The relationship between western spruce budworm defoliation levels and growth of individual Douglas-fir and grand fir trees

Regression models were fit relating foliage levels to height and basal area growth of 192 trees from 26 stands representing a wide range of site and stand conditions. The study confirmed that deteriorated tree conditions in prior years (often resulting from defoliation) affect current-year growth at specific foliage levels. Absolute height and basal area growth at specific foliage levels were found to be linearly related to absolute height and basal area growth before defoliation, indicating that proportional growth losses resulting from specified levels of defoliation do not vary between fast-growing and slow-growing trees. Various site, stand, and tree variables showed no effect on the relationship between foliage levels and proportional growth loss. Models that included predicted nondefoliated growth, foliage level, and preceding-year condition variables explained up to 61 and 91% of the variation in height and basal area growth (In-In transformation), respectively