Projected changes in global climate have important ramifications for the future of national parks and other reserves set aside to conserve ecological uniqueness. We explored potential implications of climatic changes on lifeform distribution and growth at Wind Cave National Park (WCNP), South Dakota, which lies on a climatically determined ecotone between grassland and forest. Fire, promoted by healthy grasslands, is a negative feedback limiting tree development because it kills seedlings and consumes live foliage thus reducing tree growth and survival. Historical records show that fire suppression has enhanced forest expansion. On the other hand, livestock grazing reduces grass biomass and fuel loads thus indirectly reducing fire frequency and enhancing the expansion of forests or woodlands. Natural fires and moderate grazing by native herbivores have maintained the coexistence of trees and grasses but climatic variations affecting the area's water resources can lead to dominance by either lifeform. We used a dynamic vegetation model (DVM) MC1 to simulate the interactions between climatic changes, natural fire regime, and grazing pressure and their impact on the biogeographical and biogeochemical characteristics of the park. We used one future climate projection (HADCM2SUL) which simulates warmer weather by the end of the next century: the temperature increase would constrain the growth of trees that rely on the availability of deep water, favor shrub and grass development and promote a shift from forests to woodlands. Woody encroachment of shrubs in grasslands areas, enhanced by grazing, was only held in check by frequent natural fires in the simulation.