Competitive balance among tree species altered by forest tent caterpillar defoliation

Abstract

In northern hardwood forests with a closed canopy, tree growth is largely driven by tree size and competition with neighboring trees. Canopy defoliation during insect outbreaks temporarily increases light availability and may decrease light competition among neighboring trees. Defoliation typically reduces tree growth, but canopy defoliation could also stimulate growth of trees that maintain some leaf area. We compared the growth responses of mature trees of non-host species Acer rubrum, and the three host species Acer saccharum, Quercus rubra and Tilia americana, to defoliation by forest tent caterpillar (Malacosoma disstria, Lasiocampidae), a native forest insect with a ∼10-year outbreak periodicity, in a northern hardwood forest in northern lower Michigan.We compared tree growth during the forest tent caterpillar outbreak in 2009, and in the following year, to the average growth before the outbreak in relation to the defoliation level of both the focal tree and its neighbors. Growth responses were related to focal tree defoliation, but we found little direct evidence for effects of neighborhood defoliation; relatively uniform defoliation within sites may have obscured detection of neighborhood effects. Growth of T.americana increased substantially in the year following defoliation, despite the growth reduction during the outbreak in 2009. T.americana rapidly produced new leaves after peak defoliation in 2009, which could explain increased growth in 2010. A.rubrum, which was not defoliated, decreased growth slightly during the outbreak. In 2010, however, one year after peak defoliation, A. rubrum growth was similar to pre-outbreak growth. The two other defoliated species, A. saccharum and Q. rubra, decreased growth in both 2009 and 2010. These strongly species-specific growth responses to canopy defoliation might result in changes in tree species’ interactions and shifts in species advantages over time.