Longleaf pine (Pinus palustris) savannas occur over a range of xeric to wet-mesic sites in the southern USA, and are regulated by fires that are ignited by lightning or prescribed by management. Suppression of fire changes ecosystem structure and function over time, and shifts in nitrogen (N) cycling are hypothesized to result from decreased losses in combusted N, elevated N mineralization rates from decomposing litter, increased immobilization of Phosphorus (P) in accumulating litter, and changes in N use efficiency by shifts in dominant species. This long-term study is examining the extent that fire, N, and water temporally and spatially control forest species composition, productivity, and nutrient cycling in a longleaf pine-wiregrass ecosystem. Plots are arranged across two soil moisture gradients (xeric/mesic). Treatments consist of N fertilized, irrigated, N fertilized + irrigated, burn exclusion and burn exclusion + N fertilization and a reference. Preliminary data point to landscape controls on soil moisture as a major regulator of productivity. Resource availability may differentially regulate productivity above- versus belowground. Fire exclusion from longleaf pine should shift the species composition from predominantly longleaf pine-wiregrass savanna to a hardwood/longleaf pine mixed forest. Because areas excluded from fire have greater canopy N and higher aboveground net primary productivity, litterfall quality and amounts are hypothesized to be greater than frequently burned sites. N mineralization is hypothesized to be greater with fire exclusion due to increased litter inputs, higher litter quality and turnover. These responses should vary between extremes of the hydrologic gradient, but the experimentally increased water and N availability treatments may cause shifts in species composition, productivity, litterfall, litter quality, and N mineralization. Within constraints regulated by fire regimes, N and water are hypothesized to interact to control productivity and other ecosystem processes differentially across this forested landscape, and across time.
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