Restoring forest health: The effects of biomass management on potential nutrient delivery to Lake Tahoe

Fire suppression has caused a decline in forest health and increased susceptibility to wildfire. Excess accumulation of organic debris alone has created a substantial source of biologically available N and P which can potentially move off-site into adjacent waterways. Wildfire has been found to increase the immediate mobilization of labile N, P, and S resulting in increased nutrient concentrations in surface runoff. Mechanical harvest and/or prescribed burning may offer effective restoration strategies if they can reduce wildfire hazard without adversely affecting discharge water quality. We compare the effects of wildfire, cut-to-length harvest with chip mastication, controlled burning, and untreated controls on potential hydrologic nutrient discharge. Mechanical harvest plus chipping decreased surface runoff concentrations of inorganic N, P, and S in the absence of controlled burning, but tended to increase runoff. Mechanical harvest with controlled burning, and controlled burning alone were found to decrease inorganic N and P concentrations in the surface runoff, but  S concentrations were higher after burning than beforehand. N and P discharge loads and flux for the burned and harvested treatments after burning were comparable to, but generally less than from the uburned/unharvested controls. The most ecologically significant effect of prescribed fire was the loss of N from the forest floor. Controlled burning in conjunction with mechanical harvest has the potential to improve long-term runoff water quality by reducing the N and P discharge and improving the overall health of forest ecosystems without the danger of the nutrient mobilization “shock” typically associated with a high intensity wildfire.