Can mechanical fuels treatments restore resilient forest landscapes in the Sierra Nevada?
Stresses on Sierra Nevada forests are likely to rise as changing climatic conditions increase the frequency and intensity of wildfire and drought events. Forest reconstructions and field studies indicate one role of fire in increasing forest resilience is the creation of structural and habitat heterogeneity. Recent research has suggested how mechanical thinning can augment post-burn heterogeneity in treated stands given burn window constraints that might otherwise homogenize fire severity. For forest landscapes, however, most models have focused on locating mechanical treatment to contain or suppress fire rather than facilitate post-burn forest heterogeneity. We examined the amount and spatial distribution of Forest Service land in the Sierra Nevada in which mechanical fuels reduction is allowed (including non-wilderness and non-roadless areas) and operationally feasible (considering factors such as merchantable timber, slope, distance to roads, and constraints associated with sensitive species and riparian areas). We binned our spatial analysis by 6thlevel hydrologic units (subwatershed scale) as a rough approximation of ‘firesheds’.
Mechanical treatment is highly constrained in much of the Sierra Nevada with only 15-35% of most National Forests available. The influence of different constraining factors varies across the region, but distance to roads is a particularly important factor related to both ecological risks and economic constraints. Many subwatersheds, particularly in the southern Sierra Nevada, have a small percentage of mechanically treatable land even if constraints to protect sensitive species and riparian areas were removed. Mechanical treatment alone cannot restore resilience in these forest landscapes. Targeted and innovative use of managed fire will be needed to reduce impacts of high-severity wildfire. We propose some approaches for optimizing mechanical and prescribed fire treatments. However we also suggest that new research on working with fire to increase landscape heterogeneity is urgently needed.