Long-term environmental history of two low-elevation mixed-conifer forests, Mission Valley, Montana

Abstract

Low elevation mixed-conifer forests are widespread throughout the Northern Rocky Mountains, yet there are few long-term environmental histories from these structurally and compositionally heterogenous ecosystems. We reconstructed >10,000 years of vegetation change, fire activity, and human presence (e.g., pollen, charcoal, biomarkers) for two closed-basin lakes in mixed-conifer forests in the Mission Valley, western Montana. Environmental reconstructions highlight periods of pronounced changes in climate, vegetation, and fire activity. The late glacial period (>18,000-11,000 cal yr. BP) was characterized by post-glacial warming, generally wet conditions, establishment of mixed-conifer forests and infrequent fires. Following an abrupt, short-lived return to Juniper/Douglas fir parkland associated with the Younger Dryas (~12,900-11,500 cal yr. BP), warming temperatures during the early Holocene (11,000-6,000 cal yr. BP) promoted the expansion of open parkland/grasslands and frequent fire activity until cooler summers and warm, wet winters facilitated the development of modern-day closed mixed-conifer forests. Organic biomarker analyses indicate human presence within the Rainbow Lake watershed for millennia c. 7,000-3,000 cal yr. BP. Regional fire frequency increased during this period at Rainbow Lake, suggesting a possible increased role of human influence.