How Dead Trees Sustain Live Organisms in
Western Forests1
Fred L. Bunnell,2 Isabelle Houde,2 Barb Johnston,2 and Elke Wind3
Abstract
Dead wood contributes to biological richness as substrate, cavity sites, foraging sites, and
shelter or cover. In the Pacific Northwest, 69 vertebrate species commonly use cavities, 47
species respond positively to down wood, and prevalence of both uses is related to natural fire
regimes. Almost 80 percent of nests of weak excavators are in dead trees; strong excavators
make greater use of live trees. Most bat roosts are in dead trees, whereas carnivores use
mostly declining, living trees. Selection of both cavity and foraging sites is governed by decay
patterns. Some species prefer large pieces of down wood. Management implications are
discussed.
Introduction
Dying trees, snags, and down wood are common in unmanaged forests and
required by many species, including fungi, cryptogams, invertebrates, and vertebrates
(Berg and others 1994, Harmon and others 1986). We focus on vertebrates, and note
contributions to non-vertebrates only briefly. We also focus on the Pacific Northwest,
which we define as Alaska, Alberta, British Columbia, Washington, Oregon, Idaho,
Montana, and northern Nevada and California. References to other regions are
included to indicate trends where forestry has been practiced longer, or where
particular species are well documented.
As concern for sustaining all organisms has grown, interest in natural
disturbance regimes as models for guiding forest practices has also increased
(Attiwill 1994, Hunter 1993). We first examine relations among natural fire regimes
and vertebrates that use dead wood. We then review the diverse uses forest-dwelling
organisms make of dead wood under four broad categories: substrate, cavity sites,
foraging sites, and shelter or cover (down wood). We finish by noting management
implications.
1
An abbreviated version of this paper was presented at the Symposium on the Ecology and Management
of Dead Wood in Western Forests, November 2-4, 1999, Reno, Nevada.
2
Professor and Research Technicians, respectively, Center for Applied Conservation Research,
Department of Forest Sciences, University of British Columbia, 3004-2424 Main Mall, Vancouver,
British Columbia, Canada, V6T 1Z4 (e-mail: fbunnell@interchange.ubc.ca and houde@intergate.ca).
3
Research Technician, E. Wind Consulting, 1-2817 Glenayr Dr., Nanaimo, British Columbia, Canada,
V9S-3S7 (e-mail: ewind@telus.net).
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Natural Disturbance Regimes and Use of Dead Wood
Fire is the major natural agent of disturbance in Pacific Northwest forests, and
natural fire regimes differ among forest types, influencing the amounts of dead wood
present (Agee 1993). We examined relationships of the forest-dwelling vertebrate
fauna with natural fire regimes in the 12 forested biogeoclimatic zones of British
Columbia, applying the approach of Bunnell (1995) to more recent data. We
expected predictable differences. The proportion of species positively associated with
down wood should increase as the fire-return interval increased and down wood
accumulated. Cavity users should be negatively correlated with fire size, because size
and intensity often are related, and fewer snags remain standing where fires are more
intense.
The proportion of down wood users in the fauna increased with increasing firereturn interval, and proportions of bird and mammal cavity users decreased with
increasing fire size (table 1). Species using down wood are mainly mammals
(appendix A), and both numbers of species and proportions of mammalian users of
down wood increased with increasing fire size. That is expected if larger, more
intense fires create a more reliable supply of down wood. The number of species
using cavity sites decreased significantly as fire return interval lengthened, and snags
were created less frequently (table 1). Fire regimes vary with the precipitation
regime, but associations between the vertebrate fauna and precipitation regimes were
not found. The lack of relations with total precipitation suggests that fire regime had
more influence on composition of the vertebrate fauna than precipitation itself (the
two wettest types were also higher elevation and obscured any relationship between
amphibian richness and precipitation).
Forest-dwelling vertebrate faunas appear to respond to amounts and duration of
down wood as these are influenced by the natural disturbance regime, suggesting a
mechanism for the differences in richness of dead wood users across broad forest
types.
Table 1—Significant Spearman's rank coefficients among vertebrates using dead wood and
mean annual precipitation, and characteristics of natural fire regimes in the 12 forested
biogeoclimatic zones of British Columbia. 1
Cavity users
Birds
All
N2
Precipitation
Fire Size
Fire Return
1
2
Pr2
N
- 0.85**
- 0.58*
Pr
- 0.75**
Downed wood users
All
Mammals
Mammals
N
Pr
N
Pr
0.59*
- 0.70*
- 0.79**
N
Pr
0.66*
0.68*
0.83**
* = P < 0.05; ** = P < 0.01
N = Number of species; Pr = Proportion of species in the native vertebrate fauna.
Dying and Dead Wood as Substrate
Dead wood makes its greatest contribution to biological richness as substrate for
fungi, cryptogams, and invertebrates. There are no sharp distinctions between
declining trees and snags as the most favored habitat. Some pendent lichens are
common on both, but appear more abundant on snags (e.g., Usnea longissima, Berg
and others 1994; Letharia vulpina, Bernes 1994). Berg and others (1994) reviewed
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
habitat requirements of 1,487 threatened forest-dwelling organisms in Sweden. Dying
trees were favored habitat for 89 species of fungi and cryptogams and 252 species of
invertebrates. Snags provided substrate for 21 percent of all threatened non-vertebrate
species, including 36 macrofungi and cryptogam species and 266 invertebrates. Logs
hosted more species. Berg and others (1994) estimated that about 30 percent of
threatened cryptogams and macrofungi and 28 percent of the invertebrates were
dependent on down wood.
The role of dead wood as substrate is less well known in the Pacific Northwest,
but likely is similar to the role in Sweden. Of 636 lichen species reported from
British Columbia 46 are largely restricted to old-growth stands (Goward 1999;
Goward and others 1994). Goward and Arsenault (1997) reported a snag-specific
community of lichens from Englemann spruce (Picea englemanni; scientific names
for most species are found in appendix A) and subalpine fir (Abies lasciocarpa)
forests. At least 25 lichen species are found on decaying wood (data in Goward 1999,
and Goward and others 1994). Of 93 forest-dwelling bryophytes reviewed by Vitt
and others (1988) for the Pacific Northwest, 30 species (32 percent) preferentially
grow on down wood that frequently is well rotted. Well-rotted logs also serve as foci
for dispersal of mycorrhizal fungi critical to tree productivity (Maser and others
1978). Some “saprophytic” vascular plants (e.g., Allotropa, Hemitomes) rely upon
mycorrhizal fungi that often are found in down wood for delivery of nutrients (Leake
1994). In British Columbia, 526 species of macrofungi are dependent on down wood,
including some harvested commercially (Lofroth 1998). Because some vertebrates
forage on fungi and insects in down wood, reductions of these food sources may
appear higher in the food chain.
Features of logs considered to influence non-vertebrates include tree species,
decay state, size, and distribution. Conifer logs are more durable than hardwood logs.
Natural successions of cryptogams, fungi, and invertebrates on and in down wood
(e.g., McCullough 1948, Söderström 1988) indicate the importance of a range of
decay states. Larger logs provide better substrate than smaller logs for bryophytes
and lichens, because larger logs last longer, have more surface area, and have higher,
steeper sides that discourage ground-dwelling species from invading (Samuelsson
and others 1994). Forest-floor bryophytes generally have very limited dispersal
ability (Khanna 1964, Söderström 1987), and dispersal is from log to log for epixylic
species. For these reasons, Samuelsson and others (1994) argued that logs should be
close together, but not gathered into piles. Similarly, several small logs may provide
more habitat than a single large log.
Dead wood is a critical substrate for hundreds of non-vertebrate species in the
Pacific Northwest. Large, dispersed pieces of a range of decay states are preferred.
Sustaining a range of decay requires sustained recruitment of down wood.
Dead Wood as Cavity Sites
Cavity sites are easily studied and hence well-documented. Foraging on snags is
a year round activity, is less frequently studied, and less well known. Lack of cavity
nesting sites has limited abundance of some birds in intensively managed forests
(Angelstam and Mikusinski 1994, Newton 1994).
Because of the importance of heart rot for cavity sites, most nests of primary
cavity nesters were in dead trees (table 2). Of 2,674 nesting records of weak
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
excavators, 2,154 (78 percent) were in dead trees (table 2). Although conifers are
less prone to decay than most hardwoods, the proportion of dead trees used as nest
sites by weak excavators did not change with the proportion of conifers used. Most
strong excavators located < 50 percent of their nests in dead trees (table 2), but the
proportion of nests in dead trees increased significantly with the proportion of
conifers used (Bunnell and others 2002). That relationship explains apparently
anomalous values in table 2. For example, in the largest sample for pileated
woodpecker in table 2 (Bull 1987; n = 105 nests) all available nest trees were
conifers, and 99 percent of the nests were in dead trees. Conversely, another sample
was gathered4 where hardwoods were available but scarce (< 10 percent of stems),
but six of seven nest trees were living trembling aspen (Populus tremuloides). Dead
trees were the main source of cavity sites for 16 of the 21 primary excavators (table
2). For most of the remaining species, dead trees were more commonly used as cavity
sites when nests were in conifers. Weaker excavators largely restricted to conifers
(e.g., Lewis’s woodpecker, white-headed woodpecker) may be particularly
threatened in managed, conifer forests, because trees do not become old enough for
heart rots to develop. Both Lewis’s and white-headed woodpeckers are designated 'at
risk' in the Pacific Northwest (appendix A).
Table 2―Percentages of nests located in dead trees by strong and weak excavators of the
Pacific Northwest.
Pct dead n1 Sources
Strong
Excavators
Yellow41.4
63 Scott and others 1980; BC Nest Records.
bellied
sapsucker
Red-naped
24.0
557 Campbell and others 1990; Li and Martin 1991; C.
sapsucker
Steeger2; W. Klenner and D. Huggard3; K. Martin4
4
Red-breasted
sapsucker
Williamson’s
sapsucker
55.3
132
59.3
303
Hairy
woodpecker
62.4
190
Three-toed
woodpecker
Blackbacked
woodpecker
Acorn
woodpecker
Pileated
woodpecker
42.9
161
46.7
56
7.8
238
73.2
202
Raphael and White 1984; Campbell and others 1990; Li
and Martin 1991.
Bull 1980; Scott and others 1980; Raphael and White
1984; Li and Martin 1991; Conway and Martin 1993; BC
Nest Records.
Kelleher 1963; Bull 1980; Scott and others 1980; Raphael
and White 1984; Campbell and others 1990; Li and
Martin 1991; W. Klenner and D. Huggard3; K. Martin4;
C. Steeger2
Scott and others 1980; Klenner and Huggard 1997; C.
Steeger2; K. Martin4; BC Nest Records.
Bull 1980; Raphael and White 1984; C. Steeger
(unpublished)2; BC Nest Records.
Scott and others 1980; Li and Martin 1991; Hooge and
others 1999.
Bull 1987; Mellen 1987; Campbell and others 1990; C.
Steeger 2; W. Klenner and D. Huggard3; K. Martin4; K.
Aubry and C. Raley 5
Unpublished data on file, British Columbia Ministry of Forests, Kamloops, British Columbia.
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
(table 2 continued)
Weak
Excavators
Lewis's
woodpecker
Downy
woodpecker
Whiteheaded
woodpecker
Northern
flicker
Nuttall's
woodpecker
Blackcapped
chickadee
Mountain
chickadee
Boreal
chickadee
Chestnutbacked
chickadee
Red-breasted
nuthatch
Whitebreasted
nuthatch
Pygmy
nuthatch
Pct dead
n1
Sources
62.4
367
Raphael and White 1984; BC Nest Records
60.4
109
97.4
123
Scott and others 1980; Campbell and others 1990; Li and
Martin 1991; C. Steeger2; K. Martin4
Raphael and White 1984; Milne and Hejl 1989; Dixon
1995a; Dixon 1995b.
55.9
717
94.0
48
Bull 1980; Scott and others 1980; Raphael and White
1984; Campbell and others 1990; Li and Martin 1991;
W. Klenner and D. Huggard3; K. Martin4; C. Steeger2
Miller and Bock 1972 (review within).
59.3
17
C. Steeger2; K. Martin4
65.8
433
87.3
31
58.0
132
C. Steeger2; BC Nest Records.
71.9
394
74.2
62
Raphael and White 1984; W. Klenner and D. Huggard3;
K. Martin4; C. Steeger2; BC Nest Records.
McEllin 1979; Scott and others 1980; Raphael and White
1984; Li and Martin 1991; Campbell and others 1997.
78.0
331
Scott and others 1980; Raphael and White 1984; W.
Klenner and D. Huggard3; K. Martin4; BC Nest Records.
Peck and James 1987; Campbell and others 1997.
McEllin 1979; Scott and others 1980; Raphael and White
1984; Li and Martin 1991; BC Nest Records.
1
Number of nest trees.
Unpublished data on file, Pandion Ecological Research, Ltd., Nelson, BC, Canada.
3
Unpublished data on file, BC Ministry of Forests, Kamloops Region, BC, Canada.
4
Unpublished data on file, Centre for Applied Conservation Biology, University of British Columbia,
Vancouver, BC, Canada.
5
Unpublished data for K. Aubry and C. Raley, sight unseen from Bull and Jackson (1995).
2
Figure 1 illustrates preferences by woodpeckers for trees in different stages of
decay. Decay classes in figure 1 are those of Thomas and others (1979), and classes 1
and 2 represent healthy and declining, but living, trees. We used the electivity index
of Ivlev (1961) to compare within studies, because it is largely symmetrical, ranging
from -1.0 at complete avoidance to about +1.0 when all nests are in a particular
category. Living conifers were not selected by either woodpeckers or cavity-using
mammals (primarily red and flying squirrels). Living trembling aspen trees were used
more in proportion to their availability, except by mammals that cannot excavate
their own cavities. The most strongly preferred decay classes were recently dead trees
(decay classes 3 and 4). Decay classes 4 and 5 of lodgepole pine (Pinus concorta)
were strongly avoided because they were long dead, understory trees, too small to
support cavities.
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
a)
1
0.8
0.6
Electivity
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1
2
3
4
Decay Class
5
6+
b)
1
0.8
0.6
Electivity
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1
2
3
4
5
6+
Decay Class
c)
1
0.8
0.6
Electivity
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1
2
3
4
5
6+
Decay Class
Figure 1—Apparent preference among decay classes shown by: a) woodpeckers
nesting in conifers (data of Bevis 1996); b) woodpeckers nesting in Douglas-fir [ ◆]
and trembling aspen trees [ O ] (data of Klenner and Huggard 1998); c) mammals in
lodgepole Pine [ ■ ] and trembling aspen trees [ O ] (data of K. Martin unpublished).
Preference evaluated by the electivity index of Ivlev (1961).
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
There are more secondary than primary cavity nesters (appendix A), but often 80
percent or more of their nest sites are created by primary nesters (Dobkin and others
1995, Li and Martin 1991, Schreiber and deCalesta 1992). Other nest sites are in
cavities created by rot. For both forms of nest sites, dead trees are the major source of
nesting opportunities. Several bat species also locate 70 to 100 percent of their roosts
in dead trees (table 3). Less than 50 percent of denning trees of flying squirrels,
American marten, and black bears were dead, indicating the importance of sustaining
older trees with large rot pockets. Most black bear dens recorded from coastal forests
of the Pacific Northwest were associated with wooden structures, including trees,
logs, and stumps. Den sites in southern, inland forests also were commonly in trees
(Bull and others 1996, Lindsay 1999). Mean sizes of den trees for mammals usually
exceeded 50 centimeter (references of table 3). Amphibians and reptiles make
occasional use of cavity sites (McComb and Noble 1981). Bunnell and Dupuis (1995)
reported that snags used by amphibians were recently dead with sloughing bark.
Table 3―Percentage of denning and roosting sites located in snags and dead trees by
mammals of the Pacific Northwest. Logs and stumps not included.
Species
Bats
Big brown
bat
California
myotis
Fringed
myotis
Little brown
myotis
Long-legged
myotis
Northern
long-eared
myotis
Pallid bat
Silver-haired
bat
Pct dead
n1
Sources
45.8
57
100.0
25
Rasheed and Holroyd 1995; Betts 1996; Vonhof 1996;
Kalcounis and Brigham 1998; Rabe and others 1998.
Vonhof 1996; Brigham and others 1997; Grindal 1997.
100.0
15
Rabe and others 1998.
63.2
23
90.2
54
42.9
7
Crampton and Barclay 1995; Rasheed and Holroyd
1995; Kalcounis and Hecker 1996; Grindal 1997.
Rasheed and Holroyd 1995; Ormsbee and McComb
1998; Rabe and others 1998.
Caceres 1997.
100.0
72.0
3
50
Southwestern
myotis
Western
long-eared
myotis
0.0
2
89.5
47
Caceres 1997; Grindal 1997; Vonhof and Barclay 1997;
Rabe and others 1998.
Rodents
Flying
squirrel
32.1
627
Mowrey and Zasada 1984; Carey and others 1997.
32.1
249
40.3
470
Lindzey and Meslow 1976; Noble and others 1990;
Immell and Boulay 1994; Akenson 1994; Bull and others
1996; Davis 1996; Lindsay 1999.
Spencer 1987; Martin and Barratt 1991; Jones and others
1997; Raphael and Jones 1997; Ruggiero and others
1998.
Carnivores
Black bear
American
marten
1
Rabe and others 1998.
Crampton and Barclay 1995; Rasheed and Holroyd
1995; Betts 1996; Campbell and others 1996; Vonhof
1996.
Rabe and others 1998.
Number of denning or roosting sites.
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Dying and Dead Trees as Foraging Sites
Long-term management of snag-using species requires provision of both
foraging and cavity sites. Several studies suggest that cavity sites are less often
limiting to cavity nesters than foraging habitat (Hutto 1995, Walankiewicz 1991,
Welsh and Capen 1992). Given the relative lack of data for foraging sites, a key
question is: are the kinds of trees that should be retained for foraging similar to those
that provide nesting sites?
Among larger excavators, sapsuckers feed primarily on sap and insects
associated with their sapwells. Northern flickers feed on ants on the ground (Bull and
others 1986). Several woodpeckers feed primarily by flaking bark or probing after
insect larvae in the cambium or sapwood. Pileated woodpeckers specialize on
carpenter ants excavated from decayed sap- or heartwood (Bull and others 1986). For
the latter two feeding techniques, decay state may reflect the likelihood of hosting
preferred insects and the ease of excavating. It is less clear that size of tree should
influence feeding preference, although duration of decay states and size of tree often
are correlated. If decay state indicates foraging opportunities, we expect patterns
specific to individual tree species, because species decay differently.
Figure 2 shows that conifers were avoided as feeding sites until they attained
decay class 3 (recently dead). Their attractiveness as foraging sites then increased
with further decay (see also Gyug and Bennett 1996), although that pattern differs
among cavity-nesting species (Morrison and others 1987). In Englemann sprucesubalpine fir forests, three-toed woodpeckers strongly preferred recently dead snags
(Klenner and Huggard 1997). Pileated woodpeckers use more decayed wood,
provided it hosts carpenter ants (Bull and others 1992). Among conifers, Douglas-fir
(Pseudotsuga menzieseii) is a possible exception (fig. 2), and appears to become less
attractive to woodpeckers once all bark is shed (decay class 6+). That may reflect
Douglas-fir tending to rot from the outside in, so the sapwood becomes less favorable
to breeding insects. On Bevis’ (1996) study area, Douglas-fir was not sought as a
foraging site (electivity = -0.04), and selection was shown only for western larch
(Larix occidentalis) (electivity = 0.25). Douglas-fir snags also were not selected on
Madsen’s (1985) study site (electivity = -0.21), whereas western larch and ponderosa
pine (Pinus ponderosa) were selected (electivity = +0.15 and +0.20, respectively).
Selection of trembling aspen followed a different pattern than for conifers.
Foraging woodpeckers were indiscriminate in their use of decay classes 1 through 3
(apparently healthy to recently dead trees), but tended to avoid trees of decay class 4
or greater (fig. 2). Woodpeckers tend to use more of the smaller diameter trees when
foraging than when nesting, especially when foraging on hardwoods (fig. 3).
We draw two broad points from comparisons of nesting and foraging sites. First,
when foraging on conifers, woodpeckers select dead wood. The wood need not be
standing, and several species forage on down wood when it is not snow covered (Bull
and others 1997). Second, woodpeckers will use smaller trees when foraging than
when nesting. Birds select similar decay states when foraging or nesting (compare
figs. 1, 2), but smaller snags are used when foraging (fig. 3). The tendency for nesting
trees to be larger than foraging trees makes biological sense: a cavity site must be
large enough to contain an adult bird and its young; a foraging site need only be large
enough to contain wood-boring larvae or ants. The trend is consistent across studies
(e.g., Bevis 1996, Gyug and Bennett 1996). Small snags do not remain standing for
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
as long as large snags (Morrison and Raphael 1993), and may never be used as nest
trees. Nonetheless, they do serve as foraging sites.
a)
1
0.95
0.95
0.8
0.6
Electivity
0.4
0.2
0
-0.2
2
-0.4
-0.6
-0.8
-1
1
2
3
4
5
Decay class
6+
b)
1
0.8
0.6
Electivity
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1
2
3
4
5
Decay Class
6+
c)
1
0.8
0.6
Electivity
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1
2
3
4
5
6+
Decay Class
Figure 2—Apparent preference among decay classes shown by foraging cavity
nesters. a) Woodpeckers foraging on Douglas-fir [– –◆– –], lodgepole pine [—■—],
spruce [—∆—], and trembling aspen trees [– – O – –] (from data of Klenner and
Huggard 1998). b) Cavity nesting birds foraging on conifers (from data of Madsen
1985). c) Woodpeckers foraging on conifers (from data of Bevis 1996). Values of
0.95 represent instances where specific decay or size classes were sufficiently
uncommon that they did not appear in the random sample of availability. Preference
evaluated by the electivity index of Ivlev (1961). Decay classes of Thomas and
others (1979).
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
299
Electivity
How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
a)
Douglas-fir
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
<10
20-30
10-20
30-50
>50
Electivity
Diameter class
b)
Aspen
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
<10
10-20
20-30
30-50
>50
Electivity
Diameter class
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
c)
Conifers
>15-23
>23-38
>38-53
>53
Diameter class
Figure 3—Comparisons of electivity shown by foraging [– – O – –] and nesting [—
◆— ] woodpeckers across diameter classes. Data for a) Douglas-fir from Klenner
and Huggard (1998), b) Trembling aspen from Klenner and Huggard (1998), c)
Conifers from Madsen (1985).
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Some species foraging habitats are particularly difficult to incorporate into forest
management. Both the black-backed and three-toed woodpeckers are specialized
feeders commonly exploiting conditions after fires (Apfelbaum and Haney 1981,
Hutto 1995). Three-toed woodpeckers feed primarily on larvae of bark beetles
(Murphy and Lehnhausen 1998) that respond dramatically to forest fires, laying eggs
in surviving trees and snags almost immediately after the fire. Adults emerge 2 to 3
years later, and secondary outbreaks appear rare. Although they do eat larvae of
wood-boring beetles (Cerambycidae) in other snags, food is most abundant for these
woodpeckers for only a 2- to 3-year, post-fire period. Black-backed woodpeckers
specialize on larvae of wood-boring beetles that bore into the sapwood of fire- or
beetle-killed trees. Populations of both woodpecker species are therefore irruptive
and concentrated in areas of beetle-infested trees, and both are listed “at risk”’ in the
Pacific Northwest (appendix A).
Kreisel and Stein (1999) found foraging woodpeckers in winter to be ten times
more abundant in recently burned forest than in unburned forest. Hutto (1995)
reported that 15 bird species occurred more frequently in burns than any other cover
type, including four cavity nesters: hairy, three-toed, and black-backed woodpeckers,
and the mountain bluebird. The black-backed woodpecker is the most vulnerable,
because of its specialization on wood-boring larvae (rather than bark beetles or freeflying insects). In short, 15 bird species have recent burns as their favored habitat and
at least one is dependent upon burns.
The problem for forest management is that beetle-infested stands provide the
ideal (and possibly only productive) habitat for some woodpecker species. Numbers
of black-backed woodpecker are much lower in older forests than among recent firekilled trees so that even maintenance of old stands may not be a sufficient
management tactic. Both fire suppression and salvage logging work to the detriment
of the species. The life history of the black-backed woodpecker illustrates that
commitment to maintaining all of biological diversity is also a commitment to
sustaining some areas of dying and dead forest.
Dead Wood as Shelter and Cover
Dead wood on the ground influences vertebrate abundance and richness by
providing:
•
•
•
•
Necessary substrate, energy, and nutrients for many invertebrates and fungi
upon which a wide range of amphibian, reptile, bird, and small mammal
species depend for forage (e.g., Bull and others 1997, Maser and Trappe
1984; Rhoades 1986).
Sheltered areas for reproduction in a range of vertebrates from salamanders
to black bears, and cover from aerial predators (e.g., Corkran and Thoms
1996, Harestad 1991).
A modified microclimate (cooler, moister, more stable temperature than
surrounding habitat) that is essential to species that cannot tolerate extremes
in temperature or humidity (several amphibians; Heatwole 1962).
Runways for small mammals and display or lookout posts for birds (e.g.,
Bull and Henjum 1990, Lofroth 1998).
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•
•
•
Increased habitat diversity and aeration in water by forming riffles, small
waterfalls, and pools, thereby creating habitat for amphibians and fish which
are in turn fed on by other vertebrates.
Structures exploited by near-aquatic vertebrates as cover, foraging sites, or
basking (e.g., river otter [Lontra Canadensis], mink, painted turtles
[Chrysemys picta]; Lofroth 1998).
Access routes for predators, especially under snow cover (e.g., weasels,
marten; Corn and Raphael 1992).
Among terrestrial vertebrates, strict dependence on down wood is most likely
among species breeding in rotten wood (e.g., some salamanders). Other species,
including shrews and several birds, forage on insects that are abundant in down wood
and are often more abundant at sites with more down wood (e.g., Craig 1995,
Waterhouse and Dawson 1999). Although several bird species opportunistically
exploit down wood for nesting sites (e.g., blue grouse [Dendracapus obscurus] and
ruffed grouse [Bonasa umbellus], Townsend’s solitaire [Myadestes townsendi]), only
one bird species relies largely on down wood for nesting opportunities―the winter
wren (Waterhouse 1998). Opportunistic use can be high. For example, Campbell and
others (1990) reported that 31 percent of blue grouse nests were alongside logs.
Several mammal species, ranging from little brown myotis to black bears, use down
wood as resting or denning sites, but most show flexibility across substrates. Rodents,
snowshoe hare (Lepus americanus), gray wolf (Canis lupus), and wolverine (Gulo
gulo) not only use down wood as maternal or resting dens but also use thickets or
earth dens. Hagar and others (1995) estimated that 52 species of mammals in Oregon
responded positively to greater amounts of dead wood. Among the 52, 40 were
associated with logs as cover for themselves or their prey, but it has proven difficult
to associate consistent positive responses in population size or fitness with abundance
of down wood (Bunnell and Huggard 1999, Bunnell and others 1999b). We
acknowledged that flexibility in appendix A by including only species for which a
positive response appeared likely from current literature. By using that criterion, 12
to 18 percent of terrestrial forest-dwelling vertebrate species respond positively to
increasing amounts of down wood in the 12 major forest types of British Columbia
(Bunnell and others 1999b).
The strongest responses to down wood are among terrestrial-breeding
salamanders (seven of eight salamander species in appendix A). Among habitat
variables surveyed, down wood is most consistently related to abundance of
terrestrial-breeding salamanders (reviews of Bunnell and others 1999b, deMaynadier
and Hunter 1995). Some workers reported these salamanders to be associated with
large pieces of down wood (Aubry and others 1988, Whitaker and others 1986). Corn
and Bury (1991) found that densities of clouded and western redback salamanders
were relatively constant per unit volume of down wood regardless of stand age,
indicating the benefits of retaining down wood in younger stands. Other authors have
documented positive responses of small mammals to down wood (e.g., Carey and
Johnson 1995, Corn and others 1988, Gilbert and Allwine 1991), but results are
highly variable within species and among locations. Bunnell and others (1999a)
offered four reasons for the observed variability in response to down wood, of which
the most troubling is that critical lower thresholds have not been reached.
Where forestry has been practiced longer than in the Pacific Northwest, many
organisms are threatened by reductions in down wood (e.g., Anglestam 1997, Berg
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
and others 1994). Current evidence suggests that species dependent upon down wood
in the Pacific Northwest are surviving on legacies of past practices, not the results of
current practices (e.g., Bunnell and others 1997, Spies and others 1988). We believe
that if current accumulations are not replenished, down wood accrued under past
practices will decline, as will some species.
Management Implications
In the Pacific Northwest, 69 vertebrate species consistently seek cavities in
dying or dead trees, and more use such cavities opportunistically. Another 47 or more
species respond positively to increasing amounts of down wood (appendix A). The 90
species of forest-dwelling vertebrates in the Pacific Northwest listed as “sensitive” or
“at risk,” include 30 species requiring cavities and 21 species strongly associated
with down wood. (A definition of “forest-dwelling” is problematic, and we excluded
species such as peregrine falcon (Falco peregrinus), Swainson’s hawk (Buteo
swaisoni), and barn owl (Tyto alba) whose relationship with forest cover is
marginal.) Thus, about 57 percent of listed vertebrate species are reliant upon or
strongly associated with dead wood. Many more cryptogams, fungi, and invertebrates
are dependent upon dead wood. Where the goal of management is to sustain or
restore native biodiversity, forest practices must include ways of sustaining dead
wood. Most managed stands have smaller volumes of dead wood than do unmanaged
stands (Maser and Trappe 1984, Spies and others 1988). The trend is pronounced
where forestry has been practiced longer. Angelstam (1997) reported that dead wood
comprised 30-40 percent of the total wood volume in unmanaged stands and declined
to about 20 percent after one rotation and to about 1 percent after several rotations of
intensive fiber extraction. The trend is consistent with projections of Spies and others
(1988) for the Pacific Northwest. Our review suggests that if managers desire to
sustain biodiversity they should:
•
•
•
Ensure sustained provision of dying and dead wood—Hundreds of species
depend on dying and dead logs and trees. Where the goal is to sustain all of
the biological diversity, patchwise retention incorporating all structures is
helpful.
Retain trees and snags of both hardwoods and favored conifer species (larch,
Douglas-fir, ponderosa pine), particularly where hardwood species are not
abundant. Avoid creating monocultures of less preferred species, such as
lodgepole pine―Although they are favored nesting sites and provide the
only substrate for some bryophytes, we cannot rely solely on more decayprone hardwoods. The varied needs of forest organisms include well-decayed
snags, large hollow snags, and snags with loose slabs of bark. Hardwood
species will not accommodate all these needs, nor will any one species of
conifer. Because conifers are longer-lived and provide a longer-lasting
source of cavities than do hardwoods (Erskine 1977, Harmon and others
1986), they are more likely to sustain snags late into rotations. Conifer snags
are required by species foraging on bark beetles and wood-boring beetles,
and conifer logs last longer than do hardwood logs.
Retain a range of size and age classes of dead wood―Where safety
considerations eliminate older snags at harvest, managers should ensure that
snags can develop through the rotation. Although larger diameters usually are
selected by vertebrates, smaller snags and logs are used. The desirability of a
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•
•
•
304
range of decay classes is well documented for bryophytes, insects, terrestrial
breeding salamanders, and birds. Well-decayed snags present greater safety
risks and are more easily retained in patches. Unless reserve patches are very
large, recruitment of well-decayed snags must occur outside of reserve
patches. Snags may never become well decayed if operational guidelines
require snag-falling. Either no-work zones are required during subsequent
entries, or silvicultural systems that do not require frequent entries should be
employed in at least some areas. Well-decayed snags will not develop at all
during a rotation if no trees die until late in the rotation. Retaining declining
live trees, or recently-dead snags, ensures timely onset of decay.
Ensure that some large trees or snags are retained―Although individual
birds use a wide range of tree or snag sizes, they tend to select larger ones
when available. Current data suggest that conifer cavity trees > 50 cm would
accommodate most bird species, and most hardwood trees can be smaller
(Bunnell and others 2002). Studies of vertebrate-forest relations have
concentrated where trees are larger and more valuable, so existing data
overestimate requirements where trees are smaller. A diameter > 30
centimeter will accommodate most bird species in less productive, inland
forest types (Bunnell and others 2002). Some mammals select trees or snags
> 50 centimeters in diameter (e.g., marten, black bear), and use down wood
50 to 150 centimeter in diameter (Davis 1996, Raphael and Jones 1997,
USDA Forest Service 1996). Given how larger mammals use space, large
pieces of down wood for such species can be well distributed across large
areas. Large trees and snags provide nesting or denning sites longer than do
small snags (Graham 1981, Morrison and Raphael 1993). However, smaller
snags provide foraging sites, and many more foraging sites are needed than
nesting sites.
Meet dead wood requirements for larger species in areas where the emphasis
is not on intensive fiber production―Binkley (1997) and Bunnell and others
(1999a) reviewed economic and ecological advantages of zoning the
intensity of fiber production. In some forest types, larger mammals prefer
significant amounts (100 to 200 cubic meters/hectare or more) and sizes (>
50 centimeter diameter) of down wood (review in Lofroth 1993). Needs of
those species are best provided in areas where late-successional attributes are
being maintained. Provision of some large pieces of dead wood in forests
where the dominant goal is fiber production may facilitate dispersal among
areas of more favorable habitat.
Don’t do the same thing everywhere―Retention of trees in patches reduces
safety risks of snag retention and windthrow (Coates 1997, Franklin and
others 1997) and facilitates retention of a range of size and decay classes. It
also concentrates recruitment of down wood. Debris piles are used by some
vertebrates (Morris 1984, Raphael and Jones 1997), but scattered pieces of
down wood favor other organisms. Dispersed retention of individual snags,
or declining live trees intended to become snags, may be particularly
advantageous for perching birds, and for territorial secondary users, such as,
raptors and some small birds, but impact shrub nesters negatively by
encouraging aerial predators (Vega 1993). Any single approach will
disadvantage some group of species, so a range of practices is preferable if a
range of species is to be sustained in an area.
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
•
Limit salvage logging after forest fires―Fire suppression has reduced the
area of recent burns favored by several vertebrates. If all vertebrates are to be
sustained, salvage logging should not be performed over all burns, or the
entire area of large burns.
Acknowledgments
Our research and synthesis was supported by the Canadian Wildlife Service’s
Fraser River Action Plan, Forest Renewal British Columbia, Lignum, MacMillan
Bloedel (now Weyerhaeuser BC); and Western Forest Products. R. W. Campbell
provided unpublished data from the British Columbia Nest Records Scheme; D.
Huggard, W. Klenner, K. Martin, and C. Steeger also generously provided
unpublished data or unreduced data for us to reanalyze. The manuscript benefited
from reviews by D. Huggard and B. Marcot. This is Publication No. R-36 of the
Centre of Applied Conservation Biology, University of British Columbia.
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Appendix A—Native cavity and downed wood using vertebrates breeding in forests in the
Pacific Northwest and their state or provincial status.
Common
name
Amphibians
Scientific name
Pacific giant
salamander
Arboreal
salamander
Black
salamander
Clouded
salamander
Coeur
D’Alene
salamander
Ensatina
salamander
Western
redback
salamande
Dicamptodon
tenebrosus
Aneides lugubris
Reptiles
Western skink
Western fence
lizard
Ruber boa
CA mountain
kingsnake
Racer
Ringneck
snake
Sharptail
snake
Cavity
2
DW
X
3
BC
State or Province1
AB WA OR
CA
R
X
Aneides
flavipunctatus
Aneides ferreus
X
S
X
S
Plethodon
idahoensis
X
Ensatina
eschscholtzii
Plethodon
vehiculum
X
R
X
X
Eumeces
skiltonianus
Sceloporus
occidentalis
Charina bottae
Lampropeltis
zonata
Coluber mormon
Diadophis
punctatus
Contia tenuis
X
X
X
B
S
X
X
X
R
S
Birds
Order
Anseriformes
Barrow’s
goldeneye
Bufflehead
Common
goldeneye
Common
merganser
Hooded
merganser
Red-breasted
merganser
Wood duck
314
Bucephala
islandica
Bucephala albeola
Bucephala clangula
S
S
S
S
S
Mergus merganser
S
Lophodytes
cucullatus
Mergus serrator
S
S
Aix sponsa
S
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
(appendix A continued)
Common
name
Order
Falconiformes
American
kestrel
Barred owl
Boreal owl
Flammulated
owl
Northern hawk
owl
Northern
pygmy-owl
Northern sawwhet owl
Spotted owl
Western
screech-owl
Order
Apodiformes
Vaux’s swift
Order
Piciformes
Acorn
woodpecker
Black-backed
woodpecker
Downy
woodpecker
Hairy
woodpecker
Lewis’s
woodpecker
Northern
flicker
Nuttall’s
woodpecker
Pileated
woodpecker
Red-breasted
sapsucker
Red-naped
sapsucker
Three-toed
woodpecker
White-headed
woodpecker
Williamson’s
sapsucker
Yellow-bellied
sapsucker
Scientific name
Cavity
Falco sparverius
S
Strix varia
Aegolius funereus
Otus flammeolus
S
S
S
Surnia ulula
S
Glaucidium gnoma
2
DW
3
BC
State or Province1
AB WA OR
S
S
B
S
S
S
B
S
Aegolius acadicus
S
B
Strix occidentalis
Otus kennicottii
S
S
R
R/B
Chaetura vauxi
S
Melanerpes
formicivorus
Picoides arcticus
P
Picoides pubescens
P
T
S
S
T
wP
Picoides villosus
P
B
Melanerpes lewis
wP
B
Colaptes auratus
wP
Picoides nuttallii
wP
Dryocopus pileatus
P
Sphyrapicus rubber
P
Sphyrapicus
nuchalis
Picoides tridactylus
P
Picoides
albolarvatus
Sphyrapicus
thyroideu
Sphyrapicus varius
T
CA
S
S
P
S
S
wP
R
P
R\B
S
P
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(appendix A continued)
Common
name
Scientific name
Ash-throated
flycatcher
Purple martin
Tree swallow
Violet-green
swallow
Boreal
chickadee
Chestnutbacked
chickadee
Mountain
chickadee
Plain titmouse
Siberian tit
Pygmy
nuthatch
Red-breasted
nuthatch
White-breasted
nuthatch
Brown creeper
Bewick’s wren
Myiarchus
tyrannulus
Progne subis
Tachycineta bicolor
Tachycineta
thalassina
Poecile hudsonicus
wP
Poecile rufescens
wP
Poecile gambeli
wP
Parus inornatus
Parus cinctus
Sitta pygmaea
wP
wP
wP
Sitta canadensis
wP
Sitta carolinensis
wP
Certhia americana
Thryomanes
bewickii
Troglodytes aedon
Troglodytes
troglodytes
Sialia currucoides
C
S
Sialia mexicana
S
House wren
Winter wren
Mountain
bluebird
Western
bluebird
Cavity
2
DW
3
BC
State or Province1
AB WA OR
R
S
CA
S
S
S
S
S
S
S
S
X
S
S
Mammals
Order
Insectivora
Common
shrew
Dusky shrew
Pacific shrew
Pygmy shrew
Trowbridge’s
shrew
Order
Chiroptera
Big brown bat
California
myotis
Fringed myotis
Hoary bat
Keen’s longeared myotis
Little brown
myotis
Long-legged
myotis
316
Sorex cinereus
X
Sorex monticolus
Sorex pacificus
Sorex hoyi
Sorex trowbridgii
X
X
X
X
Eptesicus fuscus
Myotis californicus
S
S
Myotis thysanodes
Lasiurus cinereus
Myotis keenii
S
S
S
Myotis lucifugus
S
Myotis volans
S
R
B
B
R
S
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
(appendix A continued)
Common
name
Scientific name
Cavity
Northern longeared myotis
Silver-haired
bat
Southern red
bat
Yuma myotis
Myotis
septentrionalis
Lasionycteris
noctivagans
Lasiurus blossevilli
C
Myotis yumanensis
S
Order
Rodentia
Creeping vole
Heather vole
Northern redbacked vole
Southern redbacked vole
Western redbacked vole
White-footed
vole
Columbian
mouse
Deer mouse
Pinon mouse
Sitka mouse
Douglas’
squirrel
Least
chipmunk
Long-eared
chipmunk
Northern
flying squirrel
Red squirrel
Sonoma
chipmunk
Townsend’s
chipmunk
Western gray
squirrel
Yellow-pine
chipmunk
Order
Carnivora
Red fox
Bobcat
Lynx
Ermine
Fisher
Least weasel
Tamias
quadrimaculatus
Glaucomys
sabrinus
Tamiasciurus
hudsonicus
Tamias sonomae
BC
B
State or Province1
AB WA OR
B
CA
S
S
S
X
X
X
X
R/B
X
X
S
X
X
X
X
S
X
R
X
S
S
X
X
Tamias townsendii
Sciurus griseus
DW
3
S
Microtus oregoni
Phenacomys
intermedius
Clethrionomys
rutilus
Clethrionomys
gapperi
Clethrionomys
occidentalis
Phenacomys
albipes
Peromyscus oreas
Peromyscus
maniculatus
Peromyscus truei
Peromyscus
sitkensis
Tamiasciurus
douglasii
Tamias minimus
2
S
S
Tamias amoenus
X
Vulpes vulpes
Lynx rufus
Lynx Canadensis
Mustela erminea
Martes pennanti
Mustela nivalis
X
X
X
X
X
X
S
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
S
S
R/B
B
S
S
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How Dead Trees Sustain Live Organisms—Bunnell, Houde, Johnston, and Wind
(appendix A continued)
Common
name
Scientific name
Cavity
2
DW
3
BC
State or Province1
AB WA OR
CA
Long-tailed
Mustela frenata
R
S
weasel
American
Martes Americana
S
X
S
marten
Mink
Mustela vison
X
Black bear
Ursus americanus
S
X
R
Raccoon
Procyon lotor
S
X
1
R = red listed; B = blue listed; S = sensitive species; E = endangered species; T = threatened species;
Sources include: Alaska Department of Fish and Game internet site as of July 1997
(www.state.ak.us/adfg/); British Columbia Ministry of Environment, Lands and Parks (1992, 1996);
Alberta Environmental Protection Status of Wildlife internet site as of December 1996
(www.gov.ab.ca/env/fw/); Rodrick and Milner (1991) for Washington; Marshall and others (1996) for
Oregon; and the U.S. Fish and Wildlife Service Division of Endangered Species internet site as of April
1999 for California. Other jurisdictions of the Pacific Northwest listed no species dependent upon cavity
sites or downed wood.
2
P = Primary Cavity Nester, wP = Weak Primary, S = Secondary Cavity Nester (obligate), C = Cave or
Crevice; (may use cavities, especially during winter).
3
Uses downed wood for breeding and/or feeding; X = Strongly Associated.
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