Effects of fuel reduction timber harvests on forage resources for deer in northeastern Washington

Fire suppression has increased the canopy cover, stocking density, and fuel loads of dry forests in the inland northwestern United States. These changes not only lead to high-severity, stand-replacing wildfires that may threaten human safety and further alter natural forest communities, but also reduce light transmission to the forest floor, which can limit production of herbaceous and woody understory vegetation that serves as forage for wild ungulates. Therefore, forest managers have implemented fuels reduction treatments that reduce the density of standing trees in mature stands to restore historic conditions, promote healthy forests, and reduce wildfire risk. To test the hypothesis that these treatments also improve the quality and quantity of forage resources for wild ungulates, we measured biomass, plant species composition, and nutritional quality of understory vegetation in 106 untreated and treated 0.5-ha plots that ranged from 1 to 22 years since treatment and 2–100% canopy cover in Douglas-fir (Pseudotsuga menziesii)/ponderosa pine (Pinus ponderosa) forests in the Colville National Forest of northeastern Washington. On average, fuels reduction treatments halved basal area and overstory canopy cover, doubling understory vegetation biomass. The most nutritious plants (i.e., composing suitable biomass) and plants most likely to be consumed by deer (i.e., acceptable biomass) increased with decreasing forest canopy cover, and increased with time since treatment for approximately 14 years post-harvest, but suitable biomass began declining thereafter. Understory plant biomass also responded to a range of site, weather, and environmental variables, especially those influencing moisture gradients. Our results indicate that active management prescriptions intended to reduce fuel loads posed by overstocked conifers can also increase the amount of high quality forage for deer and other wild ungulates, especially when carefully planning the timing, intensity, and rotation schedules of fuels reduction treatments.