CLIMATE, FIRE, AND VEGETATION LINKAGES IN THE TEMPERATE FORESTS OF THE WESTERN U.S. DURING THE HOLOCENE

A network of paleoecological sites in the northwestern U.S. provides an opportunity to examine the linkages among fire, climate, and vegetation change on multiple temporal and spatial scales. Throughout the Holocene, regional differences in fire activity are apparent in sediment and tree-ring records: the Coast Range has been characterized by relatively few fires, the Klamath Mountains experienced frequent fires, and within the northern Rocky Mountains, high-elevation sites have had lower fire frequencies than low-elevation sites. In addition, past changes in fire regimes and vegetation are strongly governed by millennial scale climate variability, including the amplification of the seasonal cycle of insolation in the early Holocene. The summer insolation maximum between 14 and 6 ka apparently enhanced monsoonal circulation causing wetter summers and decreased fire frequencies in parts of the northern Rocky Mountains, while a stronger-than-present Pacific subtropical high pressure system probably led to drier summers and increased fire frequencies in other parts of the region. The juxtaposition of these strengthened circulation features accentuated the spatial heterogeneity of precipitation, vegetation, and fire regimes at the sub-regional scale. In addition to helping us understand past fire regimes, the paleoecological record also has important implications for understanding modern and future fire regimes. In some locations, the present fire regime was established in the last century, but other sites show that near-modern conditions have persisted for several millennia. Simulations of fire-related climate variables suggest the potential for heightened fire activity in many areas of the region in the future and point to the early Holocene as a possible future “fire analogue.” Holocene records from the northwestern U.S. also provide interesting comparisons with those from similar environments in western temperate South America where the large-scale controls of Holocene climate change were different.