Is there growth after gypsy moths? Impact of an invasive defoliator

Gypsy moths began invading North American forests in 1869. Their periodic defoliations can slow the growth or sometimes kill host trees, but some native defoliators attack the same host species. I used tree rings to answer questions about growth of oak trees before and after the arrival of gypsy moths. Has tree growth been reduced more intensely since the invasion of gypsy moths? Were preferred species (white and chestnut oak) affected more than other species (red, black and scarlet oak)? I examined seven sites in Pennsylvania and New Jersey that were invaded by gypsy moths since 1972. Cores were taken from at least twenty healthy canopy oak trees at each site. Growth rings were measured to the nearest 0.025 mm, and measurements from each tree were detrended with linear or polynomial regression. Residual values were normalized and averaged for each site, and for red and white oaks within each site. Gypsy moth outbreaks reduced radial growth of all oak species at all sites. The worst outbreaks reduced growth more than any other event in the 1900s, though some reductions by native defoliators were of equivalent magnitude. There were more severe growth reductions during the 1800s, but the sample size was small. Surprisingly, red oaks and white oaks had similar patterns of growth reduction; differences that did occur between oak subgenera were not consistent among sites or outbreaks. Overall, tree ring analysis confirms the adverse impact of gypsy moth invasion on the growth of oak trees.