Holocene fire and vegetation along environmental gradients in the Northern Rocky Mountains

Holocene records of fire, vegetation, and climate were reconstructed from four sites in the Bitterroot Range region of the Northern Rocky Mountains in order to examine the vegetation and fire histories and evaluate the hypothesis proposed by Whitlock and Bartlein (1993) regarding the effects of increased summer insolation on precipitation patterns. Vegetation history in the series of sites was broadly similar. In the late-glacial period, the pollen data suggest open parkland dominated by Picea or alpine meadow, which reflect conditions cooler and drier than present. These open forests were replaced in the early to middle Holocene by forests composed mainly of Pinus and Pseudotsuga, which suggest conditions warmer than present. Modern forest compositions were in place by ca 3000 cal yr BP, and small variations in the timing of the vegetation shifts reflect local differences among sites. The long-term trends in fire occurrence support the hypothesis proposed by Whitlock and Bartlein (1993) that precipitation regimes were sharpened during the early Holocene summer insolation maximum but their location has remained unchanged as a result of topographic constraints. Sites located in areas currently summer-dry were drier-than-present during the early Holocene and fires were more frequent. Conversely, sites located in the areas that are summer-wet at present were wetter-than-present in the early Holocene, and fires were less frequent. On millennial time scales it appears that the climate boundary is controlled by topography and does not shift.