Fire frequency and tree canopy structure influence plant species diversity in a forest-grassland ecotone |
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Authors: | David W Peterson Peter B Reich |
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Institution: | (1) Department of Forest Resources, University of Minnesota, St. Paul, MN 55108, USA;(2) USDA Forest Service, Pacific Northwest Research Station, 1133 N. Western Avenue, Wenatchee, WA 98801, USA |
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Abstract: | Disturbances and environmental heterogeneity are two factors thought to influence plant species diversity, but their effects
are still poorly understood in many ecosystems. We surveyed understory vegetation and measured tree canopy cover on permanent
plots spanning an experimental fire frequency gradient to test fire frequency and tree canopy effects on plant species richness
and community heterogeneity within a mosaic of grassland, oak savanna, oak woodland, and forest communities. Species richness
was assessed for all vascular plant species and for three plant functional groups: grasses, forbs, and woody plants. Understory
species richness and community heterogeneity were maximized at biennial fire frequencies, consistent with predictions of the
intermediate disturbance hypothesis. However, overstory tree species richness was highest in unburned units and declined with
increasing fire frequency. Maximum species richness was observed in unburned units for woody species, with biennial fires
for forbs, and with near-annual fires for grasses. Savannas and woodlands with intermediate and spatially variable tree canopy
cover had greater species richness and community heterogeneity than old-field grasslands or closed-canopy forests. Functional
group species richness was positively correlated with functional group cover. Our results suggest that annual to biennial
fire frequencies prevent shrubs and trees from competitively excluding grasses and prairie forbs, while spatially variable
shading from overstory trees reduces grass dominance and provides a wider range of habitat conditions. Hence, high species
richness in savannas is due to both high sample point species richness and high community heterogeneity among sample points,
which are maintained by intermediate fire frequencies and variable tree canopy cover. |
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Keywords: | Community heterogeneity Intermediate disturbance hypothesis Oak savanna Oak woodland Prescribed fire Species richness |
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