PS 39-9 - Assessing predictors of oak tree defoliation and mortality in response to Lymantria dispar

Background/Question/Methods

Although Northern red oak (Quercus rubra) is a primary driver of productivity in New England forests, numerous factors jeopardize its future prominence. One of the major threats to oak in New England is Lymantria dispar (formerly known as the gypsy moth, now L. dispar). L. dispar has been established for over a century, but a recent outbreak from 2016-2018 was the most destructive in the past 30 years. To advance research on the impacts of invasive insects on forest health, we examined predictors of oak tree defoliation and mortality in response to the most recent L. dispar outbreak. We have data from over 450 plots that experienced differing levels of defoliation intensity across a large watershed. We will revisit these plots in Summer 2022 to assess the vulnerability of oaks to mortality post-invasion. Drawing from remote-sensing data and on-the-ground data collection, we identified variables that may predict and explain defoliation intensity. Our central research question is: how much defoliation will individual oaks experience based on their tree size, stand composition, and canopy exposure to sunlight? We hypothesized that the greater the size, amount of surrounding oak, and canopy exposure, the greater the observed defoliation for the individual tree.

Results/Conclusions

Oaks were the most abundant species group in our plots, and around two-thirds of these oaks were Northern red oak. In a survey conducted from 2017 to 2018, we found that L. dispar primarily defoliated oaks, whereas other species groups faced minor defoliation. Nearly three-quarters of the oaks sampled experienced >25% defoliation in 2017. By examining our data at the individual tree level, we found a positive relationship between tree canopy exposure to sunlight and percent canopy defoliation. Trees with a larger diameter at breast height experienced significantly more defoliation than their smaller counterparts. Trees located in plots with a higher proportion of oak tended to be more defoliated. Our hypotheses that canopy exposure, size, and amount of surrounding oak are positively correlated with oak susceptibility to defoliation were supported by these results. The next step is to investigate which biotic and abiotic factors predict vulnerability to mortality. When we revisit the plots this summer, we expect abiotic conditions (soil moisture) and competition (stand basal area) to be our key variables in explaining the vulnerability of oaks to mortality once defoliated.