Component of Plant Competition Along an Experimental Gradient of Nitrogen Availability

We tested for variation in the intensity of above— and belowground competition along a 5—yr—old experimental gradient of nitrogen availability in a Minnesota old field. Standing crop increased, species richness decreased, and species composition varied significantly as nitrogen availability increased. Transplants of the three grasses that were dominants at the three levels of the nitrogen gradient (low; Schizachyrium scoparium, intermediate: Poa pratensis, high: Agropyron repens) were grown along the gradient with no neighbors present, with only the roots of neighbors present, or with both the roots and shoots of neighbors present. Resource measurements indicated that the treatment with only neighbor roots present provided at light regime similar to that in which all neighbors had been removed but a nitrogen regime similar to that in which all neighbors were present. At low nitrogen availability, transplants grown with only neighbor roots generally did not differ significantly in biomass or growth rate from those grown with both neighbor roots and shoots, suggesting that the bulk of neighbor effects at low nitrogen were belowground. At high nitrogen availability, plants grown with only neighbor roots generally grew significantly larger than those grown with both roots and shoots of neighbors, but not as large as plants grown with no neighbors, suggesting that both above— and belowground competition occurred. At low nitrogen availability, plants grown with neighbors weighed 3—12% as much as those grown without; at the highest rate of nitrogen addition, plants grown with neighbors weighed 12—58% as much as those grown without, indicating that competition occurred on both poor and rich soils. The intensity of competition, measured as the suppression of transplants by neighbors, did not vary significantly with nitrogen availability. Further, the per—gram effect of neighbors, measured as the slope of the relationship between transplant performance and neighbor biomass, did not vary significantly with nitrogen supply rate. In total, competition was an important force at all points along the experimental productivity gradient, but competition shifted from being mainly belowground in the least productive vegetation to both above— and belowground in fertilized plots.