Spatial controls on fire regimes in three fire-prone regions of the western US

Background/Question/Methods

The ecosystem process of wildfire responds to and generates biophysical patterns. Over time and multiple fire events, this interaction between process and pattern potentially leads to a dynamic equilibrium of self-regulating or resilient landscapes. To understand this dynamic that is thought to lead to resilience, it is important to quantify the key physical and vegetative controls on fire regimes. We synthesized two studies of the process-pattern dynamic of fire regimes on relatively natural landscapes in three fire-prone regions in the western United States: the northern Rockies, the Sierra Nevada, and the Southwest. The first study sought to use pattern in the form of contemporary fire histories from each region to infer the process of dynamic equilibrium, while the second simulated the process of fire on a wilderness landscape in each region to create patterns of fire likelihood. We reconciled results from these very different studies to better understand how key spatial bottom-up controls influence fire regimes. Topographic roughness was represented as either fractal dimension or heterogeneity in elevation. Various variables were used as proxies to describe vegetation and included spatial climate normals and characteristics of fuels that affect fire spread rates.

Results/Conclusions

Both studies quantified the influence of the factors comprising the biophysical template upon which fire operates, and showed that the very nature of this influence varied greatly among the three regions. In particular, the relative importance of topography and the proxy variables for vegetation differed among the regions in ways that are at least partly attributable to the spatial structure of a variable. Furthermore, the two studies elucidated how factors interact and how these interactions differed among the three regions. Notably, in synthesizing results from the two studies, we revealed the dualistic role of topography. This dualistic role results from its ability to either promote the uphill spread of fire or to inhibit spread through dissected terrain. Results suggest that topography may thus act as a regulatory constraint on fire regimes in at least the Sierra Nevada and Southwest regions. In the northern Rockies, however, topographic roughness appeared to enhance fire spread. As such, bottom-up factors such as topography might serve as constraints on fire regimes and thus contribute to landscape-level resilience in some, but not all, cases.