Abstract
Fire and soil moisture gradients are thought to influence oak community structure in longleaf pine (Pinus palustris Mill.) woodlands. To explore these influences, we measured oak density and species composition within a longleaf pine-wiregrass (Aristida stricta Michaux) dominated landscape subjected to frequent cool-season fires for 70 yr. At 64 locations within the Joseph W. Jones Ecological Research Center in southern Georgia, oak trees, saplings and regeneration (seedlings plus sprouts) were counted in nested plots, and 20 additional measurements were taken to assess physical and chemical properties of soils, disturbance, density of other vegetation and topography. Principal components analysis of soil data revealed a soil moisture and soil chemistry gradient. Trees and saplings were sparse (mean of 37 and 81 per ha) and most common in dry and dry-mesic sites, while regeneration was abundant (mean of 110,100 stems per ha) and well-distributed across the soil moisture gradient. Fifty percent or more (depending on vegetation stratum analyzed) of cumulative species-environment relation was accounted for in the first two axes of a canonical correspondence analysis with axes representing gradients in soil moisture and several chemical and physical properties including mineralizable N, extractable Ca and Mg and soil texture. Three oak species were concentrated in the dry end of the soil moisture gradient, four in the moist end and one was common across much of the gradient. Species distributions probably reflect physiological tolerances of soil moisture extremes plus variation in fire regime caused by differences in soil moisture. Forest fragmentation and prescribed cool season burning may have increased oak densities in this landscape. If so, then this legacy of past management should be considered if managers wish to change fire regimes to better mimic natural disturbance patterns.
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