Stand characteristics of three forest types within the dry interior forests of British Columbia, Canada: Implications for biodiversity

Forest Ecology and Management 256 (2008) 114–120

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Stand characteristics of three forest types within the dry interior forests of British Columbia, Canada: Implications for biodiversity Dustin K. Oaten a,1, Karl W. Larsen b,* a b

Department of Forest Resources Management, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 Department of Natural Resource Sciences, Thompson Rivers University, PO Box 3010, Kamloops, British Columbia, Canada V2C 5N3

A R T I C L E I N F O

A B S T R A C T

Article history: Received 18 December 2007 Received in revised form 2 April 2008 Accepted 4 April 2008

Coarse wood debris and plant communities within forested ecosystems play vital roles in the life history of many wildlife species. Descriptions of the characteristics and dynamics of these ecosystem properties therefore are crucial for guiding managers interested in maintaining biodiversity and site productivity. In North America, stands of trembling aspen (Populus tremuloides) have been identified as making an important contribution to the biodiversity in certain forested landscapes. These stands are relatively rare within the dry conifer-dominated forests of interior British Columbia (western Canada), yet detailed work on their structural and vegetative properties has been lacking. To this end, we investigated coarse woody debris and plant communities in aspen stands, within a dry-forest ecosystem near Kamloops, British Columbia, Canada. Downed and standing coarse woody volumes, plant diversity, and plant abundance all were significantly higher in aspen stands, as compared to neighbouring Douglas-fir (Pseudotsuga menziesii var. glauca) and mixedwood (aspen + Douglas-fir) stands. Aspen stands also contained a relative abundance of snags and coarse woody debris. Because of these attributes, these stands likely serve as an integral part of the dry interior British Columbia forest ecosystems, representing areas of high biological diversity. This information, along with the fact that these stands may be threatened due to anthropogenic influence, make it critical that a significant component of aspen remains within these forested ecosystems. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Coarse woody debris Douglas-fir Populus tremuloides Snags Trembling aspen

1. Introduction During the past decade, forest management in Canada generally has shifted away from sustained yield timber management towards sustainable forest management (Armstrong et al., 2003), where the conservation of biological richness is recognized as an important ecological criterion of forest sustainability (Canadian Council of Forest Ministers, 1998). This shift has occurred with the understanding that ecosystems are composed of plants, animals, microbes, and the physical environment, existing as interdependent, functional units within climatically, geologically, and geographically defined boundaries (Welsh and Droege, 2001). There also has been a concomitant recognition that traditional forest management has a direct influence on the structure and dynamics of plants, snags, coarse woody debris and

* Corresponding author. Tel.: +1 250 828 5456. E-mail addresses: [email protected] (D.K. Oaten), [email protected] (K.W. Larsen). 1 Current address: Forsite Consultants Ltd., 1274 McGill Road, Kamloops, British Columbia, Canada V2C 6N6. 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.04.013

stand level attributes, and that dynamics of these attributes directly affect biological diversity (Voller and Harrison, 1998). The amount, distribution and diversity of forest attributes such as standing coarse woody debris (SCWD), downed coarse woody debris (DCWD), understory plant diversity and structure, as well as other stand attributes all play vital roles in the life history of many species (e.g. Robinson and Holmes, 1984; Ohmann et al., 1994; Hagan and Grove, 1999; Bowman et al., 2000; Carey and Harrington, 2001; Hoyt and Hannon, 2002; Lohr et al., 2002; Ulyshen et al., 2004). The amount, structure, and dynamics of SCWD and DCWD also can influence species composition, nutrient cycling and site productivity (Spies et al., 1988). Descriptions of the characteristics and dynamics of SCWD, DCWD and plant communities within forest ecosystems are crucial to guiding managers in maintaining biodiversity and ecosystem productivity (Clark et al., 1998). Understanding the different roles of coarse woody debris (CWD) in forests is very important to the effective management of forests, because removal of CWD can lead to unexpected alterations within ecosystems (Harmon et al., 1986; Graham et al., 1994). Further, knowledge of the dynamics of CWD and plant communities is important for understanding stand dynamics and variation in wildlife habitat among stands.

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The coarse woody debris and plant community components of trembling aspen (Populus tremuloides Mitchx.) stands are of particular interest, giving the apparent role these forest types play in contributing to regional biodiversity. Within western North America, trembling aspen may be a keystone species whose ecological importance to the landscape and biodiversity is only bested by riparian vegetation belts (Campbell and Bartos, 2001). Generally, the relatively large amount of woody debris within aspen stands appears to play a strong role in this pattern (Stelfox, 1995). To date, this pattern has been studied mainly in boreal forest ecosystems (Gustaffson and Eriksson, 1995; Stelfox, 1995), and the western United States (Kay, 1997; White et al., 1998; Bartos, 2001). It is unclear how and if this pattern extends to the dry inland (‘interior’) forests of British Columbia, in western Canada. When compared to the boreal forest, aspen stands within the dry forests of south-interior British Columbia present a striking contrast. Here, these stands often occur as isolated patches, embedded in montane, dry forests that are dominated by conifer species, particularly interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco). These stands also are relatively rare on the landscape and often comprise less than 5% of low-elevation montane ecosystems (White et al., 1998). Despite their uniqueness and rarity, relatively little baseline work has been done on these aspen stands, especially in comparison to the boreal ecosystem. In particular, plant community composition within aspen stands in the interior of British Columbia has received little attention (but see description in Lloyd et al., 1990). This knowledge gap may be potentially important, as the complex dynamics of these stands may allow for high species diversity, with several microhabitats likely available for different plant species (Lee and Sturgess, 2001). From a conservation standpoint, there also is significant evidence that the aspen component within North American forests is being significantly reduced by historical forest management, fire suppression, poor seedbed conditions, and increased herbivory from ungulates and cattle (Kay, 1997; White et al., 1998; Bartos, 2001). In temperate-zone landscapes dominated by coniferous forests, the amount of SCWD and DCWD as well as the density of deciduous trees (primarily aspen, Populus spp. and birch, Betula spp.) has been found to influence the composition and species richness of forest faunal communities (Angelstam and Mikuskinski, 1994). Although managers are increasingly aware that SCWD and DCWD are important components of forest ecosystems, they frequently lack information on what biomass and volume of this material is naturally present, and in what sizes, stem densities, and decay states it occurs (Clark et al., 1998). The purpose of this study was to investigate the woody debris and vegetation components of aspen stands occurring within the interior Douglas-fir region of BC. We sampled CWD and plant communities within aspen stands, in order to provide baseline data on these uncommon forest stand types, and compared these to mixedwood and Douglas-fir leading stands. This study was the basis for a larger project that included an examination of cavitynesting bird, small mammal, and carabid beetles communities (Oaten, 2007; Oaten and Larsen, unpublished). 2. Methods 2.1. Study areas Our field work was conducted near Kamloops, British Columbia, Canada (508430 N; 1208250 W). All study sites were located within the Douglas-fir forests of the Thompson River region; these forests are known as being a relatively cool and dry variant of the larger

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Fig. 1. Orthophoto of Kamloops Forest District, within the province of British Columbia, Canada (inset). The four locations of the nested study sites are as follows: B = Badger Lake, P = Pinantan Lake, M = Monte Lake, S = Stephens Meadow. As a point of reference, the City of Kamloops is situated near the centre of the photo, at the confluence of the two major river drainages.

Douglas-fir biogeoclimatic zone of interior British Columbia (Meidinger and Pojar, 1991). Stands of three types, aspen leading, mixedwood, and Douglas-fir leading, were selected within four geographically distinct areas (blocks), Monte Lake (A1, M1, F1), Stephens Meadow (A2, M2, F2), Badger Lake (A3, M3, F3) and Pinantan Lake (A4, M4, F4) (Fig. 1). Stands were identified as aspen or Douglas-fir leading if the applicable species component of these stands was 70%—mixedwood stands had a varying proportion of these trees but generally had 30% aspen and 65% Douglas-fir. The scarcity of aspen stands in this forest type, coupled with a required minimum stand size of 9 ha (a limitation imposed by vertebrate sampling), reduced the number of potential aspen study sites to a small number. From there, logistical constraints were used to make the final selection. Comparative Douglas-fir and mixedwood (Douglas-fir + aspen) stands were selected based on proximity to the aspen stands and the same minimum stand size requirements. 2.2. Stand structure Stand structure attribute sampling was carried out using three randomly located 0.04 ha plots (20 m  20 m) (McIntire and Fortin, 2006). All live trees within these plots were identified to species and measured for height with the use of a clinometer and measuring tape. A diameter tape was used to measure diameter at breast height (DBH). Tree ages were determined for mature trees within each site by increment boring (Spies et al., 1988) Douglas-fir and aspen trees at a height of 1.3 m. As aspen trees are susceptible to heart rot (Martin and Eadie, 1999), the assessment of age of these trees was augmented by cutting a cross section from recently fallen trees. Coring of aspen and Douglas-fir trees occurred during the late autumn, when seasonal tree morphology changes make increment boring more successful (DeByle and Winokur, 1985). All ages were determined in the lab using a compound microscope, and no age correction factor was applied. Five 0.05 ha fixed-radius plots were established within each study site to sample for downed CWD (DCWD) (Spies et al., 1988).

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All debris 7.5 cm diameter, whose midpoint fell within the confines of the plot, were measured. These debris were identified by species, measured for length (within the plot) and diameter at both ends. They also were given a decay-class rating based on the scale developed by Hautala et al. (2004). This scale allows for a simple identification of decay class for logs based on the hardness of the log and the amount of decay present. The volume of each piece was calculated from overall length and upper- and lower-end diameter using Smalian’s formula for cubic volume (Wenger, 1984). We also randomly established five 0.1 ha fixed-radius plots within each study site to measure and quantify SCWD (Spies et al., 1988; Bowman et al., 2000). Each dead tree (2.0 m in height) was identified to species, and DBH, height and decay class were recorded. The Maser decay class scale (Maser et al., 1979) was used to assign a decay class to each tree (1 = sound to 5 = highly decayed). Standard species specific volume equations were used to calculate overall SCWD volume (Schlaegel, 1975; Hann et al., 1987). 2.3. Understory vegetation The line intercept method was used to examine the percent cover and diversity of plant species within each study site (including the percent cover of bare ground). Line intercept sampling is a method of sampling vegetation based on measurement of all plants intercepted by the vertical plane of randomly located lines of equal length (Canfield, 1941). This line has length and vertical dimensions only. Three replicate 15 m transects were sampled in each study site. The line locations were randomly assigned, as was the direction in which they were sampled. Plant species were identified in accordance with Hitchcock and Croonquist (1973) and Parish et al. (1996). 2.4. Data analysis Data for plants and stand structure were analyzed with a twoway ANOVA using the function PROC GLM in SAS 9.1 (SAS Institute, 2006). We tested for differences in the number of, and total cover of mosses, herbs, and shrubs as well as total species richness (the number of plant species identified within a given study site) and cover between stand types. The volume of DCWD and SCWD

and the overall volume of CWD (m3/ha) were calculated and differences in these values were examined. Differences in all other stand attributes also were examined using two-way ANOVA. Pairwise t-tests were used for all post hoc tests with an associated significance level of 0.017 (0.05/number of potential comparisons) (Neter et al., 1996). All data not conforming to normality or equal variance were subject to various transformations (Zar, 1999). Percentage data were arcsine transformed to meet requirements of normality and equal variance (Zar, 1999) and all mean data are reported with standard error. 3. Results 3.1. Stand structure A summary of stand structure attributes within the different stand types is provided in Table 1. As a result of our site selection process, the aspen stands were composed of approximately 78% aspen, whereas on average the mixedwood stands were 32% aspen. The Douglas-fir stands were virtually devoid of aspen. Aspen stands were only significantly older than mixedwood stands (P = 0.013) and there were no significant differences in tree density, DBH and height (all Ps > 0.12). 3.2. Coarse woody debris A summary of DCWD characteristics in the three stand types appears in Table 2. The mean volume of DCWD was significantly different between the stand types. Aspen stands also had significantly larger numbers of wood in our largest size category (P = 0.004), and although not statistically significant (Ps = 0.07), they also had relatively larger numbers of smaller wood pieces. Although wood in all of the decay classes was more prevalent in the aspen stands, it was in classes 2 and 4 where the differences were most noticeable (Table 2). As shown in Table 3, the volume of SCWD also was significantly higher in the aspen stands, as were our estimates of snag density. Snags within the three larger diameter classes were more common in aspen, whereas the amount of small material was essentially the same across the three stand types. Snags in decay class 1 was considerably more prevalent in the aspen stands, but not significantly different within classes 2–5.

Table 1 Overview of stand structure attributes within each of the stands and the stand types Site

%Aspen

A1 A2 A3 A4

95.8 72.0 71.9 70.7

Mean

Tree age

Density (stems/ha)

DBH (cm)

Height (m)

2.8 0 15.6 24.3

96.8 100.7 103.2 101.4

477 434 430 366

19.0 25.5 27.0 28.0

17.2 23.3 22.9 23.6

77.6  3.0

10.7  2.8

100.5  0.7

427  11

24.9  1.0

21.8  0.8

M1 M2 M3 M4

36.1 33.5 36.2 34.2

62.2 62.8 59.6 65.0

86.9 91.7 88.1 88.9

360 553 254 187

23.7 20.7 33.8 26.9

21.6 19.7 25.2 17.4

Mean

32.5  1.5

65.1  1.4

88.9  0.5

339  40

26.3  1.4

21.0  0.8

284 243 270 247

32.0 29.9 30.4 27.6

21.2 23.1 24.0 19.2

261  5

30.0  0.5

21.9  0.5

D1 D2 D3 D4

0 0 0 0

Mean

0

%Fir

95.4 96.7 100.0 100.0 98.0  1.6

101.2 97.4 88.1 89.2 94.0  1.6

Note: A1–A4 refers to the 4 replicate aspen stands, and the same for M (mixed wood) and D (Douglas-fir); mean values provided with  1 SE.

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Table 2 Summary of characteristics of DCWD (volume and number of pieces of debris in diameter and decay classes) within aspen, mixedwood, and Douglas-fir stands and results of ANOVAs exploring differences in these characteristics Variable

Stands

F

P

t

P

Aspen (n = 4)

Mixedwood (n = 4)

Douglas-fir (n = 4)

Volume (m3/ha)

66.7  14.04

24.4  3.03

20.7  4.48

8.29 a

0.012

3.44b 3.16c

0.011 0.016

No. of wood pieces <10 cm 10–25 cm >25 cm

26.5  7.08 48.5  13.21 9.5  0.55

15.3  2.17 24.8  4.32 2.8  0.24

6.3  1.78 16.8  1.65 3.5  0.85

4.08 3.86 13.69a

0.067 0.074 0.004

4.24b 4.77c

0.004 0.002

Decay classes 1 2 3 4 5

4.8  3.55 46.5  17.24 23.0  9.03 8.5  1.33 4.0  0.82

2.0  0.40 25.8  2.69 9.5  3.11 3.8  2.17 1.7  1.03

1.0  0.41 11.0  1.40 5.0  1.59 4.0  0.72 3.5  2.36

0.76 7.33 a 1.69 11.34a 0.62

0.505 0.016 0.252 0.009 0.565

3.44c

0.011

3.25c

0.015

Note: Data are means  1 S.E. There were no significant differences detected between mixedwood and Douglas-fir stands (all Ps > 0.020, at a = 0.017, see Section 2). a Refers to a significant difference between stand types, at a = 0.05. b Refers to significantly higher values within aspen versus Douglas-fir. c Refers to significantly higher values in aspen versus mixedwood stands (a = 0.017).

and in Douglas-fir stands it was 16.25  0.94. Mean species richness for moss and lichens (F2,6 = 0.01; P = 0.991), grass (F2,6 = 0.55; P = 0.603), and shrubs (F2,6 = 1.74; P = 0.254) was not significantly different between stand types. See Table 4 for an overview of common herb, shrub, grass and moss/lichen species identified in these stands. Mean total percent cover for all plant species (Fig. 2) was significantly different among the three stand types (F2,6 = 20.7; P = 0.002), with aspen stands having significantly higher mean total percent cover than the Douglas-fir (t0.017,6 = 4.99; P = 0.003) and mixedwood stands (t0.017,6 = 6.01; P = 0.001). The mixedwood and Douglas-fir stands had similar mean total percent cover (t0.017,6 = 1.02; P = 0.35). There also was a significant difference in the mean percent of bare ground between the three stand types (F2,6 = 12.3; P = 0.008). Aspen stands had significantly lower percent bare ground cover than the Douglas-fir (t0.017,6 = 3.09;

3.3. Understory vegetation Mean plant species richness (total number of species identified) was significantly different between the three stand types (F2,6 = 6.87; P = 0.022) (Fig. 2). Aspen stands had significantly higher mean species richness than mixedwood (t0.017,6 = 3.18; P = 0.015) and Douglas-fir stands (t0.017,6 = 3.24; P = 0.014), whereas the latter two were similar to one another (t0.017,6 = 0.05; P = 0.959). Mean species richness for herb species was different between the two of the three stand types (F2,6 = 5.12; P = 0.043). Aspen stands had significantly higher mean herb species richness than the Douglas-fir stands (t0.017,6 = 3.14; P = 0.016) but not the mixedwood stands (t0.017,6 = 2.10; P = 0.074). Mixedwood and Douglas-fir stands were similar to one another (t0.017,6 = 1.05; P = 0.330). Mean herb richness in aspen stands was 25.00  1.21, in mixedwood stands 15.25  0.31, Table 3 Characteristics of SCWD within the three focal stand types Variable

Stands Aspen (n = 4)

Volume (m3/ha)

Total snag density

Diameter classes 7.5–10 cm 10–25 cm 25–35 cm >35 cm

Decay classes 1 2 3 4 5

F Mixedwood (n = 4)

P

t

P

Douglas-fir (n = 4)

51.0  10.69

15.4  5.83

3.5  0.72

21.59a

0.002

6.31b 4.75c

<0.001 0.003

250.0  42.32

99.2  16.35

60.0  15.33

24.67a

<0.001

5.60b 4.30c

<0.001 0.005

11.87  1.80 39.77  5.94

0.03 13.40a

0.975 0.006

9.35

a

0.011

7.65

a

0.020

4.81b 4.07c 3.89b 3.58c 4.22b 3.44c

0.008 0.012 0.006 0.009 0.010 0.007

<0.001

7.58b 6.77c

<0.001 <0.001

15.64  1.55 180.08  16.33 34.70  9.38

16.37  3.28 65.32  6.39 5.90  1.41

0

19.57  3.39

11.5  1.09

8.36  3.08

44.5  3.40

11.3  0.55

7.3  0.97

34.63a

24.0  7.60 5.3  1.34 23.0  4.51 8.0  0.84

13.0  1.74 3.0  1.02 9.5  1.56 3.8  1.09

7.8  1.09 3.0  0.74 5.0  0.96 4.0  0.35

0.81 0.33 2.77 2.54

0.489 0.733 0.141 0.159

F-statistics are for overall tests of differences between stands types. t-tests are pairwise post hoc comparisons (see note below table). Note: Data are means  1 S.E. There were no significant differences detected between mixedwood and Douglas-fir stands (all Ps > 0.020, at a = 0.017, see Section 2). a Refers to a significant difference between stand types, at a = 0.05. b Refers to a significantly higher value within aspen versus Douglas-fir. c Refers to a significantly higher value in aspen versus mixedwood stands (a = 0.017).

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no difference between mixedwood or Douglas-fir (t0.017,6 = 0.03; P = 0.98). Mean total percent cover of shrub species in aspen stands average 61.1  6.69, in mixedwood stands 25.9  0.86, and in Douglas-fir stands 25.7  2.19. 4. Discussion

Fig. 2. Plant richness and total plant cover (%) for each stand type. Note: Aspen stands had values consistently higher than either of the other stand types for both variables.

P = 0.021) and mixedwood stands (t0.017,6 = 4.89; P = 0.003). The mixedwood and Douglas-fir stands had similar mean percent of bare ground (t0.017,6 = 1.81; P = 0.12). Mean bare ground cover in aspen stands was 13.6  0.54, in mixedwood stands 47.6  3.34, and in Douglas-fir stands it was 34.8  2.74. Mean total percent cover for grass (F2,6 = 2.66; P = 0.149) and moss and lichen (F2,6 = 2.20; P = 0.192) species was similar between all three stand types. Conversely, mean total percent cover of herbs was different among the stand types (F2,6 = 7.39; P = 0.010), and was significantly higher within aspen stands than Douglas-fir (t0.017,6 = 3.15; P = 0.013) and mixedwood stands (t0.017,6 = 3.49; P = 0.013). Mean total percent cover of herb species in aspen stands averaged 43.7  6.9, in mixedwood stands 13.1  1.4, and in Douglas-fir stands 14.9  1.3. The percent cover of shrub species also was significantly different (F2,6 = 7.11; P = 0.025) with aspen stands having higher cover than mixedwood (t0.017,6 = 3.23; P = 0.02) and Douglas-fir (t0.017,6 = 3.25; P = 0.02). There was Table 4 Common herb, shrub, moss/lichen and grass species identified within the three stand types Herbs Heart-leaved arnica, Arnica cordifolia Hook. Showy aster, Aster conspicuus Lindl. Wild strawberry, Fragaria virginiana Miller Mountain sweet-cicely, Osmorhiza chilensis Hook. & Arn. False Solomon’s seal, Smilacina racemosa Desf. Common dandelion, Taraxacum officinale Weber ex. Wigg. Western meadowrue, Thalictrum occidentale A. Gray American vetch, Vicia americana Muhl. ex Willd. Shrubs Saskatoon, Amelanchier alnifolia Nutt. Twinflower, Linnaea borealis L. Tall Oregon-grape, Mahonia aquifolium (Pursh) Nutt. Falsebox, Pachistima myrsinites (Pursh) Raf. Prickly rose, Rosa acicularis Lindl. Birch-leaved spirea, Spiraea betulifolia Pall. Common snowberry, Symphoricarpos albus (L.) Blake Grasses Pinegrass, Calamagrostis rubescens Buckl. Blue wildrye, Elymus glaucus Buckl. Western fescue, Festuca occidentalis Hook. Kentucky bluegrass, Poa pratensis L. Moss and lichens Common lawn moss, Brachythecium albicans (Hedu.) Schimp. Step moss, Hylocomium splendens Hedw. Dog pelt, Peltigera canina L. Redstem feathermoss, Pleurozium schreberi (Brid.) Mitt. Electrified cat’s-tail moss, Rhytidiadelphus triquetrus (Hedw.) Warnst.

Aspen stands had significantly higher snag density and volume, CWD volume, plant diversity, and total shrub and plant cover than mixedwood and Douglas-fir stands. This is potentially important as these attributes directly affect biological diversity (Voller and Harrison, 1998) and ecosystem function (Spies et al., 1988; Loreau et al., 2001). Many forest-dwelling species are dependent on these features for all or part of their life history, and the abundance and diversity of these features may lead to increased species diversity (Van Horne, 1983). The amount and distribution of DCWD within a particular stand is influenced heavily by the dominant tree species, disturbance history, successional stage (Keddy and Drummond, 1996), and the structural characteristics of tree species (Harmon et al., 1986). As such, coniferous forests generally are expected to have greater DCWD volumes than deciduous forests (Stelfox, 1995) due to lower rates of decay in coniferous forests, potentially larger average tree sizes (Abbott and Crossley, 1982), and differences in wood structure (Wilcox, 1973). However, aspen stands in our study had significantly higher volumes of DCWD than either the mixedwood or the Douglas-fir stands. This is likely a reflection of tree morphology (such as that caused by aging), as these forests were not different in terms of size or density of trees (DeByle and Winokur, 1985). Aspen trees are more susceptible to heart rot and other decay agents than coniferous trees, and aspen snags fall over much more quickly than coniferous snags (DeByle and Winokur, 1985). Hence, current DCWD recruitment appears higher within these aspen stands than the mixedwood or Douglas-fir forests. Aspen stands also have a large component of dead trees that will continue to contribute to DCWD. The age of a forest stand often is correlated with the abundance of CWD, with older stands generally having higher volumes than mature forests (Spies et al., 1988; Sturtevant et al., 1997). Here, stand age did not play a role in these patterns as Douglas-fir stands were similar in age to the aspen stands but had significantly lower volumes of CWD. The likely cause of higher debris volumes within aspen stands likely is due to the expected difference in longevity of these tree species. Douglas-fir trees often live longer than aspen trees (Parish et al., 1996) and the rate of decay also is much slower for these trees (DeByle and Winokur, 1985). The aspen stands here are likely to be classified as old-growth stands (Lee et al., 1997), while the Douglas-fir stands would be considered as mature (Spies et al., 1988). This is likely a strong factor in the difference in CWD volumes. As expected, aspen stands had significantly higher density and volume of snags than either the mixedwood or the Douglas-fir stands. Snag recruitment is more likely to occur in aspen trees than in coniferous trees due to their higher susceptibility to heart rot and other decay agents (DeByle and Winokur, 1985). Our aspen sites also contained a high density and proportion of snags (47– 85% dead trees). Healthy aspen stands commonly have between 6 and 20% dead standing trees (DeByle and Winokur, 1985) and Lee et al. (1997) reported snag densities of 12 stems/ha for old aspendominated mixedwood forests in Alberta. These high snag densities, along with the average ages of the trees, suggest that these stands may be in a late successional stage. The volume of CWD measured here is comparable to those reported in other studies. For example, Lee et al. (1997) reported a value of 101.4 m3/ha for DCWD in aspen-dominated mixedwood

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forests in Alberta. Pedlar et al. (2002) found in their study that mixedwood and deciduous forests had CWD volumes five to eight times higher than coniferous forests. They also reported that mixedwood stands had higher volumes of CWD (160.80 m3/ha) than aspen stands. In our study, CWD volumes in mixedwood stands were three times lower than aspen stands and only slightly higher than Douglas-fir stands. This was unexpected as mixedwood stands contained a large proportion of aspen trees within them. This may have been related to stand age, as these mixedwood stands were relatively younger than the other stand types. Aspen stands elsewhere have been shown to contain a rich community of plants (e.g. Sheppard et al., 2006); this pattern also was seen in our study area. Aspen stands here had more diverse communities of plants than either the mixedwood or the Douglasfir stands. This is significant because a diversity of plants may increase animal diversity (Siemann et al., 1998) and may be related to several community and ecosystem processes (Tilman et al., 1997). The complex dynamics of these stands has probably contributed to this diversity as several microhabitats are likely available for different plant species (Lee and Sturgess, 2001). Aspen stands also had a significantly higher percent cover of shrubs and total plants. The cover of plants and shrubs may help to facilitate the survival and microhabitat selection of some animal species (Dueser and Shugart, 1978). A diverse and abundant plants and shrub community may provide a diversity of food for herbivores, and thus help support a diverse faunal community. In conclusion, aspen stands in this region possess stand characteristics that make them notably different from the more ubiquitous conifer forests, in ways other than simply their leading tree species. These characteristics likely contribute to relatively greater biodiversity found within vertebrate communities in the aspen stands (Oaten, 2007). An important next step would be to investigate the mechanisms through which aspen stands occur in a given area (e.g. soil, moisture conditions, disturbance history, etc.), similar to that done elsewhere (e.g. Stelfox, 1995). However, this knowledge gap does not belie the fact that the aspen stands in this region contribute significantly to diversity across the forest ecosystem. Given the relatively scarcity of dry-forest aspen stands within interior British Columbia, it would seem prudent to craft management plans and forestry operations that ensure the representation of aspen on the landscape is maintained or possibly even increased. Acknowledgements We would like to thank S. Symes, H. Kerr, M. Epp, B. Rozander, and T. Cobb for their help with field work. We would also like to recognize support provided by the Sustainable Forest Management Network, the National Science and Engineering Research Council of Canada, the British Columbia Forest Sciences Program (Grant #Y017020), Tolko Industries Ltd., BC Ministry of Forests, Thompson Rivers University, and the University of British Columbia. We especially would like to thank Dr. John Nelson as well as Dr. Valerie Lemay (statistical advice) at the University of British Columbia their help with this project. William ‘Wolverine’ Harrower and two anonymous reviewers provided invaluable and insightful feedback on earlier drafts of this manuscript. References Abbott, D.T., Crossley, D.A., 1982. Woody litter decomposition following clearcutting. Ecology 63, 35–42. Angelstam, P., Mikuskinski, G., 1994. Woodpecker assemblages in natural and managed boreal and hemiboreal forests—a review. Ann. Zool. Fennici. 31, 157–172.

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