Zachary Steel, Marissa Goodwin, Marc Meyer, G. Andrew Fricker, Harold Zald, Matthew Hurteau, Malcolm P. North

Climate change is amplifying the frequency and severity of droughts and wildfires in many forests. In the western U.S., fuels reduction treatments, both mechanical and prescribed fire, are widely used to increase resilience to wildfire but their effect on resistance to drought and beetle mortality is not as well understood. We followed more than 10,000 mapped and tagged trees in a mixed-conifer forest following mechanical thinning and/or prescribed burning treatments in 2001 through the extreme 2012-2016 drought in California. Mortality varied by tree species from 3% of incense-cedar to 38% of red fir with proportionally higher mortality rates in the larger size classes for sugar pine, red fir and white fir. Treatment reductions in stem density were associated with increased diameter growth and rapidly growing trees had lower rates of mortality. However, the ultimate effects of treatment on drought-related mortality varied greatly by treatment type. All species had neutral to reduced mortality rates following mechanical thinning alone, but treatments that included prescribed burning increased beetle infestation rates and increased mortality of red fir and sugar pine. Fuels reduction treatments appear to benefit some species such as Jeffrey pine, but can reduce resistance to extreme drought and beetle outbreaks in other species when treatments include prescribed burning. In a non-analog future, fuels reduction treatments may require modification to provide resistance to beetle infestation and severe droughts.