Constraints on Mechanized Treatment Significantly Limit Mechanical Fuels Reduction Extent in the Sierra Nevada

With air quality, liability, and safety concerns, prescribed burning and managed wildfire are often considered impractical treatments for extensive fuels reduction in western US forests. For California's Sierra Nevada forests, we evaluated the alternative and analyzed the amount and distribution of constraints on mechanical fuels treatments on USDA Forest Service land. With the use of current standards and guides, feedback from practicing silviculturists, and GIS databases, we developed a hierarchy of biological (i.e., nonproductive forest), legal (i.e., wilderness), operational (i.e., equipment access), and administrative (i.e., sensitive species and riparian areas) constraints. Of the Sierra Nevada Bioregion's 10.7 million acres in USDA Forest Service ownership, 58% contains productive forest and 25% is available to mechanical treatment. National forests in the southern Sierra Nevada have higher levels of constraint due to more wilderness and steeper, more remote terrain. We evaluated different levels of operational constraints and found that increasing road building and operating on steeper slopes had less effect on increasing mechanical access than removing economic considerations (i.e., accessing sites regardless of timber volume). Constraints due to sensitive species habitat and riparian areas only reduced productive forest access by 8%. We divided the Sierra Nevada Bioregion into 710 subwatersheds (mean size of 22,800 acres) with >25% Forest Service ownership as an approximation of a relevant management planning unit for fire or fireshed. Only 20% of these subwatersheds had enough unconstrained acreage to effectively contain or suppress wildfire with mechanical treatment alone. Analysis suggests mechanical treatment in most subwatersheds could be more effective if it established a fuel-reduced anchor from which prescribed and managed fire could be strategically expanded. With potential future increases in wildfire size and severity, fire policy and forest restoration might benefit if mechanical thinning is more widely used to leverage and complement managed fire.