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

Peltzer, D.A., and M. Köchy (2001)

1. Variation in the competitive ability of plant species is suggested to determine their persistence and abundance in communities. In this study, we examine competition between grasses and woody plants, a phenomenon that occurs word-wide in savannas and along the margins between grasslands and forests. Specifically, we quantified the per-gram competitive effects of grasses and woody plants in native mixed-grass prairie by manipulating the natural vegetation.
2. We separated the competitive effects of grasses, shrubs, and intact vegetation using selective herbicides to create four competition treatments: no neighbours (NN), no shrubs (NS), no grasses (NG), and all neighbours (AN). Competition treatments were applied to 2m x 2m experimental plots located in either prairie- or shrub-dominated habitats. Herbicides were applied starting one year prior to the study to create the four competition treatments. The effects of grasses and shrubs on resource availability (light, water, nitrogen) and the growth of two transplant species, Bouteloua gracilis, a perennial tussock grass, and Elaeagnus commutata, a common shrub, were measured over two growing seasons.
3. Resource availability (i.e. light, soil moisture, soil available nitrogen) was two- to tenfold higher in no neighbour (NN) plots than in vegetated plots (NS, NG, AN). Both grasses and shrubs reduced resource levels to about the same extent. Light penetration declined linearly with increasing grass or shrub biomass, to a minimum of about 30% incident light at 500 g/m² neighbour shoot mass. Soil resources (i.e. water, nitrogen) did not decline with increasing neighbour shoot or root mass for either grasses or shrubs. Thus, for soil resources, the presence of neighbours was more important than their mass (i.e. abundance).
4. Transplant growth did not decline with increasing neighbour biomass; the only exception to this pattern was a linear decline in Bouteloua growth with increasing neighbour shoot mass in plots containing only shrubs (NG). Competition was equally intense in prairie-dominated and shrub-dominated habitats for transplants of Bouteloua, but was less intense in shrub-dominated habitats for the shrub Elaeagnus. These results raise the interesting possibility that variation in the persistence and abundance of plants in communities is more strongly controlled by variation in competitive effects than by differences in competitive response ability.