Canopy defoliation by forest tent caterpillar influences mature tree growth in a northern hardwood forest

Background/Question/Methods

Gap formation due to large tree mortality, which increases light availability and releases growth of the surrounding trees, is traditionally regarded as the dominant process governing the dynamics of northern hardwood forests. However, selective canopy defoliation by insects also leads to a short-term increase in light availability. Defoliation of mature trees is expected to result in lower growth, but may also decrease light competition between neighboring trees, particularly for foliated trees that could take advantage of the increase in light. Forest tent caterpillar (FTC), a native forest insect with approximately decadal outbreak periodicity, defoliates a range of host tree species early in the growing season. Mature trees refoliate 4-6 weeks after defoliation. We compared mature tree growth of Acer rubrum, Acer saccharum, Quercus rubra, and Tilia americana in 2009 during a FTC outbreak, and in the following year, to the average growth over the previous eight non-outbreak years. We related tree growth to the actual crown openness of the tree at peak defoliation, and to the average crown openness of larger neighbor trees in four plots with distinct levels of canopy defoliation in a hardwood forest in northern lower Michigan.

Results/Conclusions

Acer saccharum, Quercus rubra and Tilia americana were heavily defoliated by FTC, whereas Acer rubrum was only slightly defoliated. All species decreased growth during the outbreak year compared to prior non-outbreak years, although for A. rubrum the decrease was marginal. For Q. rubra, growth rates in the outbreak year decreased with greater defoliation (measured as crown openness). A. saccharum, in contrast, increased growth with increasing defoliation, and for A. rubrum and T. americana no relation was observed. The average crown openness of the neighborhood did not influence growth for any of the species, likely because defoliated trees also had a strongly defoliated neighborhood. In 2010, the year following the peak outbreak, A. saccharum decreased growth further, T. americana, in contrast, strongly increased growth, and in A. rubrum and Q. rubra growth was comparable to non-outbreak growth. T. americana likely took advantage of the increased light levels through rapid refoliation, whereas A. saccharum may have spread the growth reduction across years through buffering by stored carbohydrates. Our results suggest that mature tree growth responses to FTC defoliation, and associated changes in competitive interactions, are strongly species-specific, and that growth responses may have a one-year lag.