Differences in Spruce Budworm Defoliation among Balsam Fir and White, Red, and Black Spruce

Abstract

Abstract Spruce budworm (Choristoneura fumiferana Clem.) outbreaks severely reduce growth and survival of balsam fir (Abies balsamea [L.] Mill.) and spruce (Picea spp. A. Dietr.) over much of eastern North America, resulting in major reductions in yield and disruption of harvest schedules. This is the first study to rigorously quantify defoliation differences among all four spruce budworm host tree species at a stand level. An extensive permanent sample plot data set, with more than 27,000 tree and 1,117 stand measurements from 1984 to 1992 in New Brunswick, Canada, revealed a clear and consistent hierarchy of host species defoliation susceptibility. Regardless of defoliation severity and other stand variables tested, white (Picea glauca [Moench] Voss), red (Picea rubens Sarg.), and black spruce (Picea mariana [Mill.] B.S.P.) had approximately 72, 41, and 28% as much defoliation as balsam fir, respectively. Balsam fir defoliation was a highly significant predictor of white, red, and black spruce defoliation. Of the variation in defoliation of red-black spruce hybrids, 34% was explained by balsam fir defoliation and an interaction with spruce hybrid index. This is the first model to quantify hybrid red-black spruce defoliation across a full range of severities. Quantitative models of stand-level defoliation differences among host species will improve spruce budworm stand impact forecasts and decision support prioritization of bioinsecticide or salvage operations and provide information to calibrate population-defoliation models for host species in mixed species stand types.