Plant diversity and tree responses following contrasting disturbances in boreal forest

Plant diversity and tree responses following contrasting disturbances in boreal forest

Forest Ecology and Management 127 (2000) 191±203 Plant diversity and tree responses following contrasting disturbances in boreal forest Duane A. Pelt...

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Forest Ecology and Management 127 (2000) 191±203

Plant diversity and tree responses following contrasting disturbances in boreal forest Duane A. Peltzer*, Marcy L. Bast1, Scott D. Wilson, Ann K. Gerry2 Department of Biology, University of Regina, Regina, Saskatchewan S4S 0A2, Canada Received 17 December 1998; accepted 24 March 1999

Abstract We determined the abundance and diversity of vascular plants in seven types of disturbance in mixed-wood boreal forest. Disturbance treatments included wild®re, natural regeneration after harvest and several methods of silvicultural site preparation. Relative to undisturbed forest, all disturbance treatments increased plant diversity to about the same extent. The abundance of plant growth-forms differed signi®cantly between disturbance treatments. Silvicultural treatments involving soil disturbance (disk-trenching, drum-chopping and blading) had higher cover of grasses and annual forbs; naturally regenerated and BraÈcke-cultivated treatments contained more perennial forbs and shrubs. Thus, different post-disturbance plant communities established following contrasting types of disturbance. Plant community biomass and tree growth varied among disturbance treatments. Shoot mass of aspen (Populus tremuloides Michx.) and the root mass of all species declined signi®cantly with increasing soil disturbance intensity. Aspen and white spruce (Picea glauca (Moench) Voss) differed in their response to disturbance. Aspen growth was similar among disturbance treatments. In contrast, aspen density was signi®cantly lower in disk-trenched and bladed treatments than in burned or naturally regenerated treatments, and aspen basal area was signi®cantly lower only in drum-chopped treatments. White spruce grew fastest in drum-chopped sites. Burned treatments had the highest recruitment of volunteer spruce seedlings (up to 3200 haÿ1), but not signi®cantly higher than in other disturbance treatments. Taken together these results suggest that the most intensive silvicultural treatments had the expected effects of reducing aspen abundance and increasing the growth of spruce, but also contained more grasses and forbs and had lower total root mass than burned or naturally regenerating sites. Further work is needed to examine long-term productivity and the persistence of both native and persistent weedy species following contrasting types of disturbance. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Boreal forest; Disturbance; Diversity; Growth; Management; Picea; Populus; Tree

*Corresponding author. Tel.: +1-306-585-4994; fax: +1-306585-4894. E-mail address: [email protected] (D.A. Peltzer). 1 Current address: Great Lakes Lab for Fisheries and Aquatic Sciences, Department of Fisheries and Oceans, Burlington, Ontario, L7R 4A6, Canada. 2 Current address: Saskatchewan Conservation Data Centre, 3263211 Albert St., Regina, Saskatchewan, S4S 5W6, Canada.

1. Introduction The relationship between disturbance and diversity has received increasing attention from ecologists and natural resource managers in recent years (Grubb, 1977; Connell, 1978; Grime, 1979; Huston, 1979; Oliver, 1981; Miller, 1982; Rykiel, 1985; Petraitis

0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 1 3 0 - 9

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et al., 1989; Pickett et al., 1989; Ehrlich and Wilson, 1991; Hansen et al., 1991; Roberts and Gilliam, 1995). Species diversity is often greatest at intermediate levels of disturbance intensity or frequency (Connell, 1978; Huston, 1979; Petraitis et al., 1989). Natural disturbances in¯uence species abundance, the outcome of succession, and biodiversity in many communities including forests, coral reefs and grasslands (Connell, 1978; White, 1979; Denslow, 1980, 1985; Sousa, 1984; Huston, 1994; Tilman and Downing, 1994). Large-scale disturbances such as ®re and insect outbreaks occur frequently in the boreal forest (Larsen, 1980; Suf¯ing et al., 1988; Bonan and Shugart, 1989; Johnson and Larsen, 1991; Payette, 1992). For this reason forests contain many species that may be resilient to disturbance (Larsen, 1980). Silvicultural practices also create disturbances on a wide range of spatial scales and intensities. Many studies have examined the effects of silvicultural disturbances of harvesting or post-harvest site preparation on nutrient conservation, hydrology, plant and animal diversity, soil structure, primary productivity and succession (Vitousek et al., 1979; Ezell and Arbour, 1985; Dickinson and Kirkpatrick, 1987; Halpern, 1988, 1989; Morris and Lowery, 1988; PreÂvost, 1992; Pare et al., 1993; Attiwill, 1994; Halpern and Spies, 1995). Although natural and silvicultural disturbances have been examined in many systems, few comparisons of the effects of different disturbancetypes on diversity within the same system have been made; exceptions include comparisons between ®re and harvesting in Oregon (Halpern, 1988; Halpern and Spies, 1995) and in Ontario forests (Brumelis and Carleton, 1988; Brumelis and Carleton, 1989). We examined the in¯uence of disturbance intensity on plant diversity in boreal forest. To accomplish this, we used a comparative approach to study the effects of several silvicultural practices and ®re on plant diversity at the stand level. Undisturbed forest communities are compared to those produced by various site preparation techniques and wild®re. We expected to ®nd increased plant diversity with intermediate levels of disturbance intensity in accordance with the intermediate disturbance hypothesis (Connell, 1978; Grime, 1979; Huston, 1994; Collins et al., 1995). Further, we predicted that ®re would in¯uence the species composition differently from mechanical disturbances in silvicultural treatments

due to the extent of soil disturbance (Smith, 1986). We predict that herbaceous and weedy species will make an increased contribution to community diversity with increasing soil disturbance intensity. In view of the fact that the main management goals of silvicultural treatments in this system are to decrease the abundance of trembling aspen (Populus tremuloides Michx.), and increase the growth of commercially valuable species such as white spruce (Picea glauca (Moench) Voss) (Peterson and Peterson, 1992), we also determined the abundance and growth of trees (aspen and spruce). We predict that if silvicultural treatments after harvesting are effective, they should reduce the abundance and growth of aspen while increasing the growth of spruce relative to naturally regenerating forest. 2. Methods 2.1. Study area The study was conducted in the southern boreal forest near Prince Albert National Park in central Saskatchewan, Canada (538 430 N, 1058 500 W). Forests in the region are dominated by white spruce and trembling aspen, and jack pine (Pinus banksiana Lamb.) in sandy areas. The understory is dominated by bryophytes and low shrubs, including bearberry (Arctostaphylos uva-ursi (L.) Spreng), twin¯ower (Linnea borealis L.), lingonberry (Vaccinium vitisidaea L.), and bunchberry (Cornus canadensis L.). The climate is continental with a mean annual precipitation of 406 mm. Temperatures range from a daily mean high of ÿ13.78C in January to 24.28C in July (Environment Canada, 1993). Soils in the region are classi®ed as gray luvisols on sandy loam subsoil (Agriculture Canada, 1992). 2.2. Vegetation and community biomass sampling We sampled three sites in each of the seven disturbance treatments (Table 1). Stands on sandy soils dominated by jack pine were not sampled. Naturally regenerated sites (N) were harvested but had no site preparation or planting. BraÈcke cultivation (BC) made holes through duff to expose a patch of mineral soil about every 3 m. Disk-trenching (DT) involved plow-

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Table 1 Description of disturbance treatments Treatment

Abbreviation

Wildfire

Harvested

Site preparation

Planted with spruce

Control Fire Natural regeneration BraÈcke cultivation Disk-trenched Drum-chopped Straight-bladed

C F N BC DT DC SB

no yes no no no no no

no no yes yes yes yes yes

no no no BraÈcke-cultivated Disk-trenched Disk-trenched and drum-chopped Straight-bladed

no no no yes yes yes yes

ing organic soil and litter into piles, creating trenches of exposed mineral soil separated by about 3 m. Drum-chopping (DC) is similar to disk-trenching, except that trenching was followed by chopping by using a rotating drum with blades to destroy roots and rhizomes. Straight-blading (SB) exposed mineral soil by scraping away duff with a bulldozer. White spruce was planted in mineral soil following all forms of site preparation. All silvicultural treatments had been harvested, prepared and planted between 1988 and 1990. Fire sites (F) were burned by wild®re in 1989.

Fires were intense enough to burn understory plants without penetrating deeply into the organic layer (D. Peltzer, personal observation). Control sites (C) were neither harvested nor planted. Replicate sites were scattered throughout a 150-km2 area to increase the generality of ®ndings (Fig. 1). All sites were located in stands previously containing mature mixed-wood boreal forest (aspen and white spruce dominated). We examined plant species abundance, root and shoot biomass, and light penetration in July 1994, the period of maximum community biomass. The cover of

Fig. 1. Map of study sites located north of Prince Albert, central Saskatchewan, Canada. See Table 1 for treatment descriptions.

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all vascular plant species was estimated in ®ve subplots (50  100 cm2) evenly spaced along a 30-m transect at each site. Transects were located at least 100 m from the edge of a clearcut. Cover of all plants, litter, bare ground, and bryophytes was recorded using a modi®ed Daubenmire (1959) scale by a pair of observers for increased accuracy (Nilsson, 1992). Live aboveground biomass of forbs, shrubs, grasses, and sedges were clipped in a 10  50 cm transect within each subplot, dried (708C, three days), and weighed. All harvested plants were either rooted within or hanging inside the sampling transect. Belowground biomass was determined by collecting ®ve root cores (1.9 cm diameter, 15 cm deep) within each subplot. Root cores from each subplot were pooled and roots were washed from soil, dried, and weighed. Species richness (S) was determined as the total number of vascular plant species rooted within each subplot. Diversity was calculated separately for each subplot as: X pi ln pi (1) H0 ˆ ÿ where H0 is the Shannon index of diversity and pi the proportion of the ith species in the sample, in this case the relative abundance of a species. The Shannon index was used because of its widespread application and robustness to sample size. Evenness was calculated as: J ˆ H 0 =ln S

(2)

where J is the Pielou (1966) index of species equatability, H0 the Shannon diversity index calculated above, and S the total species richness in the diversity plot (Magurran, 1988). Light penetration was determined in each subplot during July 1994, using 1  40 cm2 long integrating lightmeter (Sun¯eck Ceptometer, Decagon, Pullman, WA). Light measurements were taken 10 cm above vegetation and at the soil surface on a clear day. Light penetration was determined separately for each subplot. 2.3. Tree sampling Trees were sampled along one transect at each site in October 1994. Transects were 30 or 60 m long, with longer transects in less dense stands. We measured the stem diameter of each white spruce within 2.5 m of the

transect, except at one burned site with very high spruce densities where we measured only those spruce within 50 cm. Trees were measured 10 cm above the soil, with the exception of very small spruce in the burned sites, which were measured at 5 cm. We also measured the diameter of aspen stems 10 cm above the soil. Aspen were generally measured in ®ve plots, equally spaced along each transect. Plot size varied (1  1 m to 1  5 m) according to aspen density. At six sites, aspen were measured along the entire transect in belts 1±5 m wide. All densities were converted to number per hectare. A subset of sites also had been sampled in October 1993, which allowed us to examine variation in radial growth among some treatments. In order to determine the biomass of aspen in July, aspen stems were measured for diameter, harvested, dried and weighed. Regression analysis predicted individual aspen stem mass from diameter (log mass (g) ˆ 2.027 (log diameter (mm)) ÿ 0.397; r2 ˆ 0.76, p < 0.0001, n ˆ 32). 2.4. Statistical analyses All proportional data (diversity, light penetration, and growth) were arcsin-square root transformed and biomass data were log10 transformed prior to statistical analysis to reduce heteroscedasticity and meet the assumptions of normality and equality of variances among treatment groups (Zar, 1984). Diversity, richness, evenness, and biomass were compared between treatments using ANOVA with subplots nested within replicate sites. If the results of ANOVA were signi®cant, we used Tukey±Kramer HSD tests to contrast treatment means and to preserve an experiment-wise error rate of 0.05. Community structure was compared between disturbance treatments using MANOVA with the abundance of different plant growth forms (trees, shrubs, grasses, perennial forbs, annual forbs) as the dependent variables. Multiresponse permutation procedures (MRPP) were used to test the null hypothesis that plant species composition did not vary among disturbance treatments (Zimmerman et al., 1985; McCune and Mefford, 1997). MRPP is a nonparametric multivariate technique that has advantages over similar parametric techniques such as discriminant analysis because it does not require multivariate normality or homogeneity of variances. Disturbance treatments

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formed the a priori groups for this analysis. The test statistic (T) is the difference between the observed delta (, within group Euclidean distance) and the expected delta (mean  for all possible partitions of the data). 3. Results 3.1. Diversity and community structure Undisturbed forest (C) contained fewer species than disturbed sites (Fig. 2(A)). Naturally regenerating (N) and straight-bladed (SB) treatments contained an average of about ®ve more species than other disturbance treatments (Fig. 2(A)); however, this difference was not signi®cant (ANOVA:F6,14 ˆ 2.47, p ˆ 0.076). Species evenness and diversity were signi®cantly higher in disturbed sites than in undisturbed forest (Fig. 2(B, C)). Burned (F) and drumchopped (DC) sites had signi®cantly higher evenness than undisturbed forest, but not than other disturbances (Fig. 2(B), means contrasts). All disturbed sites had signi®cantly higher diversity than undisturbed forest, but diversity did not vary signi®cantly among disturbance treatments (Fig. 2(C), means contrasts). Dominance±diversity curves for all treatments con®rmed that lower diversity in undisturbed forest (C) resulted from both lower species richness and lower evenness (Fig. 3). Diversity in more disturbed treatments (DT, DC, SB) was similar to less disturbed treatments (F, N, BC) due to a reduction in the cover of dominant species, and increases in the cover of grasses and forbs (Fig. 3). Shrubs tended to dominate in burned, naturally regenerated, BraÈcke-cultivated and straight-bladed sites, but not in sites that were trenched and chopped (DT, DC) (Fig. 3, open triangles). Species composition varied signi®cantly among disturbance treatments for all species considered simultaneously (MRPP: T ˆ ÿ20.324, p < 0.0001). Undisturbed forest had higher cover of trees and bryophytes than disturbed treatments (Table 2). Relatively few species were found across all disturbance treatments; exceptions included Populus, Betula, Rosa, Rubus, Viburnum, Cornus, Aralia, Aster spp., Mertensia and Fragaria (Table 2). The cover of grasses and forbs increased signi®cantly with distur-

Fig. 2. Species richness (A), evenness (B) and diversity (C) for seven disturbance treatments: undisturbed control (C), fire (F), naturally regenerated (N), BraÈcke-cultivated (BC), disk-trenched (DT), disk-trenching followed by drum-chopping (DC) and straight-bladed (SB). See Table 1 for treatment descriptions. Data shown are for replicate sites; lines are means for each treatment. Lower case letters represent means contrasts.

bance intensity (MANOVA: Wilk's Lambda ˆ 0.287, F18,266 ˆ 246.81, p < 0.0001) (Table 2). The higher species diversity in disturbed treatments (Fig. 2(C)) was due to the addition of forbs and grasses and the persistence of resprouting woody species (Table 2).

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Fig. 3. Species rank abundance curves for seven disturbance treatments: undisturbed control (C); fire (F); naturally regenerated (N); BraÈckecultivated (BC); disk-trenched (DT); disk-trenching followed by drum-chopping (DC) and straight-bladed (SB). See Table 1 for treatment descriptions. Data shown are the mean species abundance from three replicate plots for the 50 most common vascular plant species.

3.2. Biomass Aspen shoot mass in drum-chopped (DC) sites was signi®cantly lower than naturally regenerated (N) or BraÈcke-cultivated (BC) sites, but not lower than burned (F), disk-trenched (DT) or straight-bladed (SB) sites (Fig. 4(A), means contrasts). The shoot mass of all other plants did not vary signi®cantly among disturbance treatments (Fig. 3(B)). Root mass was highest in BraÈcke-cultivated sites, intermediate in

burned (F) and naturally regenerated (N) plots and was lowest in the more highly disturbed silvicultural treatments (DT, DC, SB) (Fig. 4(C), means contrasts). Light penetration to the soil surface did not vary signi®cantly among disturbance treatments (Fig. 5). 3.3. Tree responses Aspen density was highest in burned (F) and naturally regenerated (N) sites, signi®cantly lower in disk-

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Table 2 Cover (%) of common vascular plant species (>5% in at least one treatment) found in seven disturbance treatments (see Table 1 for treatment abbreviations and descriptions; nomenclature follows Looman and Best (1987)) Species

Common name

Treatment C

F

N

BC

DT

DC

SB

paper birch white spruce balsam poplar trembling aspen

5 61 0 31

2 0 0 36

<1 0 18 57

<1 <1 6 60

4 <1 0 26

1 <1 0 9

1 3 5 28

Shrubs Rosa acicularis Rubus idaeus Viburnum edule Salix spp. Shepherdia canadensis Alnus crispus Rosa woodsii

prickly rose wild red raspberry lowbush cranberry willow Canada buffaloberry green alder woods rose

3 <1 2 0 0 0 0

10 25 7 2 0 0 0

13 13 5 <1 0 23 0

17 12 6 <1 5 5 1

8 15 1 1 0 4 0

2 24 2 0 0 0 4

3 10 4 11 0 24 21

Herbaceous plants Bryophytes Linnaea borealis Cornus canadensis Aralia nudicaulis Petasites palmatus Aster ciliolatus Aster puniceus Elymus innovatus Equisetum sylvaticum Maianthemum canadense Martensia paniculata Lathyrus ochroleucus Fragaria virginiana Sonchus arvensis Epilobium angustifolium

mosses twinflower bunchberry wild sarsaparilla colt's foot Lindley's aster purple-stemmed aster hairy wild rye horsetail two-leaved Soloman's seal tall lungwort wild peavine strawberry sow-thistle fireweed

32 8 3 3 2 <1 0 0 <1 3 <1 <1 <1 0 0

18 11 15 6 6 1 0 <1 4 1 7 2 <1 <1 15

2 <1 5 10 8 <1 5 0 0 2 8 2 12 <1 9

4 2 2 6 8 2 <1 0 0 4 6 2 7 0 24

15 <1 2 1 0 5 0 7 1 <1 2 6 4 3 12

14 0 1 1 <1 4 0 <1 0 <1 3 0 14 6 19

1 0 2 3 2 2 <1 0 5 2 4 <1 6 0 17

Trees Betula papyrifera Picea glauca Populus balsamifera Populus tremuloides

trenched (DT) and straight-bladed (SB) treatments, and intermediate in BraÈcke-cultivated (BC) and drumchopped (DC) treatments (Fig. 6(A), means contrasts). Aspen basal area was lowest in the more intense silvicultural treatments (DT, SB, only signi®cantly so for DC), and was highest in naturally regenerated (N) sites (Fig. 6(B), means contrasts). In contrast to the high aspen density found in burned sites (Fig. 6(A and F)), aspen basal area in burned sites was only signi®cantly higher than drum-chopped (DC) sites. Aspen growth did not vary signi®cantly among disturbance treatments (Fig. 5(C)). As for basal area, growth was low in burned plots (F) relative to other treatments, although this trend was not sig-

ni®cant (p ˆ 0.073). Growth was not recorded in straight-bladed (SB) sites because aspen suckers were not tagged in this treatment. White spruce density tended to be lower in naturally regenerated (N) sites than in other treatments, but this trend was not signi®cant (Fig. 7(A)). The density of volunteer (unplanted) spruce was highest in burned (F) sites; however, this trend was not signi®cant due to large variations in volunteer density among burned sites (Fig. 7(B)). Spruce growth was signi®cantly higher in drum-chopped (DC) sites than in burned (F) or disk-trenched (DT) sites (Fig. 7(C), means contrasts). Only a single spruce was found in naturally regenerated (N) sites, no growth was observed for this

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Fig. 5. Light penetration to the soil surface in mid-July for six disturbance treatments: fire (F); naturally regenerated (N); BraÈckecultivated (BC); disk-trenched (DT); disk-trenching followed by drum-chopping (DC); and straight-bladed (SB). See Table 1 for treatment descriptions. Data shown are for replicate sites; lines are means for each treatment. Points are staggered horizontally where values are similar for clarity. Lower case letters represent means contrasts.

Fig. 4. Mass of aspen (Populus temuloides Michx.) shoots (A), shoots of other plants (B), and root mass of all species (C) in six treatments: fire (F); naturally regenerated (N); BraÈcke-cultivated (BC); disk-trenched (DT); disk-trenching followed by drumchopping (DC); and straight-bladed (SB). See Table 1 for treatment descriptions. Data shown are for replicate sites; lines are means for each treatment. Points are staggered horizontally where values are similar. Lower case letters represent means contrasts.

individual. No spruce were tagged in BraÈcke-cultivated (BC) or straight-bladed (SB) treatments. 4. Discussion 4.1. Diversity and community structure No differences in species richness, evenness or diversity were observed between burned and naturally

regenerated sites after harvest (Fig. 2). Both, burned and harvested treatments had signi®cantly higher species evenness and diversity than unharvested forest (Fig. 2). The signi®cant increase in diversity probably represents only a short-term response of the system to disturbance. For example, diversity increased until about eight years after ®re and declined afterwards in northern Ontario boreal forest (Sha® and Yarranton, 1973). Cover of trees (Populus tremuloides and P. Balsamifera) and most shrubs was lower in burned sites than in naturally regenerated sites (Fig. 3, Table 2, F vs. N). On average shoot biomass of aspen was ca. 1000 g/m2 lower in burned sites than in naturally regenerated sites, but this result was not signi®cant due to the large among-site variation (Fig. 4(A)). As for results contrasting burned and naturally regenerated sites, diversity was not signi®cantly different among the four site preparation treatments (Fig. 2, BC, DT, DC, SB). Plant diversity was signi®cantly higher in all site preparation treatments than in undisturbed forest (Fig. 2). Increased diversity with site preparation was caused by the decreased abundance of trees and an increase in the cover of forbs and grasses, likely from the seed bank (Figs. 3 and 4, Table 2). This result was expected and is consistent with the ®nding of previous studies that there is an increase in diversity following soil disturbance resulting from site preparation in other forests. For example, Swindel et al. (1983) observed an increase in the abundance of herbaceous species (forbs and grasses)

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Fig. 6. Density (A), basal area (B) and diameter growth increment (C) of aspen (Populus temuloides Michx.) in six disturbance treatments: fire (F); naturally regenerated (N); BraÈcke-cultivated (BC); disk-trenched (DT); disk-trenching followed by drumchopping (DC); and straight-bladed (SB). See Table 1 for treatment descriptions. Data shown are for replicate sites; lines are means for each treatment. Lower case letters represent means contrasts.

Fig. 7. Density (A), number of volunteer seedlings (B) and diameter growth increment (C) of white spruce (Picea glauca (Moench) Voss) in six disturbance treatments: fire (F); naturally regenerated (N); BraÈcke-cultivated (BC); disk-trenched (DT); disktrenching followed by drum-chopping (DC); and straight-bladed (SB). See Table 1 for treatment descriptions. Data shown are for replicate sites; lines are means for each treatment. Lower case letters represent means contrasts.

and diversity after clearcutting and drum-chopping in the southeastern U.S. Similarly, higher diversity following an intense site preparation (V-blading), was the result of increased cover of herbaceous species in QueÂbec (Jobidon, 1990). Taken together, these results suggest that species diversity is not necessarily a good indicator of community integrity or development if species typically dominating after a disturbance are replaced by persistent weedy species (Angermeier and Karr, 1994).

Community structure, measured as the abundance of different plant growth-forms, shifted with increasing disturbance intensity (Fig. 3, Table 2). Grasses and forbs were more abundant in sites that were bladed or chopped than in sites that were naturally regenerated or BraÈcke-cultivated. Because grasses and forbs compete with tree seedlings for water, nutrients and light, they can slow the growth of establishing trees and may slow the recovery of canopy dominants, such as spruce (Eis, 1981; Putz and Canham, 1992; Lieffers

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et al., 1993; Hill et al., 1995; Li and Wilson, 1999). Different initial species composition after disturbance can in¯uence stand composition and structure. In a survey of northern Ontario forests, Carleton and MacLellan (1994) found coniferous species more widespread in post-®re stands, whereas hardwoods such as aspen and tall broadleaved shrubs were more frequently found in post-logged stands. This is supported by the trends of high densities of volunteer spruce and reduced aspen growth in the burned sites observed in our study (Figs. 6 and 7). Human-created disturbance regimes often increase the invasibility of herbaceous vegetation (Hobbs and Huenneke, 1992), and may also do so in boreal forest. For example, soil disturbance increases soil moisture levels and microsite availability for invasive species, such as the rhizomatous grass Calamagrostis canadensis (Michx.) Beauv., resulting in different initial community structure (Lieffers et al., 1993). As forests and other ecosystems become increasingly affected by human activities, integration of biological diversity and resource management objectives is needed to maintain the processes necessary for the continued productivity of managed systems (see, e.g. Ehrlich and Wilson, 1991; Probst and Crow, 1991; Burton et al., 1992; Angermeier and Karr, 1994; Costanza et al., 1997). In order to preserve both, species and landscape-level diversity, a full range of stand ages or seral stages and disturbance types must be maintained (see discussions in Smith, 1986; Hansen et al., 1991; Burton et al., 1992; Kohm and Franklin, 1997). 4.2. Tree responses and productivity Site preparation treatments tended to have the expected effect of decreasing aspen abundance and enhancing spruce growth. The shoot mass, density and basal area of aspen tended to be lower in the more intense site preparation treatments than in naturally regenerated sites (Fig. 4(A), Fig. 6(A and B)), as achieved for other hardwood species in temperate forests (Lanini and Radosevich, 1986; Ross et al., 1986). Aspen growth, however, was similar in all treatments (Fig. 6(C)). These results suggest that site preparation releases spruce more by decreasing aspen density than by decreasing the growth and resource demand of individual aspen stems.

Soil disturbance intended to decrease the abundance of early successional species can also result in nutrient leaching (Vitousek et al., 1979) and long-term alterations in soil structure (Ezell and Arbour, 1985). Thus, control of undesirable species, such as aspen, with intensive site preparation techniques should be balanced against the potential decrease in forest soil nutrient reserves and long-term productivity of the site (Morris and Lowery, 1988). The accumulation and mineralization of soil organic matter is a key process maintaining the productivity of forest ecosystems, and roots are one of the major sources of organic matter. Because root mass was signi®cantly lower in plots with the most intense site preparation (Fig. 3(C)), the long-term effects of soil disturbance on plant growth and rates of forest recovery after disturbance deserves further study. Spruce growth was signi®cantly higher in drumchopped sites than in disk-trenched sites (Fig. 7(C)), suggesting that more intensive site preparation produced higher growth rates of transplant seedlings, as has been found in the southeastern U.S. (Haines et al., 1973; Stransky, 1981; Morris and Lowery, 1988) and in the northwestern U.S. (Lanini and Radosevich, 1986; Ross et al., 1986). Increases in spruce growth in drum-chopped sites may be caused by lower mass of competing aspen (Fig. 4(A)), higher light penetration (Fig. 5), or by increased availability of soil resources caused by more intense soil disturbance. Further work characterizing resource availability to establishing trees is needed to determine the reasons for differences in spruce growth among the silvicultural treatments. The effects of ®re and site preparation on trees differed in several ways. The highest density of aspen was found in burned sites, but was only signi®cantly higher than disk-trenched and straight-bladed sites (Fig. 5(A)). In contrast, aspen growth tended to be lower (Fig. 6(C), p ˆ 0.073) in burned sites than in harvested sites without site preparation, suggesting that intraspeci®c competition may have limited the basal area of aspen in burned plots. Some volunteer spruce were found in silvicultural treatments (BC, DT, DC) and naturally regenerated sites. Spruce volunteers were most abundant in two burned sites (Fig. 6(B)), but were absent from a third burned site. Similarly, germinable spruce seed densities in burned forest in the region varied from 0 to 133/

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m2 (Archibold, 1979), suggesting that the distribution of seeds is extremely patchy. Suitable establishment years are also patchy through time in this region (Rowe, 1955). Spruce growth rates in burned sites were as high as those in disk-trenched (DT) sites, but were signi®cantly lower than growth in drum-chopped (DC) sites (Fig. 6(C)). High spruce establishment and growth rates on some burned sites are surprising, given the high aspen densities and presumably intense competition. These results support the idea that the dominant species in mixed stands establish nearly simultaneously following natural disturbance (Oliver and Larsen, 1990). Forest harvesting differed from ®re mainly in its effects on the establishment of volunteer spruce (Fig. 7(B)). 5. Conclusions Burned and naturally regenerated sites had signi®cantly higher plant species evenness and diversity than undisturbed forest (Fig. 2(B and C)), but not more than silvicultural disturbances (BC, DT, DC, SB). Increasing intensity of silvicultural disturbances did not change plant diversity, but more intense disturbances (DC and SB) tended to have higher cover of forbs and grasses (Table 2). Thus, different post-disturbance communities established after contrasting types of disturbance. On burned sites, aspen density was the highest (Fig. 6(A)), aspen growth was the lowest (Fig. 6(C)) and recruitment of volunteer white spruce seedlings was the highest (Fig. 7(B)). Some silvicultural treatments had the expected effect of reducing the abundance of aspen suckers (Fig. 6(A), DT, SB; Fig. 6(B), DC) and increasing the growth of spruce (Fig. 7(C), DC). Further work is needed to examine the long-term productivity and the persistence of weedy species following contrasting disturbance types. Acknowledgements We thank L. Ambrose, J. Bakker, J. Christian and H. Kleb for ®eld and technical assistance, and P. Burton and an anonymous reviewer for helpful comments on earlier drafts of this paper. This work was supported by

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