COMPLEXITY AND ECOLOGICAL RELEVANCE OF FIRE RESPONSE TO HOLOCENE CLIMATIC CHANGE

Model simulations suggest that climatic warming could increase forest fires, which would in turn alter future vegetation. Paleorecords of climate, fire, and vegetation can provide reality tests for such simulations. To assess how fire and vegetation may respond to climatic change at decadal to centennial scales, we conducted pollen and charcoal analyses of lake sediments representing four periods of major climatic change during the Holocene. (1) Paleoecologists have hypothesised that fire-regime changes at 8200 yr BP drove the expansion of important tree species, such as Fagus silvatica, in central Europe. Instead our cross-correlation analysis suggests that fire frequency variations in response to the 8200 yr BP cooling and the subsequent rapid warming did not substantially change forest composition. (2) Different disturbance agents, such as avalanches, grazing, and fire, as well as a climatic change to more oceanic conditions, have been considered as causes for the Holocene mass expansion of Alnus viridis in the Alps. Cross-correlation analysis shows that an increase in fire frequency promoted Alpine green alder when climate became warm and dry at 4200 yr BP. (3) Preliminary results from southeastern Alaska suggest that fire frequency increased in response to a climatic cooling at 2800 yr BP. This climatic change induced noticeable vegetational responses (e.g., an expansion of Alnus crispa, American green alder). However, time-series analysis shows that fire and most vegetational changes were not linked to each other. (4) In southeastern Alaska, regional fire frequency increased in response to cold and dry climatic conditions during the Little Ice Age (LIA, ca. 1500-1850 AD), whereas climatic warming after the LIA resulted in a marked decrease of fire frequency. For this period, cross-correlation analysis reveals that fire reinforced the climatically driven expansions of Alnus crispa and other disturbance-adapted species. Taken together our results suggest highly variable responses of fire and vegetation to Holocene climatic changes. The main factor influencing vegetation was climate whereas fire-frequency changes resulting from climatic variations did not always affect forest composition at regional scales.