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Forest Ecology and Management 276 (2012) 24–32
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Forest Ecology and Management
journal homepage: www.elsevier.com/locate/foreco
Effects of single-tree selection harvesting on Rose-breasted Grosbeak (Pheucticus
leudovicianus) demography in a northern hardwood forest
Sonya Richmond a,⇑, Erica Nol b, Dawn Burke c, Jay R. Malcolm a
a
Faculty of Forestry, University of Toronto, 33 Willcocks St., Toronto, Ontario, Canada M5S 3B3
Environmental and Life Sciences Graduate Program and Biology Department, Trent University, Peterborough, Ontario, Canada K9J 7B8
c
Southern Science and Information Unit, Ministry of Natural Resources, 659 Exeter Rd., London, Ontario, Canada N6E 1L3
b
a r t i c l e
i n f o
Article history:
Received 17 January 2012
Received in revised form 14 March 2012
Accepted 16 March 2012
Available online 19 April 2012
Keywords:
Single-tree selection silviculture
Hardwood forest
Rose-breasted Grosbeak
Reproductive success
Songbird
Population decline
a b s t r a c t
Single-tree selection harvesting is frequently used in the tolerant hardwood forests of North America, but
little is known about how it affects the reproductive success of migratory songbirds. Many songbirds that
breed in tolerant hardwoods, including the Rose-breasted Grosbeak (Pheucticus leudovicianus) are experiencing population declines across their breeding ranges. We studied habitat characteristics and reproductive success of Rose-breasted Grosbeaks in stands harvested 0–5, 16–20, and 21–25 years previously
and in reference stands (un-harvested for >50 years) in Algonquin Provincial Park, Ontario, Canada (n = 3
per age class). Recently harvested stands had higher cover from regenerative growth and lower sapling,
understory, and canopy cover than other treatments, whereas reference stands had higher basal area and
sapling cover. Pairing success was significantly lower in the reference stands than in all other post-harvest treatments, and the number of fledglings per successful nest was significantly lower in the reference
stands than in the 0–5 years post-harvest stands. Density and population growth rate were significantly
positively correlated, suggesting that density may be an adequate indicator of habitat quality for Rosebreasted Grosbeaks in forested landscapes. Older males were present at higher densities, initiated their
nests earlier, and produced significantly more fledglings per nest than younger males in all treatments.
Habitat characteristics did not differ significantly between nests with second-year and after-second-year
males, suggesting greater reproductive output of older males was likely due to experience rather than
monopolization of better quality territories. We concluded that single-tree selection harvesting was beneficial to Rose-breasted Grosbeaks in our predominantly forested study area. Population growth rates
were below replacement levels in the 21–25 years post-harvest and reference treatments, but populations were stable in the younger regenerating stands. Thus, in a continuously forested landscape this species’ declines are probably not attributable to single-tree selection harvesting.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
In the tolerant hardwood forests of North America (i.e., forests
dominated by deciduous, shade-tolerant trees; Hunter, 1990)
uneven-aged harvesting systems such as selection cutting are often
used because they are thought to mimic small-scale disturbances
typical of this forest region (Arbogast, 1957, Franklin, 1989, OMNR,
2000), and therefore to have minimal impacts on wildlife. However,
many migratory songbirds that breed in the hardwood forests of
northeastern North America are currently experiencing broad-scale
population declines (DeGraaf and Yamasaki, 2003; Stutchbury,
2007; Vanderwel et al., 2007). Many studies have documented a decline in the abundance of mature-forest species following selection
⇑ Corresponding author. Tel.: +1 905 239 5971.
E-mail addresses: [email protected] (S. Richmond), [email protected]
(E. Nol), [email protected] (D. Burke), [email protected] (J.R. Malcolm).
0378-1127/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.foreco.2012.03.015
harvesting (i.e., Doyon et al., 2005; Holmes and Pitt, 2007; Haché
and Villard, 2010), whereas early- and mid-successional species
generally become more abundant following this type of harvesting
(i.e., Simon et al., 2000; Jobes et al., 2004; Holmes and Pitt, 2007).
Demographic responses to selection harvesting are much less frequently studied, and results are often mixed, with nest survival of
some species apparently unaffected following harvesting (Bourque
and Villard, 2001; Gram et al., 2003; Moore et al., 2010; LeBlanc
et al., 2011), but other species experiencing reduced reproductive
success despite increased abundance (Robertson and Hutto, 2007;
Poulin et al., 2010). Determining whether silvicultural techniques
such as selection harvesting are contributing to avian population
declines is important, because much of the hardwood forest in eastern North America, which represents valuable habitat for many
songbird species, is currently subject to forest management.
Assessments of whether selection harvesting alters habitat
quality for declining songbirds typically rely on the assumptions
S. Richmond et al. / Forest Ecology and Management 276 (2012) 24–32
of the ideal free distribution model, which predicts that better
quality habitats (i.e., areas where conditions support higher
reproductive output) will be inhabited by more individuals than
poorer quality ones (Fretwell and Lucas, 1970). However, additional factors can influence where individuals settle in forested
landscapes, including recent (Schmiegelow et al., 1997) or unpredictable (Van Horne, 1983) disturbances such as logging. In some
instances, harvested habitats may contain features typically present in high quality habitats, but support lower rates of survival
or reproduction than unmanaged areas (Weldon and Haddad,
2005). For example, Olive-sided flycatchers (Contopus cooperi) settle in higher densities in selection-harvested forests, but experience much lower nest survival in these areas than in natural
burns (Robertson and Hutto, 2007). As a result, despite recent evidence that density may be an adequate indicator of avian productivity in some hardwood regions (Underwood and Roth, 2002; Bock
and Jones, 2004; Perot and Villard, 2009), additional measures of
demography should be included in assessments of habitat quality
in managed areas to determine whether harvesting is contributing
to avian declines (Sallabanks et al., 2000).
Age-related differences in acquisition and defense of high quality territories (Catterall et al., 1989; Donovan and Stanley, 1995,
Pärt, 2001) may also cause a decoupling of the predicted positive
relationship between density and productivity if older, more experienced birds out-compete younger ones and monopolize resources
(Donazar and Feijoo, 2002). In some species, nest initiation date,
clutch size (Reynolds, 1997), nest survival (Forschler and Kalko,
2006), and number of successful fledglings (Préault et al., 2005;
Brown and Roth, 2009) are influenced by the age of the breeding
adults, with after-second-year birds often performing better than
second-year birds. If older birds are more successful at obtaining
and defending territories in preferred habitats, then the ratio of older to younger birds could be used to assess which habitats are perceived as most desirable by breeding individuals. Clarifying how
selection harvesting influences population age structure, and how
age structure in turn influences productivity, is useful for better
understanding the effects of anthropogenic activities such as forest
management on declining migratory songbirds.
The Rose-breasted Grosbeak (Pheucticus leudovicianus) is a Neotropical migrant that breeds in mixed and deciduous forests across
North America (Wyatt and Francis, 2002; Smith et al., 2006) and has
been experiencing annual population declines of 0.65% across
North America since 1966 (Sauer et al., 2008). This species is
tolerant of anthropogenic disturbances (Wyatt and Francis, 2002),
and attains relatively high densities in forests with high tree densities, extensive sapling and shrub cover, and well-developed understory layers (Webb et al., 1977; Possardt and Dodge, 1978).
Although such observations suggest that this species will use
forests regenerating after selection harvesting, existing research
on the effects of partial harvesting has provided mixed results.
Depending on the study, density responses to partial harvesting
have been recorded as positive (Holmes et al., 2004; Jobes et al.,
2004; Doyon et al., 2005; Guénette and Villard, 2005), neutral
(Duguay et al., 2001; Holmes and Pitt, 2007), or negative (Smith
et al., 2006; Vanderwel et al., 2007; Thompson et al., 2009).
Demographic studies of Rose-breasted Grosbeaks, which could help
explain these mixed responses, have so far only been undertaken in
the fragmented Carolinian forest, where both density and the proportion of older territorial males were lower in harvested fragments
than in unharvested ones (Smith et al., 2006). A better understanding of how silviculture influences Rose-breasted Grosbeak demography in predominantly forested landscapes is important because
a large portion of this declining species’ range (Wyatt and Francis,
2002) falls within managed forest landscapes.
Here, we compare Rose-breasted Grosbeak demography
(density, population age structure, pairing success, nest initiation
25
dates, and population growth rates) and habitat characteristics
among stands harvested recently (0–5, 16–20, 21–25 years ago)
and stands un-harvested for >50 years that represent reference conditions. In addition, we compare demographic and habitat measures
at nests attended by second-year and after-second-year males to
determine whether single-tree selection harvesting alters population age structure or differentially affects pairing success, nest survival, and population growth rates for individuals in these two age
classes. Finally, we examine the extent to which structural habitat
features in the stands correlate with demographic responses. Based
on the hypothesis that high densities reflect highly productive populations, we predicted that Rose-breasted Grosbeak density and
population growth rates would be highest in those stands in which
the sapling and understory vegetation layers were most developed,
because these conditions represent preferred nesting habitat (Smith
et al., 2007). We also expected that after-second-year males would
outcompete younger males for territories in these stands, and that
the older males would have higher reproductive success.
2. Material and methods
2.1. Study area
Sampling of habitat characteristics and Rose-breasted Grosbeaks was carried out in twelve tolerant hardwood stands in
Algonquin Provincial Park, Ontario, Canada (45°340 N, 78°040 W) in
2006–2008 (Fig. 1). Studied stands were located in the central
and southern part of the park (mean nearest neighbor distance of
stands = 9.3 km, range = 1.0–34.2 km), which has been harvested
using the single-tree selection system since the early 1970’s
(OMNR, 1998). This method of forest management removes
approximately one-third of the over-story basal area every 15–
25 years, resulting in uneven-aged stands with a mix of canopy
gaps, regenerative growth, and mature trees (Tubbs, 1977; Hunter,
1990; Seymour et al., 2002).
Stands used in this study were dominated by sugar maple (Acer
saccharum), American beech (Fagus grandifolia), eastern hemlock
(Tsuga canadensis), and yellow birch (Betula alleghaniensis). Three
stands (mean stand size = 34 ha) were studied in each of three
stages in the cutting cycle (0–5, 16–20, 21–25) and at >50 years
postharvest. Removal or trees in the latter set of stands, which
were selectively harvested for certain tree species >50 years previously, was relatively low, and these stands were used to represent
reference conditions.
We examined the 0–5 years post-harvest treatment to evaluate
structural and demographic conditions immediately after harvesting. Recommended harvest rotations for tolerant hardwood forests
range from 15 to 25 years (Tubbs, 1977; Hunter, 1990; Seymour
et al., 2002). The 16–20 years post-harvest treatment was chosen
to provide insight into conditions at the earliest stage when a second harvest could be carried out, and the 21–25 years post-harvest
treatment was chosen to investigate whether forest structure and
songbird demography had returned to reference conditions by
the end of the recommended harvest rotation.
2.2. Habitat measurements
We measured habitat characteristics in 5 m radius, randomlyplaced circular plots and at 5 m radius plots centered on nest locations. In each year, the number of random plots surveyed in each
stand was equal to the number of nests monitored in that stand,
providing an overall average of 13 (range = 10–17) randomly located plots and an equal number of nest plots per stand. We measured vegetation layers using a modified version of the James and
Shugart (1970) method. At each plot, percent cover by (1) forbs/
26
S. Richmond et al. / Forest Ecology and Management 276 (2012) 24–32
Fig. 1. Map of sites used to study the effects of single-tree selection on the demography of Rose-breasted Grosbeaks in Ontario, Canada. The gray polygon indicates Algonquin
Provincial Park and the numbers associated with each site indicate number of years post-harvest. Solid triangles = 0–5 years post-harvest stands, solid squares = 16–20 years
post-harvest stands, solid hexagons = 21–25 years post-harvest stands, and unfilled circles = reference stands.
grasses/ferns, (2) regenerative growth (woody growth 0.5–1.3 m
tall), (3) saplings (>1.3 m tall, <2.5 cm dbh), (4) understory
(P2.5 cm dbh, <10.0 cm dbh), (5) sub-canopy (>10–20 m tall),
and (6) canopy (>20 m tall) were visually estimated, and basal area
(m2/ha) was measured using a 2-factor cruising prism.
2.3. Nest monitoring
We visited each stand every 3–4 days in May–July from 2006 to
2008 to search for and monitor nests. Mean values for nest initiation date, clutch size, number of fledglings per successful nest,
and number of fledglings per nest were calculated for each stand.
We checked nests to confirm clutch size and the number of nestlings in each, but for some canopy nests (23% of nests) the number
of eggs could not be visually confirmed, so we estimated clutch size
using the mean number of eggs per nest across all nests in our
study. For nests that already contained eggs or young when we located them (12% of nests), published lengths for nest building
(4 days), incubation (12 days), and nestling development (10 days)
were used to calculate nest initiation dates (Scott, 1998). We did
not include nests that were initiated on or after June 3rd in our analysis of nest initiation dates (range = May 18th–June 15th) due to
uncertainty as to whether they were first attempts or not. We
calculated nest survival for each stand using the Mayfield method
(Mayfield, 1975).
2.4. Density and pairing success
In 2007 and 2008, we estimated the density of territorial male
Rose-breasted Grosbeaks in each stand using a modified version
of the spot mapping technique (Bibby et al., 2000). All locations
of male Rose-breasted Grosbeaks seen or heard during eight visits
between May 10th and June 2nd were mapped. Males that were
observed in the same location on three or more occasions were
considered territorial. In territories where no nest was located, a
male was assumed to be paired if a female was observed in that
territory on at least one occasion, and unpaired if no female was
observed after a total of 90 min of observation during one or more
observation periods (Bibby et al., 2000).
Because after-second-year (ASY) males can be visually distinguished from second-year (SY) males using binoculars (Pyle,
1997; Smith et al., 2006), we also calculated the proportion of
ASY: SY males in each stand by recording the age category of each
singing male that was mapped over the course of the season. A
small proportion of territorial males (3–5%) could not be accurately
aged, but the proportion was consistent among treatments. Finally,
we calculated density (total No. of territorial males/ha) and pairing
success (No. paired males/total No. males) for each stand for all the
males together, and for each age class.
2.5. Productivity
We calculated two different measures of productivity: (1) the
mean number of Rose-breasted Grosbeak fledglings produced per
active nest in each treatment and (2) population growth rates relative to replacement levels. For a population to be self-sustaining,
the annual female mortality rate must not exceed the average number of female offspring produced per female that survive to breed;
i.e., k must be P1 (Ricklefs, 1973; Pulliam, 1988; Donovan et al.,
1995). We calculated k for Rose-breasted Grosbeaks in each treatment using a two-age-class matrix population model in the program PopTools version 3.2.3 (Hood, 2010). Confidence intervals
for population ks were calculated using the bootstrapping technique with 100 iterations. The model incorporates estimates of annual adult and juvenile survival, which we did not directly measure.
Following Moore et al. (2010), we used estimates of annual adult
survivorship ranging from 0.40 to 0.61 obtained from literature
on other Neotropical migratory songbirds. Annual juvenile survivorship for each treatment was calculated as the product of
site-specific estimates of fledgling survival up until parental independence (Richmond, 2011) and annual adult survival, based on
the assumption that once juveniles reach independence their
S. Richmond et al. / Forest Ecology and Management 276 (2012) 24–32
annual survival rate is similar to that of adults (Moore et al., 2010).
To calculate annual female fecundity (number of female offspring
per female per year) we assumed that Rose-breasted Grosbeak
young occur in a 1:1 sex ratio, that females who initially failed to
successfully nest would re-nest only once, and that the probability
of nest success for re-nesters would be the same as that experienced during first nest attempts (Smith et al., 2006; Moore et al.,
2010). Following Moore et al. (2010), we used these values to calculate population growth rates for each treatment under best case
(high adult and juvenile survival) and worst case (low adult and
juvenile survival) scenarios.
2.6. Analyses
We used the Spearman Rank correlation coefficient to test for significant (P < 0.05) correlations among habitat measures, as the vegetation data rarely met assumptions of normality. When significant
correlations were indicated among the habitat measures themselves, we omitted the measure judged to be of lesser ecological
importance to Rose-breasted Grosbeak demography from further
analysis (Burnham and Anderson, 2002; Driscoll et al., 2005). Specifically, seedling cover was omitted due to significant (P < 0.05) correlations with regenerative growth (+), saplings (), canopy cover (),
and basal area (). Percent cover in the canopy and sub-canopy layers were significantly positively correlated, so these variables were
combined into one measure (hereafter referred to as canopy cover).
Sapling cover was positively correlated with understory cover
(r = 0.56), but we retained both because they were judged to be of
importance to Rose-breasted Grosbeak ecology.
For each habitat measure we calculated a mean value for each
stand based on measures from randomly located plots. We first
used redundancy analysis (RDA; ter Braak and Smilauer, 1998) to
test for significant differences among harvest treatments. If this
multivariate test indicated significant differences, response variables were then compared among treatments individually using
one-way analysis of variance (ANOVA). In this test, each stand
mean was weighted according to the total number of habitat surveys collected in that stand during the 3 year study, using the
WEIGHT statement in SAS v. 8.02 (SAS Institute Inc., 2001). This
procedure was undertaken to account for the fact that replicates
with more subsamples are expected to have results that are less
variable than replicates with fewer subsamples. Significant differences among harvesting treatments were investigated via Tukey’s
Studentized post hoc range test (a = 0.05). For all ANOVAs, model
assumptions of homogeneity and normality were verified by
examination of residuals.
For each demographic variable we calculated a mean value for
each stand in each year, and used RDA (ter Braak and Smilauer,
1998) to test for significant differences among harvest treatments
and study years. If this multivariate test indicated significant differences, response variables were then compared among treatments individually using two-way repeated measures analysis of
variance (ANOVA). As with the analysis of vegetation variables,
each stand mean was weighted according to the number of observations collected in that stand using the WEIGHT statement in SAS
v. 8.02 (SAS Institute Inc., 2001). We first tested the full model
(treatment, year, and treatment year interaction). Because none
of the two-way interactions were close to significant (i.e.,
P > 0.15), we removed the interaction terms and re-ran the models.
Significant differences among harvesting treatments were investigated by examining the least-square differences (a = 0.05). For all
ANOVAs, model assumptions of homogeneity and normality were
verified by examination of residuals.
We tested for a significant (P < 0.05) correlation between territory density (No. territories/ha) and population growth rate (k) under the best case scenario using the Pearson Product–Moment
27
correlation coefficient. Neither territory density nor population
growth differed among years, so single mean values for territory
density and population growth rate were calculated for each stand
and used in this analysis.
To test for differences among demographic and habitat measures at nests attended by SY and ASY males we first calculated
mean values for SY and ASY males in each stand. Too few SY males
were present in most stands to allow separate comparisons for
each study year; therefore, we combined data for the 2 years. We
used separate RDAs to test for significant differences in habitat
characteristics and demographic measures among harvest treatments. If the multivariate test indicated significant differences, response variables were then compared using individual split-plot
ANOVAs with harvest treatment (0–5, 16–20, 21–25, or >50 years
post-harvest) as the whole-plot factor and male age (SY or ASY)
as the split-plot factor (Littell et al., 1996). Any among-treatment
differences were examined using Tukey’s Studentized post hoc
range test (a = 0.05).
Finally, we used redundancy analysis (RDA; ter Braak and Smilauer, 1998) to examine the overall relationship between demographic variables and habitat characteristics in the four treatments.
Correlations and model assumptions were tested in Statistica 7
(StatSoft Inc. 2004), redundancy analysis was performed in CANOCO, and all other analyses were conducted in the PROC GLM module of SAS v. 8.02 (SAS Institute Inc., 2001).
3. Results
3.1. Habitat measurements
Redundancy analysis indicated that vegetation variables differed significantly with time since single-tree selection harvesting
(F15, 39.05 = 3.642, P = 0.002). Tukey’s tests showed significantly
lower basal area in the 0–5 years post-harvest stands compared
to all other treatments (Table 1). Sapling cover was significantly
lower in the 0–5 years post-harvest stands relative to the reference
treatment, and understory cover was significantly lower in the 0–
5 years post-harvest stands than in the 16–20 years post-harvest
treatment (Table 1). As a result of the openings created by harvesting, the 0–5 years post-harvest stands also had significantly higher
cover from regenerative growth than the 16–20 and 20–25 years
post-harvest stands. By 16–20 years post-harvest the understory
layer was more developed than in the other treatments, but
among-treatment differences were not significant (P = 0.066).
Regenerative growth and sapling cover were significantly lower
in 21–25 years post-harvest stands than in the reference stands.
The reference treatment also contained significantly higher basal
area than the 0–5 years post-harvest treatment, and higher sapling
cover than was present in the 0–5 and 20–25 years post-harvest
treatments.
3.2. Demographic variables
Mean density of Rose-breasted Grosbeaks in this study was
0.212 (SE = 0.01) territories / ha, mean pairing success was 0.963
(SE = 0.01), and the mean proportion of ASY males was 0.629
(SE = 0.11), indicating that more after-second-year males successfully held territories than second-year males. Between 2006 and
2008 we monitored a total of 173 nests (20, 73, and 80 nests in
the years 2006–2008, respectively), and we were able to determine
nest initiation dates for 157 first nest attempts. Mean nest initiation
date was May 26th (range = May 15th–June 2nd). Nest contents
could be verified for 52 nests, yielding a mean clutch size of 3.81
eggs, and an average of 3.44 fledglings per successful nest. Mean
daily survival rate (DSR) for Rose-breasted Grosbeak nests was
28
S. Richmond et al. / Forest Ecology and Management 276 (2012) 24–32
Table 1
Means of habitat variables (standard error in brackets) and results from one-way ANOVAs and Tukey’s tests comparing sites from four stages in the cutting cycle: 0–5, 16–20, 21–
25, and >50 years post-harvest (n = 3 per stage).
Time since harvest (years)*
Variable
Forb cover (%)
Regeneration (%)
Sapling cover (%)
Under-story cover (%)
Canopy cover (%)
Basal area (m2/ha)
*
P
0–5
16–20
21–25
>50
7.5 (1.2)a
20.2 (2.4)a
6.3 (0.9)a
20.8 (2.8)
81.2 (6.8)
22.1 (1.2)a
2.6 (0.5)b
7.6 (1.5)b,c
16.0 (3.9)a,b
34.5 (4.1)
90.3 (4.8)
27.2 (1.2)b
8.55 (2.6)a
5.5 (1.3)b
9.9 (2.5)a
27.1 (2.9)
89.4 (3.8)
27.0 (0.8)b
6.1 (2.4)a,b
13.9 (2.8)a,c
25.9 (4.1)b
25.3 (5.4)
93. 8 (7.2)
28.1 (1.8)b
0.005
0.005
<0.001
0.066
0.376
0.006
Superscript letters indicate significant differences among silvicultural treatments from Tukey’s HSD post hoc test (a = 0.05).
0.967 (n = 173 nests), mean period survival rate (PSR) for the entire
24 day period was 0.559, and mean productivity was 2.02 fledglings
per nest.
Pairing success and the number of fledglings per successful nest
were significantly lower in reference stands than in at least one
other post-harvest treatment, and density, nest survival, and productivity were also all lowest in the reference stands, although
not significantly so (Table 2). The proportion of ASY males compared to SY males was highest in the reference stands and lowest
in the 21–25 years post-harvest treatment, although the differences were not significant (P = 0.773; Table 2). Nests were initiated
significantly later in 2008 than in 2006 or 2007.
On average, female fecundity was 1.02 female fledglings per
female (Table 3). Under the worst case scenario (low adult and
juvenile survivorship) all treatments were population sinks. Under
the best case scenario (high adult and juvenile survival), the
population growth rate was stable in the 16–20 years post-harvest
treatment and very close to stable in the 0–5 years post-harvest
stands, but all other treatments remained sinks. Both population
growth rate and density were lowest in the reference stands and
highest 16–20 years after harvesting, and at the stand level there
was a significant correlation between density and population
growth rate (r = 0.61, P < 0.05; Fig. 2).
3.3. Relationship between habitat and demographic variables
The first two axes of the RDA analysis explained 57% of the variance in demographic variables (41.6% and 15.4% for the first and
second axes, respectively; Fig. 3). High sapling cover and basal area
were strongly associated with reference stands (upper right quadrat). Regeneration and canopy cover were weakly associated with
harvested stands (lower left quadrat). All demographic variables
showed a weak positive association with harvested stands, except
for nest initiation date, the proportion of territorial ASY males com-
Table 2
Means of demographic variables (standard error in brackets) and results from two-way repeated measures ANOVAs and least-squares means tests comparing sites from four
stages in the cutting cycle: 0–5, 16–20, 21–25, and >50 years post-harvest (n = 3 per stage) and three study years (2006–2008). Standard error and sample sizes (n) are shown in
brackets, with n representing the number of observations per treatment.
Demographic variable
Time since harvest (years)
Density (No. territories /ha)
Pairing success (prop. males paired)
Proportion of territorial ASY males
Nest initiation date 2006
2007
2008
Clutch size
No. fledglings/successful nest
Mayfield period nest survival rate
Productivity (No. fledglings/nest)
*
*
0–5
16–20
21–25
>50
0.246 (0.02, 71)
0.990 (0.01, 55)a
0.598 (0.08, 54)
May 27 (5.50, 2)
May 24 (0.88, 25)
May 27 (0.46, 25)
4.000 (0.07, 21)
3.619 (0.13, 29)a
0.524 (0.09, 55)
2.305 (0.35, 55)
0.254 (0.03, 64)
0.972 (0.02, 34)a
0.634 (0.08, 34)
May 26 (1.81, 6)
May 24 (1.03, 15)
May 28 (0.56, 16)
3.667 (0.17, 27)
3.333 (0.24, 21)ab
0.598 (0.09, 34)
1.603 (0.28, 34)
0.187 (0.02, 55.25)
1.000 (0.0, 38)a
0.550 (0.09, 36)
May 24 (1.28, 7)
May 22 (0.87, 18)
May 27 (0.87, 22)
3.625 (0.12, 13)
3.500 (0.16, 25)ab
0.654 (0.12, 38)
2.282 (0.47, 38)
0.162 (0.01, 43)
0.892 (0.01, 29)b
0.725 (0.11, 27)
May 24 (1.03, 2)
May 25 (1.12, 13)
May 27 (0.76, 13)
3.833 (0.40, 6)
2.833 (0.48, 13)b
0.437 (0.09, 29)
1.383 (0.62, 29)
P (treatment)
P (year)
0.202
<0.001
0.773
0.690
0.580
0.751
0.532
0.017
0.425
0.462
0.529
0.084
0.341
<0.001
0.205
0.060
0.321
0.175
Superscript letters indicate significant differences between silvicultural treatments from Tukey’s HSD post hoc test (a = 0.05).
Table 3
Population growth rates for Rose-breasted Grosbeaks breeding in 12 tolerant hardwood stands in Algonquin Provincial Park harvested by single-tree selection. Stands represent
four stages in the cutting cycle: 0–5, 16–20, 21–25, and >50 years post-harvest. Models show intrinsic (k) and exponential (r) population growth rates under best case (high adult
and juvenile survival) and worst case (low adult and juvenile survival) scenarios. Treatments are designated as stable if k is P1 and a sink if k is <1.
Time since harvest (years)
Best case scenario
0–5
16–20
21–25
>50
Worst case scenario
0–5
16–20
21–25
>50
a
b
c
Adult survival probabilitya
Annual juvenile survivalb
Female fecundityc
k
Upper and lower 95% CI
Population status
0.61
0.61
0.61
0.61
0.337
0.417
0.249
0.198
1.019
1.012
1.022
1.012
0.966
1.025
0.893
0.848
0.864–1.070
0.922–1.128
0.790–0.996
0.745–0.951
Stable
Stable
Sink
Sink
0.40
0.40
0.40
0.40
0.221
0.274
0.163
0.130
1.019
1.012
1.022
1.012
0.713
0.760
0.649
0.614
0.610–0.816
0.657–0.863
0.546–0.752
0.511–0.717
Sink
Sink
Sink
Sink
Estimates obtained from literature on other Neotropical migratory songbirds.
Fledgling survival probability multiplied by annual juvenile survival rate, assumed to be equal to adult survival probability.
Mean No. of female offspring/successful nest/year, assuming one attempt at re-nesting following nest failure.
S. Richmond et al. / Forest Ecology and Management 276 (2012) 24–32
1.4
Population growth rate (λ)
1.2
0-5
16-20
21-25
1
> 50
0.8
0.6
0.4
0.2
0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Density (no. territories/ ha)
Fig. 2. Relationship between territory density and population growth rate (k) under
the best case scenario for Rose-breasted Grosbeaks in Algonquin Provincial Park,
Canada. Solid triangles = 0–5 years post-harvest stands, solid squares = 16–20 years
post-harvest stands, solid diamonds = 21–25 years post-harvest stands, unfilled
circles = reference stands.
pared to SY males, and clutch size, which were not clearly associated with any treatment. Nest initiation, clutch size, and the proportion of ASY males were all positively correlated with
understory cover, whereas the remaining demographic parameters
were positively associated with regenerative growth and canopy
cover and negatively associated with forbs, sapling cover, and basal
area.
3.4. Age of male parent
Redundancy analysis indicated that there were no significant
differences in percent cover by regenerative growth, saplings,
understory, and canopy cover or in basal area at nests attended
by SY and ASY males (F5, 14 = 0.42, P = 0.827). The interaction between harvest treatment and male age (SY or ASY) was also not
significant (F15, 39.05 = 0.78, P = 0.691).
29
Demographic measures for SY and ASY males did not differ significantly among harvest treatments (F15, 39.05 = 1.06, P = 0.539),
suggesting that single-tree selection did not differentially affect
SY and ASY males. The interaction between harvest treatment
and male age was also not significant (F15, 39.05 = 0.87, P = 0.669).
However, demographic measures differed significantly between
SY and ASY males (F15, 39.05 = 5.89, P = 0.010).
In all treatments, territory density was significantly higher for
ASY males than for SY males (Table 4). Nests attended by ASY
males were initiated significantly earlier than nests attended by
SY males, and productivity (No. fledglings produced per pair) was
also significantly higher for ASY males than for SY males. Differences between nests attended by SY males and those attended
by ASY males were not significant for clutch size, mean number
of fledglings produced per successful nest, or period survival rate,
but many of these measures tended to be slightly higher at nests
attended by ASY males than those attended by SY males.
4. Discussion
Habitat characteristics and demographic measures differed
among stands harvested by single-tree selection 0–5, 16–20,
21–25 years previously and our reference stands. As in other
studies, the most recently harvested stands generally had higher
cover from regenerative growth, lower basal area, and lower
sapling, understory, and canopy cover than other treatments
(Robinson and Robinson, 1999; Flaspohler et al., 2002; Jobes
et al., 2004; Moore et al., 2010; LeBlanc et al., 2011). The structural changes associated with single-tree selection appeared to
have positive effects on both Rose-breasted Grosbeak pairing success and the number of fledglings produced per nest, although
population growth rates were only stable under best case scenarios. Territory density and population growth rate were positively
correlated, and both measures were highest in regenerating
stands prior to the normal age of second entry (e.g., 20–25 years),
suggesting that single-tree selection harvesting in the continuously forested landscape of Algonquin Provincial Park is beneficial
to this species.
Fig. 3. Relationship between seven demographic variables for Rose-breasted Grosbeaks and six habitat features in four post-harvest treatments in Algonquin Provincial Park,
Canada. Demographic variables are mean territory density, proportion of territorial ASY males, pairing success, nest initiation date, clutch size, daily nest survival rate,
interval nest survival rate, number of fledglings per successful nest, and productivity. Habitat features are percent cover by forbs, regenerative growth, saplings, understory,
canopy cover, and basal area. Solid triangles = 0–5 years post-harvest stands, solid squares = 16–20 years post-harvest stands, solid hexagons = 21–25 years post-harvest
stands, unfilled circles = reference stands. Inter-sample scaling was used, and the RDA was on the correlation matrix.
30
S. Richmond et al. / Forest Ecology and Management 276 (2012) 24–32
Table 4
Means of demographic variables (standard error in brackets) and results from split-plot ANOVAs and Tukey’s tests comparing sites from four stages in the cutting cycle: 0–5, 16–
20, 21–25, and >50 years post-harvest (n = 3 per stage) and three study years (2006–2008). Results are shown for second-year (SY) and after-second-year (ASY) males.
Demographic variables are density (No. territories/ha), nest initiation date in May, clutch size, period nest survival rate, fledglings produced per successful nest, and fledgling
produced per pair.
Demographic variable
Time since harvest (years)
0–5
Density
Nest initiation date
Clutch size
Nest survival
Fledglings/s. nest
Fledglings/pair
16–20
21–25
>50
ASY
SY
ASY
SY
ASY
SY
ASY
SY
0.12 (0.04)
24 (0.35)
4.00 (0.14)
0.67 (0.09)
3.71 (0.17)
2.83 (0.47)
0.07 (0.00)
27 (1.24)
4.00 (0.00)
0.63 (0.04)
3.60 (0.23)
1.53 (0.49)
0.11 (0.04)
24 (0.78)
3.62 (0.09)
0.70 (0.11)
3.25 (0.12)
1.72 (0.57)
0.05 (0.00)
27 (0.87)
4.00 (–)
0.54 (0.03)
4.00 (–)
1.64 (0.48)
0.08 (0.02)
22 (1.49)
3.83 (0.12)
0.86 (0.05)
3.67 (0.236)
2.80 (0.27)
0.06 (0.01)
28 (0.88)
3.80 (0.283)
0.71 (0.16)
3.80 (0.283)
2.08 (0.69)
0.09 (0.03)
25 (1.65)
4.00 (0.58)
0.63 (0.03)
3.67 (0.33)
1.71 (0.18)
0.03 (0.01)
28 (1.16)
2.00 (0.58)
0.68 (0.14)
2.00 (0.58)
1.00 (0.41)
The higher pairing success of Rose-breasted Grosbeak males
and the higher productivity in harvested stands suggests that forest management may provide additional nesting habitat and/or
food resources for Rose-breasted Grosbeaks. In forest fragments,
low male pairing success is associated with low resource availability (Gibbs and Faaborg, 1990; Burke and Nol, 1998; Zanette, 2001),
suggesting that females may be more sensitive to reductions in
food resources than males during the site selection and pair formation period. Previous research has shown that while some invertebrate taxa increase in abundance following selection harvesting,
others decrease (Werner and Raffa, 2000; Vance and Nol, 2003;
Latty et al., 2006; Nol et al., 2006; Shields et al., 2008). In early
spring invertebrate prey may be more abundant or accessible for
Rose-breasted Grosbeaks in the harvested stands than in the reference treatment. Seasonal variation in arthropod availability, and
the Rose-breasted Grosbeak’s plasticity with regard to nest site
selection and foraging substrates (Smith et al., 2006, 2007) may
help explain why all demographic measures were not significantly
higher in one treatment relative to the others.
4.1. Population growth rate
Population growth estimates indicated that under the best-case
scenario, Rose-breasted Grosbeaks in our study area were replacing themselves in the 16–20 years post-harvest treatment, and in
the 0–5 years post-harvest stands under favorable conditions for
adult and juvenile survival (i.e., low predation risk, high food abundance). We did not include stands that had been harvested between 6 and 15 years previously, because when we established
our study design, we were most interested in whether, at the time
of the next cutting cycle, any negative effects of silviculture would
have disappeared. However, since we found beneficial effects, it
would now be helpful to examine the same demographic variables
for grosbeaks in stands of intermediate ages to determine whether
this species benefits for the full 20 years of regeneration prior to
the next cutting cycle. Although population growth rates have
not previously been determined for Rose-breasted Grosbeaks in
predominantly forested landscapes, this species is frequently
found in regenerating forests 15 years after harvesting (Pelletier
and Dauphin, 1996), and often increases in abundance following
logging (i.e., Jobes et al., 2004; Doyon et al., 2005), supporting
our estimates of positive population growth.
Population growth models are highly sensitive to small changes
in estimates of annual survival rates for adults and juveniles (Porneluzi and Faaborg, 1999; Powell et al., 1999; Woodworth, 1999),
which despite technological advancements in radio tracking are
still difficult to accurately obtain (Paradis et al., 1998; Anders
and Marshall, 2005). We incorporated a range of values for adult
and juvenile survival by calculating growth rates under best and
P
P
Treatment
Male age
0.375
0.335
0.620
0.312
0.127
0.142
0.005
<0.001
0.892
0.174
0.429
0.048
worst case scenarios, as well as using stand specific values for female fecundity, nest success, and fledgling survival to improve
accuracy (Anders and Marshall, 2005). Given the worst-case scenario for survival, all Algonquin hardwood forests would be considered sinks, but we did not find evidence that single-tree selection
harvesting was further contributing to the decline. Determining
accurate survival estimates will be essential for concluding anything more than the relative value of post-harvest stand age to this
species but is unlikely to allow us to determine whether populations are growing or declining at the scale of the park. Similarly, given the uncertainty in the population growth estimates under the
two scenarios, we are also not able to conclude whether other factors operating on the breeding grounds are responsible for the
overall declines observed in Ontario or not (Sauer et al., 2008).
4.2. Population age structure
Rose-breasted Grosbeak nests attended by after-second-year
males were initiated significantly earlier, and had significantly
higher productivity than nests attended by second-year males,
suggesting that more experienced breeders had a reproductive
advantage over less experienced birds. First-time breeders of other
species have shown lower nest survival than older individuals
(Pärt, 2001; Forschler and Kalko, 2006) and they sometimes occupy
poorer quality habitats (Møller, 1991; Holmes et al., 1996) or are
paired with lower quality mates (Holveck and Riebel, 2010). In
some cases, second-year birds may have limited access to highquality territories as a result of later arrival on the breeding
grounds, incomplete knowledge of where to find good territories,
and despotic distribution of more experienced breeders (Holmes
et al., 1996; Bayne and Hobson, 2001). In our study, even though
ASY males initiated their nests earlier than SY males, habitat characteristics at nests attended by SY males did not significantly differ
from those at nests with ASY males, suggesting that differences in
territory quality were probably not the main cause of higher productivity at nests with ASY males. Lack of experience, differences
in timing, or lower quality mates are more likely explanations for
the lower reproductive success of first-time Rose-breasted Grosbeak male breeders.
Our prediction that the proportion of older males would be
highest in the 16–20 years post-harvest treatment was not supported. Older, more experienced birds are sometimes more successful at obtaining and defending high quality territories than
less experienced individuals (Catterall et al., 1989, Donovan and
Stanley, 1995, Part 2001, Donazar and Feijoo, 2002). We did not detect a difference in the proportion of territorial ASY males among
treatments. The slightly higher proportion of ASY males in reference sites as compared to our other treatments may have been
the result of ASY males out-competing SY males in this treatment
S. Richmond et al. / Forest Ecology and Management 276 (2012) 24–32
for a more limited number of available territories than were offered in harvested stands. Older males were less productive in
the reference stands than in other treatments, but even in these
stands their productivity was higher than that of younger males
in most other treatments.
Although single-tree selection appeared to be beneficial for
Rose-breasted Grosbeaks in our study area, responses may be context specific. Residual basal area in forest fragments in the agricultural landscape of southwestern Ontario ranged from 23.1 to
24.5 m2/ha in harvested stands and from 24.5 to 29.0 m2/ha in
un-harvested stands (Smith et al., 2006; Moore et al., 2010), which
were similar to the basal area ranges observed in our stands. However, in the fragmented landscape the number of fledglings produced per successful nest was slightly lower in the harvested
stands than in the reference treatment (Smith et al., 2006),
whereas in our predominantly forested study area productivity
was significantly higher in the 0–5 years post-harvest treatment
than in the reference stands. As with other species (Rodewald,
2002; Phillips et al., 2005; Falk et al., 2011), Rose-breasted Grosbeak nests were parasitized by Brown-headed Cowbirds (Molothus
ater) in the fragmented agricultural landscape (Smith et al., 2006,
2007), but were not in our predominantly forested study area. Presumably, openings in the canopy created by silviculture, which
potentially expose songbird nests to visual predators and brood
parasites (Andrén, 1995; Hanski et al., 1996), had a greater impact
in the fragmented landscape than in our more forested one. These
results suggest that the effects of timber harvesting to some extent
depend on landscape context, and that adverse effects may be fewer in forested regions than in fragmented landscapes where cowbird parasitism is an added factor.
5. Conclusions
In our predominantly forested study landscape, single-tree
selection harvesting appeared to maintain or even improve habitat for Rose-breasted Grosbeaks. Many of the demographic measures we studied were lowest in the reference treatment, and
both density and population growth rates were highest in the
regenerating stands. Density and population growth rate were
positively correlated, indicating that for this species density was
an adequate indicator of habitat quality. Single-tree selection is
not equally beneficial to all migratory songbirds, and may be detrimental to some forest interior species. Nonetheless, for species
such as the Rose-breasted Grosbeak, which benefit from habitat
features characteristic of regenerating forests, this system may
present a useful management tool. Further research could investigate whether other silvicultural techniques, such as group-selection harvesting, also conducted at a small scale in Algonquin
Park (Tozer et al., 2010), provide greater advantages for this
species.
Acknowledgments
We thank K. Fletcher and the Algonquin Forestry Authority for
their participation and support of the project. P. Wilson, B. Woodworth, and G. Humphries provided invaluable assistance in the
field. Funding and in-kind support was provided by the Algonquin
Forestry Authority, Bancroft-Minden Forest Co., Canadian Forest
Service, Canadian Wildlife Service, Enhanced Forest Productivity
Science Program, Mazinaw-Lanark Forest Inc., Natural Sciences
and Engineering Research Council of Canada, Ontario Ministry of
Natural Resources, Ottawa Valley Forest Inc., Tembec Inc., and
Westwind Forest Stewardship. S.R. was supported by a Natural Sciences and Engineering Research Council scholarship.
31
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