Competitive effects of grasses and woody plants in mixed-grass prairie

1 Variation in the competitive ability of plant species may determine their persistence and abundance in communities. We quantified the competitive effects of grasses and woody plants in native mixed-grass prairie on the performance of transplant species and on resources. 2 We separated the effects of grasses, shrubs and intact vegetation containing both grasses and shrubs by manipulating the natural vegetation using selective herbicides to create four neighbourhood treatments: no neighbours (NN), no shrubs (NS), no grasses (NG) and all neighbours (AN). Treatments were applied to 2 x 2 m experimental plots located in either grass- or shrub-dominated habitats. The effects of grasses and shrubs on resource availability (light, soil moisture, soil available nitrogen) and on the growth of transplants of Bouteloua gracilis, a perennial tussock grass, and Elaeagnus commutata, a common shrub, were measured over two growing seasons. 3 Resource availability was two- to fivefold higher in no neighbour (NN) plots than in vegetated plots (NS, NG, AN) with grasses and shrubs having similar effects. Light penetration declined linearly with increasing grass or shrub biomass, to a minimum of about 30% incident light at 500 g m(-2) shoot mass. Soil resources did not decline with increasing neighbour shoot or root mass for either grasses or shrubs, suggesting that the presence of neighbours was more important than their abundance. 4 Transplant growth was significantly suppressed by the presence of neighbours, but not by increasing neighbour shoot or root biomass, except for a linear decline in Bouteloua growth with increasing neighbour shoot mass in plots containing only shrubs. Competition intensity, calculated as the reduction in transplant growth by neighbours, was similar in both grass- and shrub-dominated habitats for transplants of Bouteloua, but was less intense in shrub-dominated habitats for the shrub Elaeagnus. Variation in the persistence and abundance of plants in communities may therefore be more strongly controlled by variation in the competitive effects exerted by neighbours than by differences in competitive response ability.