All plants exist in a competitive matrix, with strong and weak interactions among their neighbors. A simplistic view of plant competition is that the larger a plant is, the more competitively effective it is. However, grassland communities indicate that this is not necessarily the case. Grasslands evolved in the presence of drought, fire and grazing. All of these external factors influence plant size, growth rate, and physiological activity. The P.I. will examine the effects of herbivory on the competitive interactions among grasses in two different grassland types. Species in the tallgrass prairie of central North America have not been found to compensate for herbivory (sensu Brown and Allen 1989) while species in the montane grasslands of Yellowstone National Park have been found to do so. Plants which respond negatively to herbivory should be at a competitive disadvantage while those which respond positively should be competitively superior. Testing this hypothesis will require a combination of field and modeling work. Community patches will be subject to different levels of simulated herbivory in the Wichita Mountains Wildlife Refuge of Oklahoma and Yellowstone National Park. Competition for a specific resource will be monitored by following uptake of stable nitrogen isotopes between pairs of clipped/unclipped plants. Several species will be involved in the analyses. Stable isotope use will be used to determine the area that a particular target plant utilizes so that the number of true competitors can be noted and their spatial distribution can be determined.
