Prescribed fire and mechanical thinning effects on bark beetle caused tree mortality in a mid-elevation Sierran mixed-conifer forest

We assessed tree mortality caused by bark beetles in a mixed-conifer forest in the central Sierra Nevada in response to fire and mechanical treatments. The treatments were: (1) no treatment, (2) prescribed fire, (3) mechanical (crown thinning-from-below followed by rotary mastication), and (4) mechanical followed by prescribed fire. Ponderosa pine (Pinus ponderosa Laws) mortality caused by the western pine beetle (Dendroctonus brevicomis LeConte), sugar pine (Pinus lambertiana Dougl.) mortality caused by mountain pine beetle (D. ponderosae Hopkins), and white fir (Abies concolor Gord. and Glend) mortality caused by the fir engraver beetle (Scolytus ventralis LeConte) was assessed pre-treatments, one-year post-treatments, and three years post-treatments. For the duration of the study, bark beetle caused mortality across all treatments for each tree species was less than 7%. Bark beetle-caused mortality of small and medium white firs increased in treatments that included fire, and bark beetle-caused mortality of medium size sugar pines was elevated in the fire only treatment compared with other treatments. Our results indicate that mechanical treatments cause little risk of mortality to residual trees from bark beetles in the short term. The higher secondary mortality in the small and medium size white firs in both fire treatments can be considered a benefit in overly dense mixed conifer forests where the understory is dominated by shade-tolerant white firs.