Grazing effects on shrub-induced resource islands and herbaceous vegetation heterogeneity in sagebrush-steppe communities

Spatial heterogeneity in plant communities promotes coexistence and diversity by providing a variety of niches for different species. In shrub-steppe communities, shrubs create distinct microsites under their canopies (canopy microsites) compared to areas between their canopies (interspace microsites). This creates spatial heterogeneity in soil nutrient availability and herbaceous vegetation. Grazing can influence spatial heterogeneity, but the effect of grazing on shrub-induced spatial heterogeneity is largely unknown. We investigated the long-term (+80 yrs.) effects of moderate grazing by cattle on sagebrush (Artemisia)-induced spatial heterogeneity in soil nutrients, herbaceous vegetation, and ground cover in sagebrush-bunchgrass steppe communities at eight sites in southeastern Oregon. Each site consisted of a long-term grazing exclosure and an adjacent grazed area. Almost all measured herbaceous vegetation (cover, density, diversity, and evenness) and ground cover variables differed between canopy and interspace microsites. Grazing did not influence the effects of microsites on most measured herbaceous vegetation characteristics and ground cover variables. Available soil nutrients were not influenced by grazing, but the majority differed between microsites. The limited effect of moderate grazing on shrub-induced spatial heterogeneity provides evidence that sagebrush exerts a strong influence on patterns of soil nutrients and herbaceous vegetation in sagebrush-bunchgrass communities. These results also demonstrate that moderate grazing by cattle can be compatible with maintaining spatial heterogeneity within shrub-steppe communities. In these ecosystems, maintaining and restoring shrubs is critical to promoting spatial heterogeneity, thereby encouraging coexistence and diversity.

Automated summary

Spatial heterogeneity in plant communities promotes coexistence and diversity by providing a variety of niches for different species. The effect of grazing on shrub-induced spatial heterogeneity is largely unknown. The limited effect of moderate grazing on shrub-induced spatial heterogeneity provides evidence that sagebrush exerts a strong influence on patterns of soil nutrients and herbaceous vegetation in sagebrush-bunchgrass communities. The results also demonstrate that moderate grazing by cattle can be compatible with maintaining spatial heterogeneity within shrub-steppe communities.