SPECIES RESPONSE TO A MOISTURE GRADIENT IN CENTRAL ILLINOIS FORESTS

Polar and Gaussian ordination applied to data collected from 37 forest sites in central Illinois resulted in a continuous and gradual change in species composition along a moisture gradient. A series of overlapping species success curves formed by plotting Importance Values over stands ordered along the gradient varied continuously in modal location and habitat width. Blackjack oak and black oak dominated upland sandy sites. Black oak, white oak, and shagbark hickory were the most important species on exposed, upper slope positions or ridge tops with silt-loam soils. Red oak, sugar maple, American elm, and bur oak dominated sheltered locations on lower slope positions and stream terraces. Sycamore, silver maple, and cottonwood were leading tree species in floodplain forests. Conversion of black, white, and red oak forests on silt-loam sites to sugar maple, white ash, and red elm dominance is evident by high densities of these shade tolerant species in the understory. Composition of forests at the extreme ends of the moisture gradient is more stable than the mesic sites. Maximum tree diversity occurred on mesic sites and decreased toward the extreme ends of the moisture gradient. However, competitive exclusion of shade intolerant species by sugar maple and other species has caused a decrease in understory diversity on mesic sites. Diversity decreased from canopy to understory strata in lowland forests and increased on xeric sites. PLANT COMMUNITY studies were traditionally based on classification and recognition of discrete units of vegetation. However, Gleason's (1926) principles of species individuality and community continuity suggested the need for other approaches based on gradient relationships and continuous community patterns. Gradient analysis deals with the study of communities in terms of gradient phenomena. In direct gradient analysis plant community samples are arranged along a measurable environmental attribute (i.e., elevation) or by comparing the samples to those representing endpoints or extremes of the environmental gradient (Bray and Curtis, 1957). Arranging community samples along abstract axes, such as a compositional gradient, that may or may not correspond to environmental gradients is indirect gradient analysis or ordination (Whittaker, 1973a). Curtis and McIntosh (1951) provided the 1 Received for publication 4 June 1979; revision accepted 31 July 1979. We gratefully acknowledge funding from the Beta Lambda Chapter of the Phi Sigma Society to D. E. Adams. Funds were also provided by Faculty and Summer Research Grants (1977-78), Illinois State University, to R. C. Anderson. Appreciation is also extended to Drs. Lauren E. Brown and Dale E. Birkenholz for their reviews of the manuscript. 2 Present address: Department of Botany and Microbiology, The University of Oklahoma, Norman, Oklahoma 73019. beginning of gradient analysis with the continuum concept which involved a unidimensional ordination of forests in southern Wisconsin based on dominant tree species. Their findings were complemented by those of Whittaker (1951, 1956) in the Great Smoky Mountains. Many aspects of the original continuum delineated by Curtis and McIntosh (1951) were clarified by subsequent studies in southern Wisconsin. McIntosh (1957) considered more than one successional pathway in species replacement patterns. Direct and indirect gradient analysis were combined into a multidimensional ordination (Bray and Curtis, 1957). Peet and Loucks (1977) separated the continuum gradient into moisture and successional gradients by incorporating environmental and dynamic factors. In spite of the development of numerous gradient analysis techniques (Curtis and McIntosh, 1951; Bray and Curtis, 1957; Loucks, 1962; Hill, 1973; Whittaker, 1973b; Gauch, Chase, and Whittaker, 1974; NoyMeir, 1974), these techniques have not been used to examine central Illinois forests in a comprehensive manner. Several descriptive studies of the vegetation of small isolated woodlots in central Illinois have been conducted (Calef, 1953; Boggess, 1964; Boggess and Bailey, 1964; Boggess and Geis, 1966, 1967; Ebinger, Phillippe, and Phillippe, 1977; Miceli et al., 1977), but determination of major compositional or environmental gradients is lacking. Bell and del Moral (1977) used a gra-