COMPETITION INTENSITY ALONG A PRODUCTIVITY GRADIENT IN A LOW-DIVERSITY GRASSLAND

Competition among plants often shifts from roots to shoots as producti vity increases and species composition changes. We examined competitio n in an old field with low diversity to test whether this shift occurr ed along a productivity gradient without species turnover. Forty plots received one of four nitrogen treatments (0, 5, or 15 g added N m(-2) yr(-1) or 400 g m(-2) yr(-1) of sawdust added to immobilize N) annual ly for 5 yr. All N levels were dominated by the perennial grasses Agro pyron cristatum and Bromus inermis. Transplants of Agropyron were grow n with all neighbors, roots of neighbors, or no neighbors present to m easure total, root, and shoot competition. Transplant growth was 22%-1 65% higher in subplots without neighbors present, which indicates that competition occurred. Competition from neighbor roots was primarily r esponsible for suppression of transplant growth over the entire produc tivity gradient. In contrast to previous field experiments that found either an increase in total competition intensity or a shift from root to shoot competition with increasing productivity, we found neither. Increases in total competition intensity or shifts from root to shoot competition found along other gradients may be caused by changes in sp ecies composition and not by increased resources or neighbor biomass. These results suggest that different competitive mechanisms may operat e in low-diversity vegetation than in more diverse natural vegetation.