Fire‐regime complacency and sensitivity to centennial‐through millennial‐scale climate change in Rocky Mountain subalpine forests, Colorado, USA

Abstract

Summary Key uncertainties in anticipating future fire regimes are their sensitivity to climate change, and the degree to which climate will impact fire regimes directly, through increasing the probability of fire, versus indirectly, through changes in vegetation and landscape flammability. We studied the sensitivity of subalpine forest fire regimes (i.e. fire frequency, fire severity) to previously documented climate variability over the past 6000 years, utilizing pollen and macroscopic charcoal from high‐resolution lake‐sediment records in Rocky Mountain National Park, Colorado. We combined data from the four lakes to provide composite records of vegetation and fire history within a 200 km2 study area. Rates of forest burning were relatively complacent to millennial‐scale summer cooling and decreased effective moisture. Mean return intervals between fire episodes, defined over 500‐year periods, generally varied between 150 and 250 years, consistent with tree‐ring‐based estimates spanning recent centuries. Variability around these long‐term means, however, was significantly correlated with variability in summer moisture (i.e. more burning with drier summers), inferred from existing lake‐level and supporting palaeoenvironmental records. The most pronounced change in fire regimes was in response to decreased subalpine forest density ca. 2400 cal. year BP, itself a response to regional cooling. This indirect impact of climate was followed by a decrease in charcoal production per fire, a proxy for crown‐fire severity, while the long‐term rate of burning remained unchanged. Over the last 1500 years, increased summer evaporation and drought frequency were associated with increased fire severity, highlighting a direct link between fire and climate. Synthesis. Subalpine forest fire history reveals complacency and sensitivity of fire regimes to changing vegetation and hydroclimate over the past 6000 years. Complacency is highlighted by non‐varying fire frequency over millennia. Sensitivity is evident through changes in biomass burned per fire (and inferred fire severity), in response to climate‐induced changes in forest density and, more recently, increased summer drought. Overall, the palaeo record suggests that (i) fire severity may be more responsive to climate change than fire frequency in Rocky Mountain subalpine forests and (ii) the indirect impacts of climate on vegetation and fuels are important mechanisms determining fire‐regime response to climate change.