ECOSYSTEM RESPONSE TO CLIMATE CHANGE AND FIRE IN SOUTHWESTERN OREGON DURING THE HOLOCENE

Oregon’s southern Cascade Range lies at the nexus of three distinct ecosystems and contains one of the world’s most biologically diverse forests. Although past ecosystem responses to fire and climate change have been well studied in the southern Coast Range, the eastern side of the Cascades and Klamath Mountains, the ecological and environmental histories of the western side of the southern Cascades are poorly understood. An 8000-year-long record from Hobart Lake (42.09935°N, 122.48170°W, 1458m), Oregon in the southern Cascades 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 an extraordinarily detailed vegetation and fire records based on pollen and macroscopic charcoal data. The chronology for the record was developed from a series of AMS radiocarbon dates and tephrochronology. From 8000 to 3500 cal yr BP, the dominance of xerophytic species, such as Pinus and Cupressaceae, and the high frequency of fires suggest that the climate 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 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 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.