HOLOCENE FIRE, CLIMATE, AND VEGETATION IN THE NORTHERN ROCKIES

Holocene fire, vegetation, and climate history was reconstructed from four lakes in the Bitterroot region of the Northern Rocky Mountains based on analyses of fossil charcoal, pollen, and macrofossils in sediment cores. The sites lie on an east-to-west gradient that reflects decreasing significance of summer precipitation. Long-term records of fire frequency reconstructed from charcoal data were used as a proxy for summer moisture. During the early Holocene, ca. 10,000-6800 cal yr BP, sites in the eastern study area experienced less fire than at present while western locations experienced more fires. The spatial variability is attributed to a heightened east-west contrast in precipitation regimes resulting from the indirect effects of greater-than-present summer insolation. Sites in the eastern region received increased summer moisture from enhanced monsoonal circulation, whereas western sites registered drought conditions created by a strengthened subtropical high-pressure system. These large-scale controls on summer moisture and fire regime attenuated in the middle and late Holocene.

The vegetation history, inferred from pollen and plant macrofossil records, indicates widespread Picea parkland and alpine vegetation before ca. 12,000 cal yr BP. The late-glacial vegetation was replaced by Pinus-dominated forests after ca. 12,000 cal yr BP, and most sites indicate increased abundance of Pseudotsuga in the early and middle Holocene (ca. 10,000-5000 cal yr BP) as a result of warmer temperatures than present. Modern forests were established by ca. 3000 cal yr BP, reflecting the onset of cool, moist conditions. High fire frequency was associated with plant taxa that indicate low effective moisture and high temperatures throughout the Holocene. Dendroctonus ponderosae remains at two sites indicate past outbreaks of mountain pine beetle at ca. 5000 and 8000 cal yr BP. These outbreaks occurred during periods of low fire frequency suggesting that past infestations were associated with wet conditions that likely resulted in dense homogeneous forests susceptible to attack. This analysis of long-term records demonstrates the interplay between the effects of large-scale climate changes at the regional scale and the embedded local responses of vegetation, fire, and insect infestation at the watershed scale.