Effects of fire and grazing on smallscale spatial heterogeneity in a tallgrass prairie

1. Grazing and fire are historically two important factors in the development and maintenance of many grassland systems. Most previous studies investigated their effects separately. We conducted a study to gain insight in the effects of fire, grazing and their interaction on small-scale (0.25 m2) vegetation heterogeneity and distribution of resources. In four different treatments (IU = annually burned, ungrazed, 4U = 4-year burned, ungrazed, I G = annually burned, grazed, 4G = 4-year burned, grazed) we measured plant species composition and resource availability. 2. Grazing had a positive effect on species richness, diversity and evenness, while it decreased the dominance of C4-grasses (e.g. Andropogon gerardii). Fire had an opposite effect, it reduced species richness, diversity and evenness and enhanced the cover of C4- grasses. The effect of fire was stronger in the absence of grazing. 3. Heterogeneity in the plant community was increased under grazing, whereas fire or a fire-grazing interaction had no significant effect on a small scale (0.25 m2). The increased heterogeneity in grazed sites was strongly correlated with increased resource availability, and not with enhanced patchiness in resources distribution. 4. The availability of resources (light and N) was only affected by the grazing treatment. No spatial patterns in resource distribution were found in the present study. 5. We conclude that grazing is the most important factor on the small scale, I determining the resource availability, vegetation composition and heterogeneity.