Influence of Topography on Stand Composition in a Midwestern Ravine Forest

Abstract

-The influence of slope steepness, position (upper, middle and lower), aspect and other site variables on vegetational patterns was examined in 26 relatively undisturbed forest stands in central Illinois. Detrended correspondence analysis (DCA) was used to ordinate stands based on tree species importance values. Stand ordination scores for DCA Axis 1 were positively correlated with a synthetic index (site protection rating) derived from slope variables (r2 = 0.69, P < 0.001). Axis 2 scores were not correlated with any of the measured environmental variables. For stands on slopes, Axis 3 scores had a weak positive correlation with transformed aspect (r2 = 0.15, P < 0.05). The dominant tree species on high, gently sloping ridges were Quercus velutina and Q. alba. Acer saccharum and Q rubra dominated sheltered slopes and ravines, and dominant species on stream terraces were Celtis occidentalis, Fraxinuspennsylvanica, Platanus occidentalis and Juglans nigra. All species of Quercus had reduced abundance in the sapling stratum compared to their abundance in the tree stratum, whereas Acer saccharum, Prunus serotina, Fraxinus americana and Ostrya virginiana had low importance values in the tree stratum, but high importance values in the sapling stratum. The decline in abundance of Quercus species from the tree to the sapling stratum and the relative increase in abundance of more mesophytic species is probably related to fire cessation since European settlement. INTRODUCTION Gleason's individualistic concept of plant associations (1926) has formed the basis for a large number of research studies over the last 40 yr (Curtis and McIntosh, 1951; Whittaker, 1951, 1956; Curtis, 1959; Goff and Cottam, 1967; Peet and Loucks, 1977; Adams and Anderson, 1980). A variety of ordination techniques (Whittaker, 1973; Hill and Gauch, 1980; Ter Braak and Prentice, 1988) locate sites with similar species composition and abundance near each other, whereas sites supporting dissimilar vegetation are placed farther apart, regardless of their spatial distribution on the landscape (Austin and Smith, 1989). Because species segregate according to their environmental requirements (Ter Braak and Prentice, 1988), their ordering usually sequences sites along an environmental gradient. Species are distributed continuously, with each species having its own characteristic distribution, influenced by site conditions and competitive interactions. The influence of topography, including slope characteristics such as percent slope and aspect, on the distribution of vegetation has been studied extensively in the United States, especially in mountainous regions (Whittaker and Niering, 1965; Haase, 1970; Peet, 1978a,