Postglacial Fire Frequency and its Relation to Long-Term Vegetational and Climatic Changes in Yellowstone Park

Abstract

The paleoecologic record provides unique insights into the response of communities to environmental perturbations of different duration and intensity. Climate is a primary agent of environmental change and its long-term effect on the vegetation of the Yellowstone/Grand Teton region is revealed in a network of pollen records. Fire frequency is controlled by climate, and as climate changes so too does the importance of fire in shaping spatial patterns of vegetation. The prehistoric record of Yellowstone's Northern Range, for example, shows the response of vegetation to the absence of major fires in the last 150 years (Whitlock et al., 1991; Engstrom et al., 1991, 1994). In longer records spanning the last 14,000 years, periods of frequent fire are suggested by sediments containing high percentages of fire-adapted trees, including lodgepole pine and Douglas-fir, and high amounts of charcoal (Whitlock, 1993; Whitlock and Bartlein, 1993; Whitlock et al., 1995; Millspaugh and Whitlock, 1995). The primary research objective has been to study the vegetational history of the Yellowstone region and examine the sensitivity of vegetation to changes in climate and fire frequency on different time scales. This information is necessary to understand better the relative effects of climate, natural disturbance, and human perturbation in the present and future. Fossil pollen and plant macrofossils from dated-lake sediment cores provide information on past vegetation and climate. The frequency of charcoal particles and other fire indicators in dated lake-sediment cores offer evidence of past fires. Studies of future climate and vegetation in the Yellowstone region were based on climate simulations produced by general circulation models that incorporate a doubling of carbon dioxide and an equilibrium model to project potential range displacement of selected tree taxa. Relations between present distributions of tree taxa and climate were established by the use of response surfaces. The study has been divided into three parts: (1) an investigation of the vegetation history of Yellowstone National Park (YNP), (2) an analysis of charcoal accumulation in lakes following the 1988 fires and a reconstruction of fire history in central YNP on long time scales, and (3) a study of the potential ranges of tree taxa in the future with global increases in atmospheric carbon dioxide. A final report to the UW-NPS Research Center describes the methodology and findings in detail (Whitlock et al., 1994).