PARENT SESSION
Contributed Oral Session 129: Biodiversity: Disturbance; Landscapes; Heterogeneity
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 513 E, Level 5, Palais des congrès de Montréal

Fire frequency and tree canopy cover effects on plant species diversity in oak savannas and woodlands.

Peterson, David^*,1, Reich, Peter², ¹ USDA Forest Service, Wenatchee, WA, USA² University of Minnesota, St. Paul, MN, USA

ABSTRACT- Disturbances, competition, and environmental heterogeneity are all potential factors influencing plant species diversity in terrestrial ecosystems. We surveyed understory plant communities and measured tree canopy cover on permanent plots spanning an experimental fire frequency gradient (0-8 fires/decade). Our objective was to study fire frequency and tree canopy effects on plant species richness and community heterogeneity in oak savannas and woodlands in Minnesota, USA. Species richness responses were assessed within major plant functional groups and for all groups combined. Woody plant species richness was highest in unburned woodlands and declined with increasing fire frequency and declining tree canopy cover. Forb species richness was highest in biennially burned grasslands (and open savannas) and declined with increasing tree canopy cover and declining fire frequency. Grass species richness was highest in open grasslands and declined with increasing tree canopy cover. Grass species richness was positively correlated with fire frequency, but this may have been due to fire frequency effects on tree canopy cover. Community heterogeneity was highest in biennially burned savannas and was positively correlated with variability in tree canopy cover. Total plant species richness was highest at biennial fire frequencies, consistent with predictions of the intermediate disturbance hypothesis. However, plant species richness was also maximized in savannas and woodlands with 30-70% tree canopy cover. Our results suggest that biennially burned savannas and woodlands may have supported the highest species richness because of high environmental heterogeneity and reduced rates of competitive exclusion; partial shading may reduce the ability of grasses to exclude species through competition for soil resources, while frequent fires reduce the ability of woody plants to exclude species through competition for light.

Key words: species richness, oak savanna, fire frequency, tree canopy cover