Multiple disturbance interactions and drought influence fire severity in rocky mountain subalpine forests

Disturbances such as fire, insect outbreaks, and blowdown are important in shaping subalpine forests in the Rocky Mountains, but quantitative studies of their interactions are rare. We investigated the combined effects of past disturbances, current vegetation, and topography on spatial variability of the severity of a fire that burned approximately 4500 ha of subalpine forest during the extreme drought of 2002 in northwestern Colorado. Ordinal logistic regression was used to spatially model fire severity in relation to late 1800s fires, a 1940s spruce beetle outbreak, forest cover type, stand structure, and topography. The late 1800s fires reduced the probability of burning in 2002, and the 1940s beetle outbreak slightly increased the probability of fire, particularly at high severity. Aspen (Populus tremuloides) and lodgepole pine (Pinus contorta) stands, which established after the late 1800s fires, were less likely to burn, whereas Engelmann spruce (Picea engelmannii)–subalpine fir (Abies lasiocarpa) stands were more likely to burn. The highest elevations (≥3100 m) had the lowest probability of burning, whereas intermediate elevations (2900–3100 m) had an increased probability of burning at high severity. The influences of the late 1800s fires and 1940s beetle outbreak on stand structure and forest cover type may be more important than their direct effects on fuels. The most important predictors determining fire severity were stand structure, forest cover type, the late 1800s fires, and elevation. Although, in other studies, the effects of pre‐burn stand conditions and topography declined with increasingly severe fire weather, in the case of the 2002 fire in Colorado, these predictors explained 42% of the variability of fire severity. Thus, these results suggest that pre‐burn stand conditions are important influences on burn severity even for fires burning during extreme drought.