Climate, fire and Grazing Effects at Wind Cave National Park, S.D.

Climatically, Wind Cave National Park is at the ecotone between grassland and forest so that small climatic variations can lead to dominance by either system. Natural fires promoted by productive grassy areas and moderate grazing by native herbivores have maintained a system where trees and grasses coexist. However, it is a fragile equilibrium that can be greatly affected by management practices such as fire suppression or livestock grazing and also by climatic changes. We used a dynamic vegetation model MC1 that simulates vegetation distribution, associated biogeochemical cycles and natural fire occurrence to test the sensitivity of the system. Simulated fire suppression enhances the expansion of forests. Fire, promoted by healthy grasslands, acts as a negative feedback on tree development since it consumes seedlings and live foliage thus reducing tree growth and survival. Simulated grazing reduces grass biomass and fuel load thus indirectly reducing fire frequency and enhancing the expansion of forests or woodlands. Future climate projections simulate warmer and drier weather by the end of the next century. This would constrain the growth of trees which rely on the availability of deep water resources and favor shrub and grass development and a shift from forests to savannas. To prevent shrub encroachment over grassland areas and conserve a source of forage for herbivores, Park managers will need to restrict the grazing pressure and maintain a frequent fire regime that can prevent woody seedlings establishment.