VEGETATION ECOLOGY OF FLATWOODS ON THE ILLINOIAN TILL PLAIN

Midwestern flatwoods are open woodlands of level uplands and terraces that occur on impervious subsoil horizons and have seasonally wet and dry soils. Vegetation and soil data from six flatwoods remnants on the Illinoian till plain in Illinois were examined to identify what biotic and edaphic factors control vegetation structure, composition and diversity. Soils at all sites are characterized by the presence of argillic horizons (clay pans). Two soil ordination groups were clearly defined, each characterized by separate parent material origin. Canopy and subcanopy composition are dissimilar at sites with soils developed entirely in loess; more similar at sites on lacustrine soils (including aeolian sand). Quercus stellata was by far the most important tree species followed by Q. marilandica. Q. stellata importance was negatively correlated with Ca, particularly in the A-horizon. Q. marilandica shows a positive correlation to sand and magnesium, particularly in the B-horizon, and was most frequent and abundant on soils where Ca/Mg ratios are lower than 1.0 in the E and B-horizons. Total stem density of woody plants was positively related to total available soil moisture as influenced by depth to clay pan and soil textural differences. Ground-cover species richness and diversity were inversely related to tree and total woody stem density and tree diversity. One site with a recent history of annual prescribed fire had significantly greater ground-cover species diversity compared with the unburned sites. Evidence from stand structure and similarity between canopy and subcanopy strata, if reflecting stationary processes of canopy recruitment, suggest that disturbances historically maintained the importance of oak species in most flatwoods. Prescribed fire appears to maintain or enhance ground-cover species diversity at one flatwoods remnant, suggesting that fire could be used to maintain compositional stability at sites with greater available soil moisture.