High-elevation fire regimes in subalpine ribbon forests during the Little Ice Age and Medieval Period along the Continental Divide, Colorado, U.S.A.

Fires in high-elevation subalpine forests have been rare, making estimates of fire-return intervals and influences of climate on fire in these forests difficult. Lake sediment charcoal provides an opportunity to extend fire records into the past and to compare them with long-term climate reconstructions. Here, we reconstruct fire histories from two high-elevation subalpine lakes that are surrounded by fragmented spruce-fir ribbon forests. We then compare the fire histories to independent temperature and moisture reconstructions. Fire episodes at the two lakes have been rare for the last millennium, but were more frequent when the climate was warm and dry, a period from ∼1000 to 3000 Before Present (BP). Variations in fire-episode frequency at individual lakes rarely exceeded the stochastic range of variability estimated by resampling the fire-episode distributions, although variations at a site with few topographic firebreaks were more significant than at a site in rough terrain. When fire-episode frequencies from both lakes were summed, fire-episode frequencies declined significantly relative to the stochastic range when the climate was cool and wet, suggesting that climate exerts a more meaningful influence at larger spatial scales than individual lake records (>3000 ha). Temperature and moisture were significant predictors of fire frequency, but, overall, climate had a weak influence on burning; regression showed that the two climate variables significantly explained 34% of the variance in the summed frequency record. Based on the results, climate change is an important driver of fire frequency in high-elevation forests, but stochastic influences may overprint the climate controls and determine patterns at local spatial scales.