Effects of Fire and Fire Surrogate Treatments on Bark Beetle-Caused Tree Mortality in the Southern Cascades, California

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Forest Science

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We examined bark beetle responses to fire and fire surrogate treatments 2 and 4 years after the application of prescribed fire in a mixed-conifer forest in northern California. Treatments included an untreated control (C), thinning from below (T), and applications of prescribed fire (B) and T + B replicated three times in 10-ha experimental units. A total of 1,822 pine and fir trees (5.1% of all trees) were killed by bark beetles. Mountain pine beetle (Dendroctonus ponderosae Hopkins) was found infesting ponderosa pine (Pinus ponderosa Dougl. ex Laws.), sugar pine (Pinus lambertiana Dougl.), and lodgepole pine (Pinus contorta Dougl. ex Loud.); western pine beetle (Dendroctonus brevicomis LeConte) was found infesting ponderosa pine; and fir engraver (Scolytus ventralis LeConte) was found infesting white fir (Abies concolor [Gord. & Glend.] Lindl. ex Hildebr.). Significantly higher rates of bark beetle-caused tree mortality occurred on B (9.2%) than on C (3.2%), T (<1%), or T + B (3.3%) cumulatively during the 4-year period. A total of 723 pines (4.4% of all pines) were killed by bark beetles, primarily mountain pine beetle. Attacks resulted in significantly more pine mortality on B (5%) than on C, T, or T + B (all <1%) 2 years after the application of prescribed fire. No significant treatment effects were found during the second sample period or cumulatively during the 4-year period. A total of 1,098 white fir trees (5.8% of all white fir) were killed by the fir engraver. Attacks resulted in significantly higher rates of fir mortality on T + B than on T during both sample periods but not cumulatively during the 4-year period. Overall, bark beetle-caused tree mortality was concentrated in the smaller diameter classes. The implications of these and other results to forest management are discussed.