Ecosystem response to Holocene fire and climate change at Hobart Lake, southwestern Oregon
White, Alicia Lauren
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Oregon's southern Cascade Range contains one of the world's most biologically diverse forests. Past ecosystem responses to fire and climate change have been well studied in the Coast Range, the eastern Cascade Range, and the Klamath Mountains but the ecological and environmental histories of the southern Cascade Range are poorly understood. Therefore, an 8000-year-long record from Hobart Lake (42.09935°N, 122.48170°W, 1458m), Oregon in the southern Cascade Range was examined to better understand past changes in vegetation and fire activity, conifer biogeographic distributions, and regional climate variability. Hobart Lake is exceptional in that it has an unusually fast sedimentation rate, resulting in a detailed vegetation and fire records based on pollen and macroscopic charcoal data. From 8000 to 3500 cal yr BP, the dominance of xerophytic species, such as Pinus and Cupressaceae, and the high frequency of fires are consistent with a climate that was warm and dry. Late-Holocene vegetation from 3500 cal yr BP to the present day was characterized by an abundance of mesophytic taxa, such as Abies and Pseudotsuga, and the decline of xerophytic taxa such as Pinus. These changes, along with reduced fire frequency, suggest that the climate became cooler and wetter. In addition to the Hobart Lake record, changes in the abundance of Abies and Pseudotsuga pollen at multiple sites throughout Oregon and northern California were examined. Abies was abundant during the late glacial, its range and/or abundance contracted during the early Holocene, and it gradually became more widespread and abundant during the mid- and late-Holocene. Pseudotsuga became more abundant at northern low-elevation sites during the warm dry conditions of the early Holocene and then flourished in more southern mid-elevation sites when the climate became cooler and wetter in the late Holocene. The vegetation history at Hobart Lake and other sites is consistent with large-scale variations in regional climate related to slowly varying changes in the seasonal insolation cycle and the indirect effects of insolation on the size and strength of the northeastern Pacific subtropical high-pressure system.