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Accurate dating of spruce budworm infestation using tree growth anomalies

Abstract Spruce budworm feeding causes stem and branch deformities that alter the architecture of host trees in a permanent way, easily recognized several years after the defoliation event. Dating of budworm-related growth anomalies using tree rings provides a new means to reconstruct recent infestations of this defoliator. We tested this dendro-architectural method in the southernmost black spruce-lichen woodlands in eastern North America, which were affected by spruce budworm from 1976 to 1985. In these open stands, moribund black spruce trees support high loads of the epiphytic lichen Bryoria arranged in unusual patterns, a phenomenon known as black spruce decline. Because Bryoria was restricted to the dead and deformed parts of the trees, we hypothesized that the lichen only colonized those parts of the trees defoliated by spruce budworm. To test this hypothesis, we dated i) three types of growth anomalies associated with spruce budworm feeding (death of terminal buds, death of axes, and reiteration, i.e., production of new leaders and epicormic shoots) and ii) maximum extent of Bryoria on stem and branches of three spruce trees showing contrasting lichen cover patterns. All trees showed peaks of growth anomalies during periods of known spruce budworm activity. Also, Bryoria was almost exclusively located on internodes that were defoliated by spruce budworm. A model is also proposed to explain the different lichen cover patterns observed. Dating of tree growth anomalies (terminal bud mortality) has better resolution in time and space than tree-ring patterns to determine the first year of insect defoliation.