Using Indicator Plants to Assess Susceptibility of California Red Fir and White Fir to the Fir Engraver Beetle

Book excerpt: Using a Vegetation Drought Index (VDI) for estimating the susceptibility of California red and white firs to the fir engraver beetle (Scolytus ventralis) was evaluated in northern California forests where these true firs (Abies species) occur in mixed conifer and true fir stands. Midway through the summer drought, true fir moisture stress-a known predisposing factor-was highest on sites with highest VDI values (most xeric). In mixed conifer stands, the susceptibility of white firs-as indicated by the mean percentage of their basal area killed annually-was highest on sites with highest VDI values. But in true fir stands, the susceptibility of mixed red and white firs was highest on sites with the lowest VDI values. Regression models with VDI, stand basal area, and percentage of true fir as regressors lacked the required predictive range and precision, suggesting that VDI will likely have to be used in combination with predictors other than, or in addition to, those analyzed.