The forests of the Sierra Nevada have been altered for millennia by climate, natural disturbances, and more recently by the activities of humans. Management of these forests and their resources as ecosystems to meet diverse objectives, requires an understanding of the conditions under which existing forests developed and how they have changed through time. Recently accumulated information suggests that the interactions of factors such as geographic position, topography, and shifts in climate have resulted in substantial spatial and temporal variation in the vegetation composition and structure of the forests of the Sierra Nevada. These forests are substantially different today than they were a century ago. The climate of the last 1,000 years appears to have been drier and wildfires were more frequent than at present. Consequently, the forests, especially pine and mixed conifer, were generally more open with fewer trees, but a greater proportion of larger diameter trees and fewer shade-tolerant or fire-intolerant trees. Although the evidence supporting this description of the average forest condition is persuasive, there is currently little information on the variation in forest structure and composition across the landscape and through time. Nor is there much information on the relations of other plants and animals to the changing forests. As forest managers seek to manage forests as ecosystems, information on the structural and compositional variation of forests in the Sierra Nevada through space and time should be used as the basis of management objectives.
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