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 environmental gradients. Ordination by canonical correspondence analysis separates plots into two relatively distance communities, floodplain and upland forests. Within each community there is continuous compositional variation in response to environmental gradients. The composition of upland forests is primarily correlated with stand age, soil nutrients, and topography (presumably a surrogate for soil moisture status). Although topography/moisture has typically been cited as the primary environmental 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 interactions in later stages of succession, thus late successional forests exhibit compositional sorting along soil nutrient gradients. Floodplain forest composition varies along a gradient of proximity to the channel, apparently as a response to variation in flood frequency and duration (with associated influences on soil physical and chemical properties). Patterns of species affinities to floodplain and upland sites reveal an important group of transitional species associated with disturbance (e.g. Liquidambar styraciflua, Quercus nigra, Ulmus alata), occurring in floodplain forests and as early successional colonists of upland sites.