Mountain ecosystems, with their geographic compression of climatic gradients and biological communities, offer unique opportunities for exploring the relationships among climate, disturbance, and forest response. Taking advantage of these opportunities, The Sierra Nevada Global Change Research Program began in 1991. Originally funded by the National Park Service, and now funded by the Biological Resources Division of the U.S. Geological Survey, the program has involved more than 20 scientists from ten research institutions. The program set out to explore the fundamental character and significance of forest changes driven by the two most powerful agents of change in the Sierra Nevada: climate and fire. Studies are organized around three time periods: past, present and future. This organizational approach--modern mechanistic studies and extensive paleoecological studies informing one another under the integrative framework of state-of-the-art computer models--is a uniquely powerful way of exploring the character and significance of forest change. The diagram below summarizes the project organization.