Fires and Climate in Forested Landscapes of the U.S. Rocky Mountains

Scattered reports indicate that the number of fires or area burned has increased recently in parts of the northern temperate zone, but is climatic change responsible? Annual number of fires and area burned have generally increased since about 1950 in Canada and Sweden (Stocks 1991), the Rocky Mountains (Qu and Omi 1994; Fig. 5.1a) and the western United States (Arno 1996; Fig. 5.1b). However, trends in fire statistics may in part reflect increasing ability to monitor fires (Ryan 1976; Qu and Omi 1994). Moreover, in Canada and in Yellowstone National Park, trends are dominated by a few exceptional fire years in the 1980s (Stocks 1991; Balling, Meyer, and Wells 1992a). Also suppression of fires decades ago may have increased fuel loads, leading to the larger fires seen now (Covington and Moore 1994). Finally the landscape may shape potential responses to climatic change, leading to disequilibrium between climate and fires (Baker 1995). Identifying a climatic signal in historical fire data may thus require more understanding of how climate, fuels, the landscape, and land-use practices separately and jointly shape fire regimes. To organize a discussion of the present state of understanding in the Rocky Mountains, I contrast a view that emphasizes how broad-scale patterns of climate and fuels control fire regimes, with a contingent view in which local spatial constraints and historical legacies may limit general trends. While these perspectives on what is important underlie models, empirical studies, and theories, they are seldom explicit. Models that represent the broad-scale view, for example, suggest that fires may hasten the response of vegetation to climatic