PLANT COMPETITION AND RESOURCE AVAILABILITY IN RESPONSE TO DISTURBANCE AND FERTILIZATION

Variation in the intensities of above- and belowground competition is proposed to underlie changes in community structure associated with variable fertility and disturbance. Here we report measurements of light, soil nitrogen, and above- and belowground competition in vegetation that had received four combinations of fertilization and disturbance for 3 yr. Two levels of nitrogen addition (none and 17 g. m-2 . yr-1) and two of soil disturbance (none and annual tilling to remove all vegetation) were applied in a factorial design to 5 x 5 m plots (10 replicates) in a 31-yr-old field in southeastern Minnesota. During the 3rd yr of the experiment, community standing crop, light penetration, and soil ammonium and nitrate concentrations were measured every 3-4 wk. Fertilization significantly increased productivity and tilling significantly decreased community biomass at the start of the growing season, indicating that the experimental treatments created variation in stress and disturbance. Transplants of Schizachyrium scoparium, a native perennial grass, were grown with no neighbors, or with the roots of neighbors, or with all neighbors in each combination of fertility and disturbance in order to examine competition intensity. In both undisturbed and tilled plots, belowground competition was the dominant interaction in unfertilized plots, whereas both below- and aboveground competition controlled growth in fertilized plots. The intensity of aboveground competition was greatest in plots with the lowest light penetration and declined significantly with increasing light supply. Similarly, belowground competition was most intense in plots with the lowest nitrogen availability and decreased significantly with increasing nitrogen availability. The intensities of above- and belowground competition were significantly negatively correlated. The total (above- and belowground) intensity of competition did not vary with fertility, but was significantly reduced by disturbance. Similarly, the effect of neighbor mass on transplant growth was significantly reduced by disturbance.