Components of plant competition along an experimental gradient of nitrogen availability

Along a gradient of N availability in a Minnesota old field, standing crop increased, species richness decreased, and species composition varied significantly as nitrogen availability increased. Transplants of the 3 grasses that were dominants at 3 levels of the N 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 neighbor present. The treatment with only neighbor roots present provided a light regime similar to that in which all neighbors had been removed but a N regime similar to that in which all neighbors were present. At low N 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 N were belowground. At high N 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 N availability, plants grown with neighbors weighed 3-12% as much as those grown without; at the highest rate of N addition, plants grown with neighbors weighed 12-58% as much as those grown without, indicating that competition occurred on both poor and rich soils. -from Authors