Effects of gypsy moth on forest composition and structure in the Midwest United States

Background/Question/Methods

Gypsy moth (Lymantria dispar L.) was brought to North America and accidentally released in 1869, and has since continued its spread across the eastern United States from east to west.  Only 25 percent of the area susceptible to gypsy moth is infested, and the rate of gypsy moth spread has been fairly slow because female gypsy moths are generally incapable of flight. Although gypsy moth prefers oak (Quercus spp.), aspen (Populus spp.), birch (Betula spp.) and willow (Salix spp.) foliage, during population outbreaks, nearly all forest trees are at least partially consumed.  Therefore, gypsy moth outbreaks can cause extensive defoliation and severe damage to forest ecosystems.  Given the slow spread rate and the fact that gypsy moth is a resource generalist, monitoring vegetation changes in the core infested area is important for management purposes. The goal of this study was to compare the structure and composition of impacted ecosystems before and after gypsy moth arrival to determine how gypsy moth affected oak forests in northern Michigan and Wisconsin from 2000-2010. To this end, remeasurement data from 228 US Forest Service Forest Inventory and Analysis (FIA) plots were examined along with gypsy moth population data interpolated to FIA plots.

Results/Conclusions

Our results suggest gypsy moth infestation led to increasing height and diameter diversity and increasing tree species richness. Infestation apparently led to an increase in overall sapling density and a decrease in sapling density of white oak and preferred species groups. Infestation also appears to have led to decreased importance value of white oak and preferred species groups. Gypsy moth may be contributing to a decrease in historically dominant woody species such as oaks by accelerating mortality of trees from upper crown classes (i.e., thinning from above or crown thinning). In turn, this may be opening the forest canopy and encouraging development of a diverse group of less preferred associated woody species. Given oak regeneration difficulties and given the lack of fire which is thought to be crucial for oak regeneration, the trends we found are not surprising. However, the trends were linked to the timing and geographical extent of gypsy moth infestation, indicating gypsy moth could quickly become another culprit behind diminishing North American oak forests. Continued analysis of FIA plots in proximity to the gypsy moth infestation front could help describe the potential effect insect defoliators can have on species composition in forest ecosystems.