Multiple Disturbance Interactions and Drought Influence Fire Severity in Rocky Mountain Subalpine Forests

Abstract

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 (Ն3100 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.

Disturbance interactions can impact resilience mechanisms of forests

“…However, interacting disturbances can lead to a surprising lack of resilience, creating an event of extraordinary magnitude and may cause shifts to alternate stable states. It is conceivable that other disturbances, especially those with structural effects, could also interact with unusually dramatic and long-term consequences (e.g., Kulakowski et al 2003, Bigler et al 2005, Sibold et al 2007). Because many ecosystems are adapted to the disturbances common to their biome, compounding disturbances that create atypical conditions may impact them in unique and surprising ways, potentially exceeding ecosystem resilience.…”

Modelling natural disturbances in forest ecosystems: a review

“…Still, we find an inclusion in our disturbance-focused review valuable since drought is an important factor in the interaction with other disturbance agents in forest ecosystems (e.g., Bigler et al, 2005) and the simulation of drought conditions remains challenging for current ecosystem modelling approaches (Hanson et al, 2004). In line with the scope of this review we focus on models addressing distinct drought periods leading to tree mortality (see McDowell et al, 2008 for a recent review of ecological mechanisms), whereas gradual effects of water stress on processes such as growth are not the focus here.…”

“…These effects were modelled statistically applying logistic regression approaches (e.g., Fleming et al, 2002;Bigler et al, 2005;Sieg et al, 2006), classification tree models (Kulakowski and Veblen, 2007) as well as Markov chain Monte Carlo approaches (Lynch et al, 2006). Notably, Lundquist (2007) used a structural equation modelling approach to assess the effect of numerous disturbance agents on fuel loading, finding the greatest interactions for wind (indirect) and root rot (direct).…”

“…Models of such interactions mainly focused on descriptive, statistical approaches, including various logistic regression models (Bebi et al, 2003;Bigler et al, 2005;Breece et al, 2008), generalized linear models with different link functions (Peltonen, 1999;Eriksson et al, 2005;Hood and Bentz, 2007) and classification tree models (Kulakowski and Veblen, 2007). As an alternative approach for evaluating hypotheses and conceptual understanding about fire-bark beetle interactions, Youngblood et al (2009) demonstrated the utility of structural equation modelling.…”

The moisture content and opening of serotinous cones from lodgepole pine killed by the mountain pine beetle

“…Most notably, the beetle is affecting about half of the merchantable lodgepole pine forests of British Columbia and Alberta (Natural Resources Canada 2013) and threatens to expand east into Saskatchewan (Cullingham et al 2011). This unprecedented MPB outbreak has raised several concerns for forest managers (e.g., Bigler et al 2005, Negron et al 2008, Raffa et al 2008, Simard et al 2011, namely whether these MPB-killed forests will successfully regenerate naturally with or without fire.…”