Climate change is amplifying the frequency and intensity of drought and fire stress in many forests. In the western United States, 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 (Teakettle Experimental Forest) following mechanical thinning and/or prescribed burning treatments in 2001 through the extreme 2012-2016 drought in California. Using a Bayesian multi-level and multivariate model, we evaluated the direct and indirect drivers of drought mortality among five common conifer species.
Results/Conclusions
Mortality varied by 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 and white fir. Reductions in stem density were associated with increased diameter growth and 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 ultimately increased mortality of red fir and especially sugar pine. Our research suggests that fuels reduction treatments benefit some fire-resistant species such as Jeffrey pine but can reduce resistance to drought and beetle stress in others when treatments include prescribed burning. These findings improve our understanding of how multiple disturbances (anthropogenic and natural) interact in forested ecosystems, and how shifts in disturbance regimes are likely to influence forest health and community composition in the future.