Long-term patterns in vegetation-site relationships in a southern Appalachian forest

We used permanent plot inventories from 1969-1973 and 1988-1993 to describe forest species distribution patterns of the Coweeta Hydrologic Laboratory, a 2,185 ha basin in western North Carolina, USA. We used canonical corresp ondence analysis to explore the vegetation-site patterns for the 1970s and 1990s inventories combined. Site variables were determined by direct measur ements or calculated by digital geographical information system mapping met hods. Site variables were percent slope, elevation, terrain shape, precipit ation, modified azimuth, soil organic matter content, soil depth, soil clay content, depth of A-horizon, potential solar radiation, and mean temperatu re during the growing season. Fifty percent of the variation in the vegetat ion distribution was explained by the site variables used in the canonical correspondence analysis. Soil organic matter, terrain shape, and elevation were the variables most strongly related to vegetation distribution. Specie s associated with convex terrain (upper slopes and ridges), such as Pinus r igida, Quercus coccinea, and Quercus velutina, decreased in abundance from the 1970s to the 1990s; species associated with soils having high organic m atter content and deep A-horizons, such as Liriodendron tulipifera, Rhodode ndron maximum, and Tsuga canadensis increased in abundance. Individual spec ies responded differently to site gradients. For example, Acer rubrum, Quer cus prinus, Oxydendrum arboreum, and Nyssa sylvatica were located in the ce nter of the ordination space (i.e., their occurrence was not related to any of the site variables), which suggests that these species are habitat gene ralists.