CONTRASTING PATTERNS OF RESOURCE LIMITATION IN TREE SEEDLINGS ACROSS A GRADIENT IN SOIL TEXTURE

Resource competition theory for terrestrial plant communities explains gradients in species composition among various habitats as the outcom e of differential supply and demand for limiting resources. To examine one group of assumptions of this theory, for forest vegetation in sou theastern Texas, we grew seedlings of three broadly distributed tree s pecies, Acer rubrum L. (red maple), Liquidambar styraciflua L. (sweetg um), and Pinus taeda L. (loblolly pine), in intact soil cores collecte d from three sites located along a gradient in soil texture. We tested resource limitation in a temperature-controlled greenhouse with nutri ent fertilization, light, and moisture treatments applied in a resourc e-minus or screening design. This design provided most resources at le vels not limiting for growth while simulating (potentially limiting) f ield conditions for a single resource or group of related resources. L ight was most limiting with clay-rich floodplain soil cores under simu lated floodplain forest conditions. Soil nutrients were most limiting in sandy upland soil cores under upper slope forest conditions. Both l ight and soil nutrients were strongly limiting with loamy intermediate soil cores under simulated mesic forest conditions. The most limiting soil resource was phosphorus. The pattern of reciprocal limitation by aboveground or belowground resources in extreme habitats and joint li mitation in intermediate habitats was consistent with requirements of the theory.