FIRE REGIME SENSITIVITY TO DECADAL- THROUGH MILLENNIAL-SCALE CLIMATE VARIABILITY IN ROCKY MOUNTAIN SUBALPINE FORESTS, COLORADO, U.S.A

A key uncertainty in anticipating future fire regimes is the degree to which climatic change will impact fire regimes directly, through changing the frequency and magnitude of summer drought, vs. indirectly, through broad-scale changes in vegetation and landscape flammability. We studied the sensitivity of subalpine forest fire regimes to the direct and indirect impacts of climatic variability over the past 6000 years, utilizing four high-resolution lake-sediment records from subalpine forests in Rocky Mountain National Park, Colorado. We reconstructed fire-event frequency and total biomass burning using macroscopic charcoal analysis, and changes in forest vegetation using fossil pollen analysis. The sensitivity of fire regimes to climatic variability was evaluated through statistical comparisons between fire history metrics and existing paleoclimate records. The most pronounced change in the fire regime was in response to the indirect impacts of millennial-scale climatic cooling on vegetation: decreased forest density ca. 2400 yr BP, coincident with evidence of a lowering treeline regionally, was immediately followed by a decrease in biomass burning and inferred fire severity. This shift in fire severity was not accompanied by a shift in the rate of burning. Rather, the average rate of burning over the past 6000 years was largely non-varying, with mean fire return intervals ca. 150-250 yr, statistically similar to tree-ring based estimates from the past 300 years. Variability around these long-term mean return intervals was significantly and positively correlated with summer moisture deficits, inferred from lake-level and oxygen isotope records, and tree-ring reconstructions of drought frequency. Fire regimes were thus sensitive to the direct impacts of climatic variability at decadal to centennial scales, despite their complacency to climatic variability at millennial scales. Our results highlight a varying sensitivity of fire regimes to climatic change across multiple time scales, and they imply that future fire regimes will be particularly sensitive changes in forest vegetation.