Plant competition and resource availability in response to disturbance and fertilization

The authors report measurements of light, soil N, and above- and belowground competition in vegetation that had received four combinations of fertilization and disturbance for 3 yr. Two levels of N addition (none and 17 g^.m^-2.yr) and two of soil disturbance (none and annual tilling to remove all vegetation) were applied in a factorial design in a 31-yr-old field in SE Minnesota. Over 3 yr, 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 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. Intensity of aboveground competition was greatest in plots with the lowest light penetration and declined significantly with increasing light supply. Belowground competition was most intense in plots with the lowest N availability and decreased significantly with increasing N availability. Intensities of above- and belowground competition were significantly negatively correlated. The total intensity of competition did not vary with fertility, but was significantly reduced by disturbance, as was the effect of neighbor mass on transplant growth. -from Authors