Persistence of Tree Cavities Used by Cavity-Nesting Vertebrates Declines in Harvested
Forests
Author(s): AMANDA B. EDWORTHY and KATHY MARTIN
Source: The Journal of Wildlife Management, Vol. 77, No. 4 (May 2013), pp. 770-776
Published by: Wiley on behalf of the Wildlife Society
Stable URL: http://www.jstor.org/stable/23470724
Accessed: 25-08-2016 21:49 UTC
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The Journal of Wildlife Management 77(4):770-776; 2013; DOI: 10.1002/jwmg.526
HpT*l
pj.v
- L&
Habitat Relations
Wr.MiAf
Persistence of Tree Cavities Used by
Cavity-Nesting Vertebrates Declines in
Harvested Forests
AMANDA B. EDWORTHY, 1,2 Centre for Applied Conservation Research, Department of Forest Sciences, University of British Columbia,
2424 Main Mally Vancouver, BC V6T1Z4 Canada
KATHY MARTIN, Centre for Applied Conservation Research, Department of Forest Sciences, University of British Columbia, 2424 Main Mall,
Vancouver, BC V6T 1Z4 Canada, Environment Canada, Pacific Wildlife Research Centre, Robertson Road, RR1, Delta, BC V4K 3N2,
Canada
ABSTRACT An abundant supply of cavity-bearing trees is important for maintaining wildlife communities
in harvested forests. During harvesting, suitable trees and cavities are directly removed, and the longevity of
cavities in retained trees may be reduced by increased exposure to wind and other disturbance factors. We
examined patterns of cavity survival in retained trembling aspen (Populus tremuloides) trees in harvested stands
compared with those in unharvested mature stands by monitoring the persistence of individual cavities. We
followed 930 cavities in 3 harvest treatments for up to 17 years in pre-cut and uncut forest, and up to 13 years
post-harvest (reserve patches and dispersed retention), in temperate-mixed forests of interior British
Columbia, Canada. Average annual cavity loss rates were 5.6% in pre-cut and uncut forest, 7.2% for cavities
in trees retained in reserves, and 8.1% for cavities in retained trees dispersed throughout cuts.
Correspondingly, median cavity longevity was 15 years for cavities in pre-cut and uncut forest, 10 years
for cavities retained in reserves, and 9 years for those in dispersed retention. Risk of loss increased most for
cavities in living trees (factor of 2.17), but we found no detectable difference for cavities in recently dead trees
and trees with advanced decay. We suggest retention of a range of aspen size and decay classes to allow for
future cavity-tree recruitment in harvested stands. Inclusion of wildlife reserves as part of an overall forest
management plan will also help to mitigate the effects of windthrow and maintain long-lived cavity resources
required by a large portion of forest wildlife. © 2013 The Wildlife Society.
KEY WORDS British Columbia, forest harvesting, Populus tremuloides, tree cavities, tree hollows, variable retention,
wildlife reserves.
Several studies have suggested that general guiding principles
subsequent increase in loss of cavity-bearing trees retained in
for forest wildlife conservation should include retention of
cuts due to windthrow or other disturbance factors may occur.
Tree cavities for nesting and roosting are a vital multi
processes (Franklin et al. 2002, Lindenmayer et al. 2006, annual resource that are created by excavators and decay
Bergeron et al. 2007). Structural complexity is especially im processes, and are reused by a range of secondary cavity
portant when it includes persistent resources that are used over users for breeding and roosting (Martin and Eadie 1999,
several years or decades by a large number of species, such as a Edworthy et al. 2012). Reuse of a single cavity has been
structural complexity and maintenance of natural-disturbance
supply of tree cavities used for nesting and roosting by verte
brates (Tews et al. 2004). Tree cavities are at risk in harvested
documented up to 17 times in sequence in 13 years
(K. Martin, University of British Columbia, unpublished
landscapes because they are removed during logging and re data). Many forest and savannah ecosystems support large
stands helps to maintain structural complexity and mimics the
numbers of cavity-using species (Cockle et al. 2011). In
interior British Columbia (BC), Canada, more than 40
patchy effects of regular disturbance events in the landscape. In
species, or about 30% of forest vertebrates, use cavities for
generate slowly. Retention of cavity-bearing trees in harvested
addition to direct loss of cavity-bearing trees from harvesting, a nesting and shelter (Bunnell and Kremsater 1990, Martin
et al. 2004). Experimental studies in both managed and
primary forests show that cavity-nester abundance increases
Received: 15 September 2011; Accepted: 19 November 2012
Published: 4 February 2013
when nest boxes are added, and thus some cavity-nesting
species appear to be limited by nest cavity availability in these
forests (Newton 1994; Holt and Martin 1997; Aitken and
1 E-mail: amanda. edworthy@anu. edu.au
2Present address: Australian National University, Research School of Martin 2008, 2012; Cockle et al. 2010). The density of nest
Biology, Building 116, Canberra, ACT 0200, Australia.
770
The
Journal
of
cavities is determined by rates of excavation and cavity
Wildlife
Management
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•
77(4)
persistence (Cockle et al. 2011). In temperate forests of BC,
temperate and boreal forests of North America show that loss
long-lived cavities excavated by woodpeckers are the primary
rates increase following harvest (Huggard et al. 1999, Ruel
source of cavities for secondary cavity nesters (94% of nests;
et al. 2001, Garber et al. 2005, Mascarua-Lopez et al. 2006,
Russell et al. 2006). Detailed studies of windthrow loss show
Aitken and Martin 2007, Edworthy et al. 2012). Thus, the
abundance and longevity of tree cavities has a strong influ
ence on cavity nest-site availability for cavity-nesting com
that landscape features and harvest treatment methods also
affect risk of loss (e.g., Scott and Mitchell 2005).
munities. Long-lived tree cavities should be protected by
The risk of tree fall of retained trees or snags has been
examined in harvested stands (e.g., Angers et al. 2011), but
few studies have examined the risk to individual cavities,
forest management plans to support cavity-using species and
overall forest biodiversity.
Variable retention has been applied in harvested sites to
improve wildlife conservation by retaining cavity-bearing
trees as individuals spread throughout a cut (dispersed re
which may have different loss dynamics (see Everett and
tention) or grouped in wildlife reserves, which contain a
majority of non-cavity-bearing trees (Lance and Phinney
greater risk of collapse than trees without cavities in har
vested eucalypt forests. We used a 17-year study of cavity
formation and longevity, in both harvested and unharvested
forest stands, to address the following 2 questions: 1) Does
forest harvesting reduce the longevity of cavities in retained
Otter 2004). In south-eastern Australia, Gibbons et al.
(2008) found that cavity-bearing trees were at 2.6 times
2001). In general, the results of variable retention are positive
for wildlife; species richness in stands where variable reten
tion harvest techniques are applied is often greater than in
uncut forest, mainly because of increases in edge species (e.g.,
trees compared with those in uncut forest? and 2) What is the
Lance and Phinney 2001, Leupin et al. 2004). Drever et al.
(2008) found that both woodpecker richness and general
forest bird richness were greater in harvested sites with
risk of loss of cavities with regard to the type of tree retention
(dispersed retention vs. group retention in wildlife reserves)?
STUDY AREA
retention of trembling aspen (Populus tremuloides) and large
Douglas-fir (Pseudotsuga menziesii) than in uncut forest.
Despite these initial positive results, trees retained in har
vested stands may have an elevated risk of loss from wind
throw, additional to those lost directly from harvesting
activities, resulting in longer term declines in cavity abun
We monitored the persistence of individual cavities for up to
17 years between 1995 and 2011 at 20 study sites in interior
BC, Canada, all within 50 km of the City of William's Lake
(51° 51'N, 122° 21'W; Table 1). Sites ranged in size from 7
to 32 ha, and were in the warm, dry interior Douglas-fir
bioregion (Meidinger and Pojar 1991). Prior to harvest, all
dance. Studies of tree and snag dynamics after harvesting in
Table 1. Harvest dates, site area (ha), area harvested (ha), proportion of total basal area removed (ha), proportion of aspen (Populus trem
retained (ha), reserve types, and sample sizes of cavities in retained aspen trees at 20 sites in interior British Columbia, Canada (1995-2011).
Proportion Proportion Reserve type Cavity in
Years Year of Site Area basal area aspen basal (no. of Total Cavities in dispersed
Site monitored harvest area harvested removed area retained reserves) cavities3 reserves3 retention3
Uncut sites
Solitary Woods
Tongue
Y
1995-201lb
1995-2011b
NAC
15
NA
NA
NA
NA
21
NA
NA
16
NA
NA
NA
NA
41
NA
NA
1995-2011
NA
20
NA
NA
NA
NA
51
NA
NA
NA
South Hawks Control
1996-2100
NA
25
NA
NA
NA
NA
7
NA
NA
Mailbox Control
1997-2010
NA
15
NA
NA
NA
NA
5
NA
NA
Little Till 2
1998-2011b
1995-2011b
NA
20
NA
NA
NA
NA
20
NA
NA
Military Gate
NA
20
NA
NA
NA
NA
33
NA
NA
7 Mile
1997-2011
NA
15
NA
NA
NA
NA
51
NA
NA
Doc English
1995-2001
NA
15
NA
NA
NA
NA
17
NA
NA
56
4
23
96
1
18
55
0
44
78
0
68
69
8
48
arvested sites
South Hawks Clear-Cut
1996-2011
1998
14
14
0.83
1
Dingwall 1
Coldstream Triangle
1997-2011
2000
20
11
0.3
0.86
1996-2011
2001
7
7
Fork
1996-2011
2001
15
14
0.79
1
Knife
1996-2011
2001
15
12
0.95
0.42
Dingwall 2
1997-2011
2001
22
8
0.29
0.96
1997-2011
2001
22
13
0.57
0.89
Missed Moose
0.77
0.06
Little Till 1
1998-2011b
2002
9
9
0.74
0.77
Mailbox Cutd
1996-2011
2002
32
22
0.61
0.98
Hermit Hill
2002-20llb
2004
26
19
0.44
1
Rock Lake
1995-2011
2010
15
6
0.34
1
Patch (1), Edge (1)
Edge (1)
Patch (1), Edge (1)
Strip (2), Edge (1)
Patch (1), Edge (1)
Edge (1)
Strip (1), Edge (1)
Edge (1)
Strip (1), Edge (1)
Patch (1), Edge (1)
Edge (1)
58
3
36
67
21
28
64
71
0
20
0
18
63
8
29
51
4
7
a If a cavity tree was first in a pre-cut or uncut site, and then the site was harvested, the cavity tree will be counted in both applicable columns.
b Parts of these sites were severely burned shortly after the field season of 2010 in a human-started wildfire. The last year monitored for cavity trees in burned
sections was 2010.
c NA: not applicable as site was not harvested.
d Mailbox Cut was harvested a second time in 2005, increasing proportion of the basal area removed to 0.85, and decreasing the proportion aspen basal area
retained to 0.76.
Edworthy and Martin • Tree Cavity Persistence in Harvested Forest
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771
sites were mature, mixed-continuous forest, and were domi
nated by lodgepole pine (Pinus contorta\ 42% by stem count),
interior Douglas-fir (28%), hybrid spruce (Picea engelmannii x
glauca\ 18%), and trembling aspen (12%).
Of our 20 sites, 11 were harvested between 1998 and 2010,
0.10 -I
0.081
0.08 -
0.072
which involved removal of pine and/or hybrid spruce,
with retention of most trembling aspen and large or small
(pre-commercial) Douglas-fir, either as part of reserves or
0.06 -
0.056
trees dispersed throughout the harvested area (Table 1).
Three types of reserves occurred at our study sites: 1) patches
0.04 -
of trees approximately 1 ha in size, 2) strips of trees
50-100 m wide, along roads or creeks, and 3) forest edges
(up to 50 m into the forest from a cut). The remaining 9 sites
-
0.02 -
were classified as uncut, although 2 of these sites had selective
removal of large interior Douglas-fir 60-80 years ago.
0.00 -1
Pre-cut
and uncut
METHODS
Data Collection
Reserves Dispersed
retention
Figure 1. Mean annual loss rates of cavities in aspen trees (Populus tremu
loides) in 3 harvest treatments in interior British Columbia, Canada (1995
2011). Harvest treatments included 1) pre-cut and uncut (n — 13 yr), 2)
reserves (n = 10 yr), and 3) dispersed retention (n = 11 yr). Error bars
To locate active cavity nests, we systematically searchedrepresent
the
standard error.
study sites for signs of fresh excavation and existing cavities
(May-July, 1995—2011). We visually monitored cavities up
to 5.2 m high using ladders, mirrors, and flashlights, and
and was used in our models to represent the general level of
tree decay.
starting in 2006, we monitored cavities up to 15 m using
video cameras on extendable poles. We monitored nest
Statistical Analysis
cavities in subsequent years until they were no longer usable.
We considered cavities unusable in 3 cases: 1) the entire To
tree
determine general patterns in cavity loss rates across
or the section of tree containing the cavity broke off or
fell treatments, we estimated annual loss rates by dividing
harvest
down, 2) the cavity chamber decayed, or 3) the cavitythe
ennumber of cavities lost by the number of cavities standing
trance healed over in a living tree. The primary source
in of
the previous year. We averaged these annual loss rates for
cavity loss at our sites was windthrow or stem breakage each
(92%;
harvest treatment (pre-cut and uncut, reserve, dispersed
retention). To assess temporal trends at harvested stands, we
Edworthy et al. 2012).
We classified cavities into 3 harvest treatment types: 1)modeled
pre mean cavity loss rates as a function of number of
cut and uncut, which included all cavities >50 m into the
years since harvest, and used a simple linear regression to test
forest interior, 2) reserves, which included cavities in retained
for a relationship. We only included time since harvest
patches and forest edges <50 m from a cut, and 3) dispersed
categories with at least 10 cavities at risk of loss in the
retention, which included cavities in individual trees spread
analysis. We did not include cavities in the reserve treatments
throughout the harvested area. We also indexed tree decay
in the temporal analysis because of their low sample size.
class during the first year we found a cavity containing Our
an long-term data following the persistence of individual
active nest. We defined an active nest as a cavity containing
at enabled us to fit Cox proportional-hazards survival
cavities
(CPH) models, which model risk of loss based on time to
least 1 egg or nestling. We did not monitor cavities that never
held an active nest during the study period (although event
some data (cavity age at loss). These models allow the
were checked occasionally), as we could not be certain inclusion
that
of multiple covariates and the use of right-censored
data (observations with a minimum survival time, but where
they provided suitable nest sites, especially if they were
formed by decay rather than by excavators. We assigned
the exact time of loss is unknown); for example, a cavity that
decay class using the decay scale developed by Thomas
persisted to the end of the study has a minimum survival
et al. (1979) and adapted for aspen trees by Martin et
al.but not a time to loss. The hazard ratios produced by
time,
(2004). For this analysis, category A includes live trees
CPH models represent the proportional risk of cavity loss
(healthy or with signs of decay), category B includes recently
compared with baseline hazard level (e.g., pre-cut and uncut
forest).
dead trees, and category C includes dead trees with advanced
decay (see Fig. 1 in Edworthy et al. 2012). Decay class does
Because we first found some of the cavities when secondary
potentially change throughout the lifespan of a cavity; cavity-nesters
how
used them, rather than the year they were
ever, we only measured cavity decay class each year a cavity
excavated, their exact age was unknown. We tested for a
was used, not in years when a cavity was unoccupied. Thus,
difference in persistence between cavities of known versus
the measure of decay class taken when we first found a cavity
minimum age, and for an interaction of this variable with
active is the only measure we have for all cavities in the study,
harvest treatment and decay class, but found none. Thus, we
772 The Journal of Wildlife Management • 77(4)
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1.0
included all cavities in the analysis and dropped the age
certainty variable from our final model.
Because sample sizes were low in reserves (n = 49), we first
0.8
fit a simple survival model, which included only the effects of
harvest treatment (pre-cut and uncut, reserve, dispersed reten
tion). We then fit a more detailed model excluding the reserve
treatment. This detailed CPH regression modeled hazard of
cavity loss as a function of harvest treatment (pre-cut and
uncut, dispersed retention), decay stage (A, B, C), and an
interaction of harvest treatment and decay stage. Harvest
treatment was included as a time-dependent covariate, which
allowed the covariate value to change partway through the
ro
gj 0.6
3
W
>
5 0.4
co
O
Pre-cut and uncut
0.2
Reserve
lifespan of a cavity (e.g., when forest surrounding an existing
cavity was harvested). The interaction term of harvest treat
ment and decay stage allowed us to test whether cavities in
Dispersed retention
0.0 -
more decayed trees were more susceptible to loss after harvest
0
5
than those in less decayed trees. We completed all analyses
R (Therneau and Lumley 2009, Fox and Weisberg 2011; R
RESULTS
15
Cavity
using the base and survival packages in the statistical program
Version 2.14.2, www.r-project.org, accessed 1 Jun 2012).
10
ag
Figure 2. Simple model of survival of cavities in aspen trees (Populus tremu
loides) in 3 harvest treatments in interior British Columbia, Canada (1995
2011) Harvest treatments included 1) pre-cut and uncut (n = 383 cavities),
2) reserves (n = 49 cavities), and 3) dispersed retention (n = 560 cavities).
Shaded areas represent 95% confidence intervals. Median survival (repre
sented by the dashed line) was 15 years for cavities in pre-cut and uncut
forest,
10 years
We monitored the fates of 930 cavities in aspen trees
for
up
to 17 years in total and up to 13 years after harvesting. We
also located cavities in lodgepole pine (23), interior Douglas
in reserves, and 9 years in dispersed retention.
fir (29), hybrid spruce (11), and birch (Betula spp.; 1),
We
but
further
we
investigated patterns in cavity persistence
did not analyze survival for these tree species becauseacross
of their
harvest treatments using CPH survival analysis, a
low sample sizes. Of the cavities in aspen trees, 383technique
were in that uses time to event data (age at cavity
Because we did not detect a difference between
dispersed retention, 49 were embedded in wildlife loss).
reserves,
and 560 were in pre-cut or uncut forest. Sixty-two
cavities
cavities of known and minimum age, we pooled all
(6%) of the cavities counted in harvested forest (reserves
cavitiesor
in these analyses (/Jage certainty = 0.012, SE =
dispersed retention) were initially in uncut forest,0.028,
and then
^age certaintyxharvest treatment 0.011, SE — 0.027,
were retained in a cut during their lifespan. More /^age
than certainty
90%
X decay class A 0.012, SE 0.032, /^age certaintyx
of cavities in harvested stands were excavated by woodpeck
decay class B = —0.019, SE = 0.033). Because our sample size
ers after harvesting occurred.
in reserves was limited (n = 49), we first fit a simple model
estimate
The average annual rate of cavity loss in pre-cut to
and
uncut differences in risk of loss across all 3 harvest
forests was 0.056 ± 0.020 (mean ± SE), whereas
cavities Median cavity longevity was 15 years in pre-cut
treatments.
retained in reserves had loss rates of 0.072 ± 0.023
(27% forest, 9 years in dispersed retention, and 10 years
and uncut
increase), and cavities in dispersed retention had loss
in rates
reserves
of (Fig. 2). Risk of loss for cavities in pre-cut and
0.081 ± 0.024 (45% increase; Fig. 1). These differences
uncut forest (median longevity = 15 yr) was used as the
to compare to the other harvest treatments. Risk
among harvest treatments were non-significant atbaseline
an alpha
level of 0.05 (F = 1.62, P — 0.21; Fig. 1). Annual loss
of loss
rates
for cavities in dispersed retention increased by a factor
ranged from 0.020 to 0.068 at pre-cut and uncut
of stands,
1.46 (95% CI = 1.16—2.11; equivalent to a 46% increase),
and from 0.032 to 0.13 in dispersed retention. We
and risk
found
of loss for cavities in reserves increased by a factor of
no trend in loss rates with increasing time since harvest
1.30, though this was not significant (CI = 0.81-2.11;
(P = -0.0016, F = 0.30, P = 0.60).
Table 2, Fig. 2).
Table 2. Model parameters for Cox proportional-hazards model predicting hazard rate in relation to harvest treatment for 930 cavities in retained asp
(.Populus tremuloides) in interior British Columbia, Canada (1995-2011).
Variable
Coefficient
Hazard ratio* (e"^)
95% CIb
1.08
0.28
0.81-2.11
3.17
<0.001
1.16-1.86
Z
1.30
0.25
1.46
0.12
0
1
Reserve
0.27
0.38
Dispersed retention
P
SE of the coefficient
Pre-cut and uncut
a The hazard ratio is equal to exp(estimated coefficient) and represents the change in hazard compared to a baseline hazard rate (represented by cavities in pre
cut and uncut forest in this analysis). A hazard ratio of 1 indicates no change in hazard, a hazard ratio above 1 indicates an increase in hazard (shorter
lifespan), and below 1 indicates a decrease (longer lifespan).
b When the 95% confidence interval for the hazard ratio does not include 1, the coefficients differ significandy from 1 at the 5% level.
Edworthy and Martin • Tree Cavity Persistence in Harvested Forest
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773
Table 3. Model parameters for Cox proportional-hazards model predicting hazard rate in relation to harvest treatment and decay stage for 881 tree cavities in
interior British Columbia, Canada (1995-2011).
Variable" Coefficient Hazard ratio (^coe*)b SE of the coefficient Z P 95% CIC
0
Pre-cut and uncut
1
Dispersed retention
0.77
2.17
Decay A
Decay B
Decay C
0
1
Pre-cut and uncut X decay A
Dispersed retention x decay B
Dispersed retention x decay C
0.22
3.48
<0.001
1.4-3.35
1.68
5.39
0.25
6.82
<0.001
3.32-8.75
1.85
6.35
0.24
7.69
<0.001
3.97-10.18
0
1
-0.39
0.68
0.32
-1.23
0.22
0.37-1.26
-0.61
0.54
0.32
-1.91
0.06
0.29-1.02
aTree decay categories are defined as A: alive (healthy or with signs of decay), B: recently dead, and C: dead with advanced decay.
b The hazard ratio is equal to exp(estimated coefficient) and represents the change in hazard compared to a baseline hazard rate (represented by cavitie
cut or uncut forest, decay category A). A hazard ratio of 1 indicates no change in hazard, a hazard ratio above 1 indicates an increase in hazard (sh
lifespan), and below 1 indicates a decrease (longer lifespan).
c When the 95% confidence interval for the hazard ratio does not include 1, the coefficients differ significandy from 1 at the 5% level.
dispersed
retention, and a 27% increase in cavities in reserv
Our more detailed analysis of cavity survival accounted
for
compared with pre-cut and uncut stands. This finding,
effects of cavity-tree decay stage, in addition to harvest
statistically non-significant, suggests that rese
treatment. Harvest treatment and cavity-tree decay though
class
provide some protection from windthrow. Our stud
were significant predictors of risk of loss, but an interaction
persistence
of tree cavities builds on past studies of sn
between these variables was not significant (Table 3, Fig.
3).
longevity after harvesting and the general dynamics of cav
Cavities initially in living trees (decay category A) in pre-cut
losstoto directly address the effects of harvesting on ca
and uncut forest had the lowest risk of loss, corresponding
longevity
(Huggard et al. 1999, Ruel et al. 2001, Everett
long lifespans (median longevity > 15 yr), and we used
this
Otter 2004, Garber et al. 2005, Scott and Mitchell 2
combination as a baseline hazard level. Multiple comparisons
Edworthy et al. 2012). But our study is the first to use
among harvest treatments and decay class factors showed
control-impact design with several harvest t
that risk of loss after harvesting increased significantly before-after
in live
trees (factor of 2.17; Z = 3.47, P < 0.001), but we did
not to measure impacts of harvesting on a key habit
ments
detect an effect for cavities in recently dead trees or trees
attribute
with
for cavity-dependent vertebrates.
Our survival models, which incorporate cavity age, pr
advanced decay (respectively, Z = 1.92, P = 0.055 and
Z = 0.53, P = 0.60; Fig. 3). Cavities in recently deadduced
trees similar results, with risk of loss and cavity longe
the most in dispersed retention (46% increa
(decay category B) and trees with advanced decay (C) decreasing
had a
risk
much lower overall longevity (6-7 yr) than cavities in
live of loss, longevity = 9 yr), and to a lesser exten
reserves (30% increase in risk of loss, longevity = 10 y
trees (>15 yr; Fig. 3).
in comparison to baseline survival in pre-cut and uncut fo
DISCUSSION
(median longevity = 15 yr). Our more detailed surv
which accounted for cavity-tree decay class, show
As expected, we found elevated loss rates model,
and decreased
although
cavity longevity in harvested compared withthat
pre-cut
and an interaction between decay class and har
treatment
was not significant, including decay class impr
uncut stands, and this effect was intermediate
in reserves,
the model;
the estimated increase in risk of loss for cavitie
although data in reserves were limited. Our analysis
of annual
loss rates showed a 45% increase in loss rates for
in
livecavities
trees isolated
in dispersed retention was 177%, sub
Figure 3. Survival curves showing the effects of harvest treatment and decay class on survival of tree cavities in aspen trees (Populus
Columbia (1995-2011). These predictions are based on a model of risk of cavity loss as a function of cavity-tree decay class, harves
of decay class and harvest treatment. Median survival is represented by the dotted line. Shaded areas represent 95% confidence i
774
The
Journal
of
Wildlife
Management
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•
77(4)
tially greater than the general estimate produced by the
simpler model for the same cavities (46%). Despite the
than in mature aspen forest in southwestern Colorado (Scott
and Crouch 1988). In a study of woodpeckers in Appalachian
relatively large increase in loss rates of cavities in live trees,
live trees survive longer than dead and decayed trees in both
uncut and harvested stands. Thus, cavity loss rates would
have been significantly greater if only dead trees (snags) were
forests, Conner and Crawford (1974) found that woodpeck
ers foraged intensively on insects under the bark of fallen
trees and branches left in clearcuts (slash). Within 5 years
after clearcutting, this food source disappeared and wood
pecker abundance at harvested stands declined (Conner and
retained in harvested plots (Angers et al. 2011, Edworthy
et al. 2012). Also, the impact of decay class should be
Crawford 1974). Our system is probably more similar to
considered in survival models because some species depend
on trees in advanced stages of decay for excavation (e.g., red
breasted nuthatch \Sitta canadensis], black-capped chickadee
\Poecile atricapillus\, and downy woodpecker \Picoidespubes
cent), whereas others prefer live trees (e.g., red-naped sap
Conner and Crawford (1974), but woodpecker populations
at our sites remained high for more than 5 years (Edworthy
sucker [Sphyrapicus nuchalis\\ Martin et al. 2004).
Increased risk of loss for retained wildlife trees and the
et al. 2011). Over the long term, cavity availability will largely
depend on the growth and decay rates of the retained trees.
With variable retention harvesting, a range of tree age and
decay classes can be retained to ensure a steady supply of
suitable cavity trees as the forest regenerates.
potential for windthrow at newly created edges is a well
documented phenomenon; trees at newly created edges haveMANAGEMENT IMPLICATIONS
greater exposure to wind but may not have developed the
In light of the modest, but measurable reduction of cav
structural integrity to withstand the wind (Ruel et al. 2001,
survival in harvested sites, we suggest retention of cav
Scott and Mitchell 2005, Mascarua-Lopez et al. 2006). In
bearing trees should be as comprehensive as possible
the southern interior of BC, loss rates of subalpine fir (Abies
ensure the maintenance of an adequate supply of nest c
lasiocarpa) trees increased by a factor of 2.4 in harvested sites
ties. Given the greater persistence of cavities in retaine
compared with uncut stands and Engelmann spruce
reserves compared with those in dispersed retention, inc
(P. engelmatinii) loss rates were 20 times greater in harvested
ing reserves as part of an overall retention plan will ben
forest (Huggard et al. 1999). The high variation in rates of
cavity-dependent wildlife, both in terms of greater cav
loss of retained trees likely reflects variation across forest
supply and other habitat values. Additionally, the increas
systems in factors that affect windthrow susceptibility, such
woodpecker abundance at our sites, and subsequent recr
as topography, climate, soil composition, and tree species.
ment of new cavities, emphasizes the importance of retai
Studies of cavity persistence in relation to tree and forest
suitable non-cavity-bearing trees for future recruitmen
stand factors are more limited, but factors related to wind and
the short term, a range of decay classes, including living tr
stability, including distance to edge, tree diameter, and tree
to trees with advanced decay should be retained, as stro
decay stage, strongly affected loss rates of cavity trees in some
excavators (woodpeckers) prefer live trees, whereas w
forest systems (Everett and Otter 2004, Lindenmayer and
excavators (e.g., chickadees and nuthatches) most of
Wood 2010, Edworthy et al. 2012). Many of the cavities at
use soft, decaying trees (Martin et al. 2004, Drapeau et
our sites were in live aspen trees, which persist the longest,
2009). Retention of young, healthy trees and saplings wi
and thus contribute the most to nest-cavity availability for
important for cavity recruitment in the long term.
the majority of species (Edworthy et al. 2012). However,
cavity-nesting species that rely on more quickly decaying
ACKNOWLEDGMENTS
dead trees might suffer greater losses of available nesting
substrates in harvested stands.
Many field assistants and graduate students contribute
At our sites, retention of potential cavity trees, includingdata collection, including A. Adams, A. Norris, M. Hun
H. Kenyon, and A. Koch in recent years. Thanks t
most aspen and large interior Douglas-fir, after harvesting
provided nest sites, which allowed woodpecker populationsWiebe, V. LeMay, P. Drapeau, S. Converse and 1 an
to increase, even at harvested sites (Edworthy et al. 2011). mous reviewer for their constructive and valuable commen
on this manuscript and to J. Goheen for early discussi
Thus, despite an increase in loss of cavities due to harvesting,
The Natural Sciences and Engineering Research Council
excavation of fresh cavities by woodpeckers produced an
increase in cavity density (Edworthy et al. 2011).Canada (NSERC) provided funding in the form of a Can
Woodpeckers tend to select excavation sites in living trees,Graduate Scholarship to A. Edworthy and a Spe
which are relatively persistent, and the majority of wood Strategic Grant to K. Martin. Other sources of fun
were the Sustainable Forest Management Network, For
peckers excavate new cavities annually and do not reuse their
cavities from previous years (Aitken and Martin 2004,'BlancRenewal BC, FIA Forest Sciences Program of BC, a
and Martin 2012, Edworthy et al. 2012). Thus, the increaseEnvironment Canada. A. Edworthy received UBC sup
in woodpecker populations in harvested stands with reten from the Mary and David Macaree Fellowship, the Don
S. McPhee Fellowship and the Bert Hoffmeister Scholar
tion of potential nest trees may facilitate the maintenance or
increase of cavity nester populations (e.g., Drever et al. 2008,in Forest Wildlife, as well as a Junco Technologies Awa
Drever and Martin 2010). The response of woodpeckers to from the Society of Canadian Ornithologists. T
Industries Limited (Cariboo Woodlands) provided logist
harvesting is mixed among forest systems. Densities of nest
ing woodpeckers were lower in 6- to 10-year-old clearcutsand financial support from 1996 to 2003.
Edworthy and Martin • Tree Cavity Persistence in Harvested Forest 775
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