Spatially-patchy disturbance often creates spatial heterogeneity in vegetation structure that has long been recognized as key to increasing biodiversity and recently shown to reduce variability in aboveground plant biomass across years. While the magnet effect -- the attraction of recently-burned areas to grazers who find high-quality forage in the succulent regrowth -- is widely recognized as the mechanism that maintains contrast among patches in the season after the initial burn, less research has explored within-season variability in forage quality, availability, and grazing distribution. Also of interest in extensive landscapes in which productivity is limited by precipitation is whether the imposed pattern of disturbance-driven heterogeneity can overcome inherent landscape heterogeneity, and how persistent disturbance-driven effects are under variable precipitation. We explored these questions in two experimental rangeland landscapes in the Northern Great Plains. At each location, sampling points were stratified within ecological sites in both burned and unburned patches across replicated pastures. Samples were collected at monthly intervals throughout the grazing season.
Results/Conclusions
Rainfall patterns diverged across the rangeland landscapes reported here: one location began the growing season wet and ended in drought, while the other had the opposite trend. Regardless of uneven precipitation, spatially-patchy disturbance created and maintained contrast at both locations throughout the growing season. Based on fecal densities around forage sampling points, grazers showed a strong preference for burned patches, indicating that the disturbance-driven heterogeneity imposed by the initial prescribed patch burn overrode inherent heterogeneity in soils to create a patch-level magnet effect that persisted in both landscapes despite . But across ecological sites, both forage quality and forage production varied and even appeared to switch through the grazing season, suggesting that even though management-driven heterogeneity can override inherent environmental heterogeneity, drought resilience and within-season stability are likely maximized when ecological site diversity is relatively even across patches. Forage quality -- measured here as crude protein -- declined sharply as biomass increased, indicating that repeated grazing is critical to maintaining high forage quality throughout the season following fire.