Livestock grazing is the most extensive land use in drylands, and this is true for millions of hectares of big sagebrush plant communities, the largest dryland ecosystem in North America. However, much of our understanding of the long term effects on plant community composition and structure are based on total grazer exclusion experiments, which explicitly avoid the natural grazing gradients that occur in water-limited landscapes. We sampled plant communities along grazing intensity gradients associated with distance from 17 artificial water sources in the upper Green River Basin in Wyoming, USA. We used distance from water as a proxy for grazing intensity and used a nested sampling design to explore grazing effects across 6 spatial scales to answer our guiding questions:
1) How do total plant cover, functional type cover, and bunchgrass biomass respond along a grazing intensity gradient?
2) How do patterns of species and functional type richness, composition, and proportions of native/exotic species change along a grazing intensity gradient?
Results/Conclusions
Livestock grazing did not dramatically affect the structure and composition of plant communities, and the largest effects we observed were at intermediate distances from water sources. In these zones, there is less bunchgrass and total vegetation cover and more exotic species.
Total vegetation cover, total grass cover, cover of native species, and bunchgrass biomass were negatively related to grazing intensity, and cover of annual forbs, exotic cover, and exotic richness were positively related to grazing intensity. The changes in grass cover we observed were small and may not be ecologically meaningful. This builds on existing evidence that the balance in grasses and shrubs in big sagebrush systems may be relatively stable with current grazing practices.
Species richness, shrub density and size, bare ground, litter, and biological soil crusts did not vary along our grazing intensity gradients at large (1000 m2) and small (1 m2) spatial scales. Composition, similarly, did not vary along our grazing gradients. Our results suggest that the long term effects along our grazing intensity gradient are relatively small, and the big sagebrush plant communities we sampled are resistant to negative effects of livestock grazing.