The response of plant communities to disturbance may be influenced by the presence of non-native species, prior disturbances or anthropogenically enhanced fertility. Grazing and fire are two common ecological disturbances in grasslands and wetlands but the influence of management intensity, grazing, fire, and their interactions on community responses have not been well studied. An understanding of these interactions is necessary to maximize ecosystem services such as biodiversity, wildlife habitat, and forage production.

We evaluated the response of plant communities to fire and grazing in forty isolated, depressional wetlands embedded in two distinct management intensities in subtropical pastures: intensively managed and semi-natural.  Treatments were: 1) ungrazed and unburned, 2) grazed and unburned, 3) ungrazed and burned, and 4) grazed and burned, replicated five times in each pasture type. We hypothesized that grazing and fire effects on these communities would vary with management intensity. We predicted that release from grazing would increase exotic species and decrease diversity in wetlands in intensively managed pastures, but would increase dominant natives and lower diversity in wetlands in semi-natural pastures.  We also hypothesized that combined treatments of fire and grazing would have the highest diversity in both management intensities and that native and exotic species would respond differently to fire and grazing treatments.

Results/Conclusions

Release from grazing resulted in lower native plant richness across both management types.  Non-native plant richness was higher in intensively managed wetlands compared to semi-natural wetlands but was unaffected by grazing and fire treatments. Release from grazing resulted in a much larger increase in the abundance of the invasive native species, dogfennel (Eupatorium capillifolium) in intensively managed pasture wetlands than in the semi-natural pasture wetlands in the first growing season following grazing exclusion. Biomass of exotic grasses increased and that of native species biomass decreased with burning in wetlands embedded in intensively managed pastures but not in semi-natural pastures. Biomass of exotic forbs was decreased by fire in both management types. Our results demonstrate that the response of wetland plant communities to disturbance can be altered by surrounding management, such as nutrient inputs and upland alteration.