Holocene fire reconstructions in Patagonia and the western U.S.: providing a context for recent catastrophic fires in temperate forests
by
Cathy Whitlock
Dept. of Earth Sciences, Montana State University, Bozeman MT 59717 USA
Coauthors: Patrick Bartlein (Dept. Geography, Univ. Oregon,USA), Vera Markgraf (INSTAAR, Univ. Colorado, USA), Maria M. Bianchi (CONICET-CRUB, Universidad Nacional del Comahue, Argentina) and Jennifer Marlon (Dept. Geography, Univ. Oregon, USA)

Holocene fire-history records from southern Argentina and Chile and the western U.S. provide an opportunity to compare the linkages among fire, climate, and vegetation on multiple temporal and spatial scales in the two hemispheres. The reconstructions, based on high-resolution charcoal and pollen records, suggest that climate variations were the primary control of past fire occurrence at the regional scale, with human influences potentially important at the local scale. In Patagonia, sites from lat 40-44S, and south of lat 55S register high fire occurrence between 12 and 6-8 ka, when summer insolation was less than at present but winter insolation was greater. High fire occurrence may be explained by the location of winter and summer storm tracks between lat 46 and 50S and a pattern of interannual variability that favored recurring drought in the early Holocene. In northwestern North America, fire history data also indicate heightened fire occurrence between 12 ka and ca. 5-7 ka, when summer insolation was greater than present and winter insolation was less. Higher-than-present fire frequency is attributed to environmental changes arising from the amplification of the seasonal cycle of insolation, including the enhancement of the subtropical high-pressure system and increased summer drought. A network of sites in the western U.S. reveals regional differences in fire-climate relationships: the northern Coast Range has been consistently characterized by relatively few fires, whereas the southern Coast Range has experienced frequent fires. Fire regimes in the northern Rocky Mountains have been strongly governed by millennial and centennial climate variability and subregional differences in summer precipitation. In both hemispheres, the association between intensified drought episodes, increased fire occurrence, and xerophytic vegetation suggests that such long-term linkages should be considered when assessing the effects of present-day climate, human actions, and forest management practices on current fire regimes.

Date received: September 6, 2004