EFFECTS OF DEFOLIATION BY DOUGLAS -FIR TUSSOCK MOTH ON RING SEQUENCES OF DOUGLAS -FIR AND GRAND FIR

Increment cores were collected from 10 stands in mixed-conifer forest stands which had suffered varying levels of Douglas-fir tussock moth defoliation during 1946, 1964, and 1973 infestations in north central Idaho. Ring-width measurements, standardized to remove inherent growth trends, were compared between host (Douglas-fir and grand fir) and nonhost (western larch, ponderosa pine, western white pine) species for evidence of growth losses due to defoliation. Heavy defoliation caused growth of host species to decrease 75 %-90% in one year. Normal growth rates returned within 3-4 years after maximum defoliation, however. The effect of moderate defoliation could not be reliably identified in the data.