A Gradient Analysis of Oak Savanna Community Composition in Western Wisconsin

J. MEISEL, N. TRUSHENSKI AND E. WEIHER (Department of Biology, University of Wisconsin-Eau Claire, Eau Claire, WI 54702). A gradient analysis of oak savanna community composition in western Wisconsin. J. Torrey Bot. Soc. 129: 115-124. 2002. Oak savannas were historically abundant, but they are now rare. Although there is great interest in oak savanna restoration, there is still debate about the nature of the understory composition of oak savannas, especially in lowlands. We sampled 168 quadrats (0.25 m2) in remnant floodplain oak savannas along the Chippewa River, WI, to produce a gradient analysis of vegetation by both taxonomic and guild composition. Community composition varied along two main gradients, as indicated by correlations between DCA scores and environmental factors. The communities mainly varied along a soil quality gradient (primarily nitrogen and organic matter), from sand barrens to wet-mesic savannas, and along a tree canopy cover gradient from open prairie-like areas to areas with heavy canopy. Although significant, fire frequency was of minor importance, but we believe this was partly a sampling artifact. Seven general community types were described, and MANOVA tests showed they differed in terms of guild composition. Forbs tended to dominate most community types, with Solidago species usually among the most abundant species, while C4 graminoids (mostly Andropogon gerardii and Sorghastrum nutans) were dominant only in the poorest soils and relatively open areas. Stepwise regression showed relatively weak relationships between relative guild cover and the measured environmental variables. Only C4 graminoids showed close correspondence to the main gradients affecting taxonomic composition. The other guilds responded to disturbance and distinct soil factors.