Plant Competition and Herbivory in Relation to Vegetation Biomass

We tested whether effects of competition and herbivory on plant growth depend on the aboveground biomass of vegetation. Our objective was to test theoretical predictions about the site-dependence of biotic mechanisms of plant population regulation. Biomass ranged from a mean of 64 to 776 g/m^2 at the eight study sites, which included six old fields and two herbaceous plant communities near water. To measure effects of competition, 4-5 wk old plants of a perennial grass (Poa compressa) were transplanted into plots where neighboring plants were either left intact or removed and after 4 mo the shoot mass of transplants was compared in the two treatments. Transplants were caged in both treatments to exclude herbivores. Removing neighbors increased transplant shoot mass significantly at all sites, indicating that transplants experienced less competition for resources when their neighbors were removed. Regression analysis indicated that the increase in shoot mass due to neighbor removal was significantly greater at sites with higher biomass. This result supports theories that predict competition should increase with increasing biomass. To measure effects of herbivory, the growth of caged and uncaged plants was compared. Caging transplants did not increase their shoot mass significantly at the two sites with the lowest biomass. However, shoot mass was significantly greater for caged transplants than for uncaged transplants at all other sites. Caged transplants lost less tissue to herbivores such as small mammals and molluscs. Regression analysis indicated that the increase in shoot mass due to caging was significantly greater at sites with higher biomass. The combined effect of herbivory and competition was measured by comparing transplant growth in plots where transplants were caged and neighbors were removed with the growth of transplants in plots where transplants were not caged and neighbors were left intact. Transplant shoot mass was significantly greater where herbivores were excluded and neighbors were removed, especially at sites with high biomass. Our results indicate that competition and herbivory each have a greater effect on plant growth at sites with higher biomass and that herbivory has less effect than competition on plant growth at sites with relatively low biomass.