ENVIRONMENTAL GRADIENTS IN SECONDARY FORESTS OF THE GEORGIA PIEDMONT,USA

Nearly all of the forest cover on the Georgia Piedmont was removed for agriculture during the nineteenth century. Subsequently, much of this land was abandoned and has regrown to forest. A study of seventy-five 0.1 ha plots distributed across the central Georgia Piedmont relates species distribution within these secondary forests to prominent envir onmental gradients. Ordination by canonical correspondence analysis se parates plots into two relatively distance communities, floodplain and upland forests. Within each community there is continuous composition al variation in response to environmental gradients. The composition o f upland forests is primarily correlated with stand age, soil nutrient s, and topography (presumably a surrogate for soil moisture status). A lthough topography/moisture has typically been cited as the primary en vironmental determinant of mature upland Piedmont forest organization, soil nutrients are found to be of particular importance here. Species ' nutrient demands appear to mediate the outcome of competitive intera ctions in later stages of succession, thus late successional forests e xhibit compositional sorting along soil nutrient gradients. Floodplain forest composition varies along a gradient of proximity to the channe l, apparently as a response to variation in flood frequency and durati on (with associated influences on soil physical and chemical propertie s). Patterns of species affinities to floodplain and upland sites reve al an important group of transitional species associated with disturba nce (e.g. Liquidambar styraciflua, Quercus nigra, Ulmus alata), occurr ing in floodplain forests and as early successional colonists of uplan d sites.