Patterns and controls of ecosystem function in longleaf pine - wiregrass savannas. I. Aboveground net primary productivity

Longleaf pine - wiregrass (Pinus palustris Mill. - Aristida stricta Michx.) woodlands occupy sites ranging from deep, xeric sandhills to the edge of w etlands in the southeastern United States. Aboveground net primary producti vity (ANPP) of the overstory and understory were determined for three repli cate sites of three site types (xeric, intermediate, and wet-mesic) that sp an a wide environmental gradient. In addition, soil moisture (at 30 and 90 cm) and N mineralization (in situ buried bag incubations) were measured thr ough an annual cycle. Longleaf pine - wiregrass ecosystems varied by nearly twofold in ANPP across complex gradients. Overstory and understory and tot al (overstory and understory) ANPP were positively correlated to soil moist ure at 30 and 90 cm. The proportion of understory ANPP relative to the tota l ANPP did not increase across the environmental gradient as predicted by h ypotheses that invoke niche differentiation in rooting habits of grasses an d trees. Contrary to expectations, cumulative net N mineralization was nega tively related to soil moisture. All ANPP estimates were significantly and negatively related to cumulative N-mineralization. Further work is needed t o explore the mechanisms by which soil moisture regulates productivity acro ss space, time, and for individual species. Additional experimentation thro ugh resource addition would allow for investigations into multiple resource limitations and how resource limitations vary depending on gradient positi on.