Interactions between biotic and climatic disturbances are likely to amplify the aggregate effects of disturbance within forests. Insect and pathogen outbreaks are challenging to anticipate, and ecological forecasting can play an important role for rapidly understanding the potential near-term outcomes of an outbreak. There was an unexpected outbreak of gypsy moth (Lymantria dispar) in southern New England from 2015-2018, which has led to widespread defoliation concurrent with a period of drought. Most otherwise healthy but defoliated trees can produce a second flush of leaves by drawing on stored carbon after caterpillars leave the larval stage, but repeated defoliation and interactions with climatic stress are likely to create more severe and lasting changes, making the magnitude and sensitivity of ecosystem responses to this disturbance uncertain. In this study, we combined a near-real-time defoliation data product produced from Landsat satellite imagery with an ecosystem model in an ecoinformatic framework. Using the Ecosystem Demography Model version 2.2 (ED2), we conducted baseline simulations that did not include gypsy moth defoliation and we compared these results to defoliation simulations that assimilated the Landsat data product into ED2. We started simulations in 2012 and forecasted recovery through 2023 (five years after the last defoliation).
Results/Conclusions
Defoliation at the start of the growing season reduced primary productivity and transpiration and increased albedo, and these effects altered the annual carbon, water, and energy exchange within impacted forests. We found that forests that were defoliated in three successive years had rates of NPP that were 75-83% below baseline simulations at the end of our forecast period, whereas NPP recovered within 1-4 years in forests that were defoliated in just one year. Forest composition and structure, which closely tracks soil water availability in sandy soils in coastal ecosystems, also influenced rates of recovery, where NPP recovered fastest in less dense, inland, hardwood stands with larger trees. Our initial forecasted results will be tested through additional data collection in Summer 2019.