1) large, individualistic Douglas-fir trees with coarse branch systems and deep
crowns, (2) large standing dead trees, or "snags," (3) large logs in various stages of
decay on the ground, and (4) large logs in streams. Other characteristics mentioned
were high coefficients of variation in tree sizes (i.e. a multi-layered canopy), and
patchiness of the understory. Franklin et al. (in press) stated that these components
begin to appear in a Douglas-fir stand in western Oregon after 200 to 250 years.
Undoubtedly, the length of time required to attain old-growth characteristics, and the
characteristic components themselves change among forest types and growth sites.
Therefore, different definitions based on general structural components and specific
needs of old-growth-dependent wildlife may be needed for different forest types in
different areas.
Due to the lack of a suitable general definition of "old-growth," the 11 censuses
were selected simply on the basis that they were conducted in old (usually 200+ years),
358
generally undisturbed stands of trees. The sample of censuses is admittedly small,
and by no means represents all old-growth forests in western North America. Data for
such a complete analysis are not available. What I hope to gain by an examination of
these censuses is some suggestion of the differences and similarities among assemblages of bird species in old-growth timber.
Species Richness and Total Bird Density
Results of the selected censuses showed substantial differences in species richness (total number of species) and total bird density (Table 1). Some (much) of these
variations may be attributed to differences in the type of census method employed, the
size of the study area examined, and identification skills of the observers (Table 1).
Variability in these factors probably invalidates any close comparison of community
parameters among the censuses. Nevertheless, the data appear to suggest (as ·might be
expected) that bird species richness and total bird density are not necessarily uniform in older age classes of various forest types in western coniferous forests. If
this is so, any assessment of the relative magnitude of these parameters in an oldgrowth forest may best be based on a comparison with the results of bird censuses conducted in younger forest age classes of the same forest type, in the same area, using
the same census technique and observer.
Foraging and Nesting Guilds
Despite the wide range of values of bird species richness and total bird density
among the selected old-growth forests, there were similarities among proportions of
species and individuals occupying various foraging and nesting "guilds" (Root 1967).
Grouping species into categories based on ecological and behavioral patterns (i.e.
guilds) may provide some insight into the importance of various forest components.
Among the foraging guilds (Bock and Lynch 1970), the number of species and individuals
in the "tree-foliage-searching" and "ground-brush-foraging" categories were most abundant (Table 2). Species and individuals in the "timber-gleaning" and "timber-drilling"
categories were less abundant, while birds in the "aerial-searching" and "hawking"
guilds comprised only small proportions of the selected censuses (Table 2). A similar
distribution of birds among foraging guilds was noted by Wiens (1975) for western coniferous forests in general.
An examination of nesting guilds (Mannan 1977) revealed that coniferous-treenesting birds and hole-nesting birds (i.e. birds that nest primarily in dead trees)
comprised, on the average, 37.6 and 29.7 percent of the species, and 42 and 30 percent
of the total number of individuals, respectively, in the selected censuses (Table 3).
Smaller proportions (less than 16 percent) were occupied by ground- and bush-nesting
birds, and other tree-nesting birds (Table 3).
If foraging and nesting guilds are good indicators of the relative importance of
various forest components to birds, then, not surprisingly, it appears that coniferous
trees are one of the more important structural components to birds in old-growth
forests. Snags, or standing dead trees, also appear to support a large porportion of
the bird populations in old-growth forests (Meslow and Wight 1975, Mannan et al. in
press). Other components, such as the understory tree and brush layers may not support
a large number of species or individuals, but nevertheless contribute to the structure
of bird species assemblages (see Kilgore 1971).
Questions that arise upon examination of these very general conclusions are (1)
how do forest components that are important to birds differ between old-growth stands
and younger stands? (2) how does timber management affect these important components?
and (3) are there any bird species or groups of species that depend upon structural
components found only in old-growth forests?
359
;··
I
I.
TABLE 1.--Bird species richness and total density in assemblages of bird species in 11
old-growth coniferous forests in western North America.
Dominant tree species!/
location
Census technique
(area covered)
Total number of
bird species
Density
birds/40.5 hectares
TSHE/THPL - British Columbia
(Webster 1969)
spot-map
(8.1 ha)
19
435
TSHE - Oregon
(Wiens and Nussbaum
variable
circular plot
(undefined)
12
1060
TSHE - Oregon 3 /
(Anderson 1972)-
sample count
(3.2 ha)
25
819
THPL/TSHE/PSME - Oregon
21
(Wiens and Nussbaum 1975)-
variable
circular plot
(undefined)
15
1170
PSME/TSHE - Oregon
(Wiens and Nussbaum
1975)~/
variable
circular plot
(undefined)
12
572
PSME - Oregon
(Wiens and Nussbaum
1975)~/
variable
circular plot
(undefined)
12
720
PSME - Oregon
(Mannan 1977)
sample count
(5.0 ha)
33
667
PSME - Califo~nia
(Hagar 1960)if
spot-map
(10.1 ha)
13
238
PIJE-ABCO - California
(Bock and Lynch 1970)~/
spot-map
(8.5 ha)
24
184
ABMA/PICO - California
(Robert 1966)
spot-map
(10.1 ha)
14
304
PSME/PIPO - Arizona
(Franzreb 1977)~/
spot-map
(15.5 ha)
36
750
1975)~/
1/
- ABCO - Abies concolor; ABMA - Abies magnifica; PICO - Pinus contorta; PIJE Pinus jeffrey!; PIPO - Pinus ponderosa; PSME - Pseudotsuga menziesii; THPL - Thuja
plicata; TSHE - Tsuga heterophylla.
~/Raptors were excluded from census results.
3/
-Stand selectively cut in 1960s (Anderson 1970).
4/
-Results expressed are means of 2 breeding seasons.
i 1stand selectively cut in early 1800s (pers. comm. Carl E. Bock). Results
expressed are means of 3 breeding seasons.
360
TABLE 2.--Foraging 7uilds of birds in the 11 selected old-growth coniferous
forests •.!
Foraging
Mean percent of total
species (range)
guild~/
Hawking
Tree-foliage-searching
Aerial-searching
Timber-gleaning
Timber-drilling
Ground-brush-foraging
10.0
35.8
0.5
10.4
10.8
32.3
Mean percent of total
density (range)
(6-17)
(26-43)
(0-3)
(5-17)
(0-22)
(25-42)
10.6
44.7
0.1
13.8
3.9
26.7
(1-16)
(36-56)
(0-1)
(8-25)
(0-11)
(17.:.35)
.!/See Table 1.
~/After Bock and Lynch (1970).
TABLE 3.--Nesting guilds of birds in the 11 selected old-growth coniferous forests •.!/
Mean percent of total
species (range)
Nesting guild~/
Ground-nesting
Bush-nesting
Tree-nesting (either
conifers or deciduous)
Coniferous-tree-nesting
Deciduous-tree-nesting
Hole-nesting
.!1see Table 1.
~/After Mannan
Mean percent of total
density (range)
11.9 (5-17)
11.3 (0-26)
15.2 (8-26)
7.2 (0-13)
8.8
37.6
0.4
29.7
5.5
42.0
0.2
30.0
(1-18)
(27-50)
(0-3)
(17-41)
(0-14)
(29-54)
(0-2)
(21-46)
(1977).
TIMBER MANAGEMENT, OLD-GROWTH FORESTS , AND BIRDS
The effects of complete, or nearly complete overstory removal on the structure of
bird species assemblages in old-growth forests has been investigated by Hagar (1960)
and Franzreb (1977). Disturbances of this magnitude obviously alter the availability
of food sources, nest sites, and shelter; all of which are important factors in habitat
selection in birds (Klopfer and Hailman 1965, Hilden 1965, Verner 1975). As might be
expected, both Hagar (1960) and Franzreb (1977) noted differences between the cut and
uncut areas in total bird density, species richness, and species composition. Hagar
(1960) found that in the third year following harvest, total bird density in a cut-over
area was greater than in an old-growth stand of Douglas-fir. The high density in the
cut-over area was due primarily to a greater number of dark-eyed juncos (Junco
hyemalis) and mountain quail (Oreortyx pictus); both species nest and forage on the
ground. However, species richness in the cut-over area (9 species) was lower than in
361
1.·
'
the old-growth stand (13 species). The difference was due primarily to a lower number
of birds that nested and foraged in coniferous trees and snags.
Franzreb (1977) found that total bird density was greater in a mixed-conifer oldgrowth stand than in a similar area where most of the overstory had been removed. Of
the 13 species Franzreb (1977) listed as being adversely affected by the logging, 42
percent were species that nested in snags, and 33 percent were species that nested and
foraged in coniferous trees. One species, the hermit thrush (Catharus guttatus),
nested in dense brush or small trees, and foraged on the ground.
Conversely, of the 10 species Franzreb (1977) listed as being beneficially
affected by the logging, 50 percent were also birds that nested in snags. These
species included the American kestrel (Falco sparverius), yellow-bellied sapsucker
(Sphvrapicus varius), violet-green swallow (Tachycineta thalassina), house wren
(Troglodytes aedon), and western bluebird (Sialia mexicana); all of which generally
prefer open areas for nesting and foraging. A more complete harvest (i.e. removal of
most snags) would probably have reduced the numbers of these species as well.
Hagar (1960) and Franzreb (1977) studied the effects of one aspect of timber
management on bird communities - harvesting. Another pertinent avenue of study is a
comparison of species assemblages in old-growth stands and managed stands near or at
rotation age. Because old-growth stands may eventually be replaced by managed stands,
this comparison should suggest some of the differences that will exist between assemblages of bird species in natural forest systems (i.e. those with old-growth forests)
and managed forest systems. This sort of information will also indicate what components of old-growth forests need to be actively maintained in managed forest systems.
Unfortunately, few comparisons have been made between assemblages of bird species
in managed and old-growth forests; possibly because few older-aged managed stands
exist. Mannan (1977) found that bird species richness during the breeding period was
greater in a stand of old-growth Douglas-fir (31 species) than in 4 thinned and unthinned stands of Douglas-fir 33 to 70 years of age (18-27 species). Total bird density was also greater in the old-growth stand than in all but one of the 4 younger
stands. These differences were due primarily to the absence of several hole-nesting
birds in the younger stands. Manuwal and Munger (1978)l/ also found a low number of
hole-nesting bird species (0-3) in managed Douglas-fir stands 34 to 64 years of age.
The studies by Mannan (1977) and Manuwal and Munger (1978) suggest that in
Douglas-fir forests, species richness and total bird density are greater in old-growth
stands than in younger, managed stands. Some evidence supporting this conclusion can
be found in the results of studies of avian communities and plant succession (mostly
from eastern forests). General trends in birds species richness and total bird.density
through succession suggest that the replacement of old stands of timber with mediumaged stands (usually dense, pole stands) may result in decreased numbers of species and
individuals (Johnston and Odum 1956, Martin 1960, Haapanen 1965, Conner and Adkisson
1975). However, a note of caution must be issued. With the possible exception of the
study by Manuwal and Munger (1978), none of the above studies examined intensively
managed forest stands. Therefore, any general conclusions about total bird density and
species richness in unmanaged old-growth stands compared to younger, intensively
managed stands would be speculative at best.
The foregoing discussion has concentrated on inter-stand differences in 2 parameters that characterize species assemblages. Perhaps more important than these comparisons is a consideration of how these inter-stand differences may affect the avifauna in
l 1unpublished
report submitted to the U.S.D.A. Forest Service, Pacific Northwest
Forest and Range Experiment Station, LaGrande, Oregon 97850.
362
an entire managed forest system (i.e. all forest age classes). This consideration involves examination of individual population densities and should seek to determine if
any species will be eliminated (or greatly reduced) by the complete liquidation of oldgrowth forests. The term "elimination" as it is used here can be thought of in two
ways (1) complete absence, or (2) a reduction in density to the point where a species
is unable to fulfill its functional role in the system.
One example of a species that apparently depends heavily upon old-growth coniferous forests for its habitat requirements is the northern spotted owl (Strix
occidentalis caurina). Based on a roadside call count, Forsman et al. (1977) estimated that spotted owls were 12 times more abundant in old-growth Douglas-fir forests
than in second growth stands 40 to 50 years of age. Two of the 5 owls detected in
second-growth stands were associated with small patches of old-growth habitat. These
results supported Forsman's (1976) earlier conclusion that spotted owls depend on oldgrowth forests for preferred nest and roost sites.
Several other species, or groups of species, while not dependent on old-growth
forests for their existence, do reach maximum densities in old-growth habitat. In
many of the studies mentioned above, differences between bird species assemblages in
young stands and old-growth stands were due to differences in population levels of
hole-nesting birds. In Douglas-fir forests in the Oregon Coast Range, Mannan et al.
(in press) found that hole-nesting birds preferred large snags with broken tops as
nest sites. McClelland and Frissell (1975) noted similar preferences by hole-nesting
birds in Douglas-fir and western larch (Larix occidentalis) forests in Montana.
Mannan et al. (in press) also found that breeding densities of hole-nesting birds
increased with stand age. They attributed these results to the greater number of
large, decayed snags in older forest age classes (Cline et al. in press). One holenesting bird that is especially dependent on large snags is the pileated woodpecker
(Dryocopus pileatus) (Bull and Meslow 1977).
Lists of other species that may reach maximum breeding densities in old-growth
forests have been developed (Meslow and Wight 1975, Franklin et al. in press).
Included on these lists, in addition to the spotted owl and pileated woodpecker, are
the goshawk (Accipiter gentilis), Vaux's swift (Chaetura vauxi), Townsend's warbler
(Dendroica townsend~), and hermit thrush. These species, and many others, require
further examination before conclusions can be drawn about their relationships with oldgrowth forest habitat.
j
Effects of Simplifying Forest Systems
The well-known idea (dogma) that complexity begets stability is no longer accepted
as a generalization for ecological systems (if indeed it ever was) (May 1976, Ricklefs
1979). However, several investigations have suggested that simplification of natural
systems often results in reduced stability (see Ricklefs 1979) ("stability" is used
here to indicate low population fluctuations) . Several theories have been put forth to
explain the reduction in stability following simplification. For example, May (1976)
stated that although simple, stable systems exist naturally (e.g. the marsh grass,
Spartina), many man-made monocultures may be unstable due to the lack of coevolution
between pests and pathogens. A reduction in species richness combined with the absence
of predators has also been suggested as a condition that leads to instability (Pimintel
1961).
Timber management certainly simplifies forest systems by eliminating older forest
age classes, reducing the structural complexity and tree species richness in existing
age classes, and potentially reducing the number and density of predators (e.g. insectivorous birds). At what point these manipulations affect the stability of the system
is not known. However, the occasional (and sometimes frequent) outbreaks of insects
363
.·•·
in managed forests may evidence instability. Ricklefs (1979) stated that large increases in herbivorous species are characteristic of simplified systems.
Whether the elimination of old-growth coniferous forests will markedly reduce the
total number of bird species in managed forest systems is a question that at present
can not be answered. Certainly densities of some species will be greatly reduced.
What will be the effects of these changes and potential changes on the forest system
as a whole? How much perturbation can a system absorb and still exhibit the compositional and organizational characteristics of its undisturbed state?
The role of birds in regulating forest insects has been reviewed by Bruns (1960),
Franz (1961), Thomas et al. (1975), and Wiens (1975). The conclusion of these reviews
is that insectivorous birds in general, and perhaps hole-nesting birds in particular,
play an important role in the reduction of insect populations at endemic levels.
Birds are probably important, therefore, in damping the number and size of ins~ct outbreaks. The reduction of hole-nesting birds and other insectivorous species via elimination of older forest age classes may thus reduce the stability of managed forest
systems. Potential effects of reducing or eliminating forest raptors such as the
spotted owl and goshm.;rk are unknown.
MANAGEMENT CONSIDERATIONS
Given that complexity does not necessarily lead to stability, it seems evident
that management for high species richness or diversity, with little regard for natural
species composition, is unwarranted. A particular density, or species richness, or
diversity value is not, in and of itself, "good." A managed stand could conceivably
have an exceptionally high species richness or total bird density and still lack an
entire group of specie~ that plays an important role in the natural system.
One alternative course of action, as suggested by Balda (1975), is to attempt to
maintain a forest system in a condition that is as close as possible to the natural
state (at least as close as possible to our conception of the natural state).
Similarly, Wiens (1978) advocated managing for "suites" of species that occur together
naturally.
Included in the generalized management schemes suggested above should be the
retention of a certain percentage of land in old-growth timber. Practical questions
that immediately confront timber management agencies and wildlife managers are (1) how
much old-growth habitat is needed? (2) what are the required sizes and distribution
of patches of old-growth habitat? (3) is there a need for concern regarding the age
of trees in stands adjacent to patches of old-growth? and (4) how should the distribution of old-growth patches be managed over time? Our lack of knowledge about the
abundances and interrelationships among most species in old-growth forests, as indicated by the low number of bird censuses in old-growth stands, leaves little alternative but to attempt to answer these questions on an individual species basis. The
current management plan for old-growth timber in western Oregon was initiated by concern for the habitat requirements of the spotted owl. This plan has come under criticism because it does not approach old-growth management from a community perspective.
Ideally, management plans for old-growth forests should be more holistic, but the lack
of information on "community interactions" renders the community approach difficult to
defend and implement at this time.
In areas for which few data exist on the abundances of species in various forest
age classes, justification for retaining old-growth habitat must be based on reasons
other than habitat management for wildlife. Such reasons could include maintenance of
the natural level of habitat diversity, maintenance of water and soil quality, preservation for recreational and aesthetic purposes, and retention to allow study of the
ecology and structure of these old forest stands. The latter point is especially
364
important when one considers the lack of information on these topics. Answers to the
above questions about "how much, where, and of what size" must then be based on economic and political considerations. Hopefully, both approaches will retain enough oldgrowth habitat so that options will remain open for alternative, and perhaps more
biologically-based plans in the future.
For the present, an intensive effort should be made to gather information about
wildlife in old-growth forests. Initially, a general inventory should be conducted.
The wide range of bird species richness and total bird density in the 11 old-growth
forests (Table 1) indicates that the inventories should be conducted regionally by
forest type. Ideally, the inventories should be conducted in a series of age classes
extending from recently established stands to old-growth forests. However, given time
and fiscal constraints, it is unlikely that this type of intensive program can be
initiated in all forest types. One alternative is to conduct the inventories in oldgrowth stands and managed stands that represent what will be the oldest age class of
intensively managed forests of the future.
Development of an ecological information base on species that inhabit old-growth
forests also seems to be of critical importance. Initial efforts should be on those
species that are closely associated with the older stands (as determined by the
inventories). Emphasis should be placed on examination of nest site selection, productivity, foraging behavior and ecology, territory and home range size, and dispersal
capabilities of the young. In addition, some assessment of the lower limits of viable
population size must be made. All of these factors are important in the development
of sound management programs for old-growth forests and their associated fauna.
ACKNOWLEDGEMENTS
I am grateful to E. L. Bu~l, B. G. Marcot, E. C. Meslow, and J. W. Thomas for
their suggestions and comments on initial drafts of this paper. I also thank D. A.
Sweeney for typing the manuscript.
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