Plant diversity and tree responses following contrasting disturbances in boreal forest
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 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 influence species abundance, the outcome of succession, and biodiversity in many communities including forests, coral reefs and grasslands (Connell, 1978; White, 1979; Denslow, 1980, Denslow, 1985; Sousa, 1984; Huston, 1994; Tilman and Downing, 1994). Large-scale disturbances such as fire and insect outbreaks occur frequently in the boreal forest (Larsen, 1980; Suffling 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, Halpern, 1989; Morris and Lowery, 1988; Prévost, 1992; Paré 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 disturbance-types on diversity within the same system have been made; exceptions include comparisons between fire 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 influence 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 fire on plant diversity at the stand level. Undisturbed forest communities are compared to those produced by various site preparation techniques and wildfire.
We expected to find 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 fire would influence 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.
Section snippets
Study area
The study was conducted in the southern boreal forest near Prince Albert National Park in central Saskatchewan, Canada (53° 43′N, 105° 50′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), twinflower (Linnea borealis L.), lingonberry (Vaccinium vitis-idaea L.), and bunchberry (Cornus canadensis L.).
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 five more species than other disturbance treatments (Fig. 2(A)); however, this difference was not significant (ANOVA:F6,14 = 2.47, p = 0.076). Species evenness and diversity were significantly higher in disturbed sites than in undisturbed forest (Fig. 2(B, C)). Burned (F) and drum-chopped (DC) sites had significantly higher
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 significantly higher species evenness and diversity than unharvested forest (Fig. 2). The significant increase in diversity probably represents only a short-term response of the system to disturbance. For example, diversity increased until about eight years after fire and declined afterwards in northern Ontario
Conclusions
Burned and naturally regenerated sites had significantly 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
Acknowledgements
We thank L. Ambrose, J. Bakker, J. Christian and H. Kleb for field and technical assistance, and P. Burton and an anonymous reviewer for helpful comments on earlier drafts of this paper. This work was supported by the Prince Albert Model Forest Association, the Canada–Saskatchewan Agreement in Forestry, and the Natural Sciences and Engineering Research Council of Canada.
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- 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, 326-3211 Albert St., Regina, Saskatchewan, S4S 5W6, Canada.