Multiple disturbance interactions and extreme drought influence fire severity in a Rocky Mountain subalpine forest landscape.
Bigler, Christof*,1, Kulakowski, Dominik1, Veblen, Thomas1, 1 University of Colorado, Boulder, CO
ABSTRACT- Large-scale, severe disturbances such as fire, insect outbreaks, and blowdown are important in shaping subalpine forest landscapes in the Rocky Mountains, however, quantitative studies of their spatial and temporal interactions are rare. We investigated the combined effects of past disturbances, current vegetation, and topography on spatial variability of a fire that burned 4500 ha of subalpine forest during the extreme drought of 2002 in northwestern Colorado, USA. 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 strongly reduced probability of burning in 2002, and the 1940s beetle outbreak slightly increased 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. Old-growth stands increased the probability of burning at low or moderate severity, and saplings/poles were more likely to burn at high fire severity. 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 forest cover type and stand structure may be more important than their direct effects on the amount of dead 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.
Key words: disturbance ecology, fire, insect outbreak, spatial modeling
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