Holocene environmental history of the northwestern Great Basin and the analysis of modern pollen analogues in western North America

An analysis of present-day pollen, vegetation, and climate in western North America was used to interpret the environmental history at three sites from the northwestern Great Basin. A comparison of modern pollen and vegetation from 1884 sites in western North America indicated that different vegetation types were characterized by distinct pollen spectra. Relationships between modern pollen spectra and present-day climate showed that arboreal pollen types were associated with cool wet conditions and short growing seasons, whereas non-arboreal pollen types dominated in warmer drier environments with longer growing seasons. These modern data were used to interpret Holocene pollen data from Patterson Lake in subalpine forest, Dead Horse Lake in middle-elevation lodgepole pine forest, and Lily Lake in low-elevation mixed conifer forest. The pollen records suggest that sagebrush grasslands and open subalpine forests dominated middle and high elevations between ca. 11,500 and 6500 cal yr BP and were replaced by subalpine pine forests after ca. 6500 cal yr BP. At low elevations, open conifer forest was present between 9500–7500 cal yr BP after which the forest became more closed. Climate reconstructions based on modern pollen-climate relationships suggest a trend toward decreased growing season length, cooler summers, warmer winters, and increased annual precipitation from 9500 cal yr BP to present. The fire history indicates high fire activity at ca. 7000–8500 and 3000–4000 cal yr BP, and increased fire activity after 2000 cal year BP. These vegetation reconstructions were consistent with regional climate changes that occurred in response to variations in the seasonal cycle of solar radiation. In contrast, variations in fire activity were apparently more sensitive to climate variations on century and millennial time scales.; This dissertation includes unpublished coauthored materials.