Invasive gypsy moth caterpillars (Lymantria dispar), introduced to New England in the mid-1800s, defoliate trees, causing stress and sometimes mortality with multiple years of defoliation. Nitrogen is a critical nutrient for caterpillar growth and plant recovery from defoliation. Comparing the interactions among defoliation, recovery, and ecosystem nitrogen cycling across scales can help us understand forest recovery and resilience following a gypsy moth outbreak. This study assessed the extent to which N availability impacted gypsy moth defoliation intensity and influenced tree recovery at the tree, plot, and landscape spatial scales. Ecosystem processes can vary across different scales, so investigating scaling relationships of gypsy moth defoliation can help to predict large-scale ecosystem dynamics. To span these spatial scales, we combined field data and remotely sensed imagery. To assess tree defoliation and recovery we conducted field surveys of individual trees and forest plots nearby the Quabbin Reservoir, MA, and we measured leaf and soil N pools and soil N mineralization rates at individual trees and across our network of forest plots. We combined these field data with remotely sensed data from Landsat and the National Ecological Observatory Network (NEON) to assess relationships between landscape-level defoliation and recovery and metrics of canopy N content.
Results/Conclusions
We found that leaves with lower nitrogen content experience more extensive herbivory by caterpillars than leaves with higher nitrogen. At the individual tree scale, there was a positive correlation between soil and foliar N content as well as between leaf herbivory and tree-level defoliation severity. At the plot scale, areas with lower soil nitrogen were correlated with more severe gypsy moth defoliation, and at the landscape scale, there was more intense defoliation in areas with lower canopy nitrogen. The relationship between higher defoliation and lower N levels is consistent from the leaf to the landscape scale. Results suggested that higher nitrogen levels could promote oak resistance to gypsy moth defoliation through an herbivory feedback where caterpillars consume less overall leaf mass for leaves with higher nitrogen and through a recovery feedback where trees with higher nitrogen have more resources to rebound from herbivory. This research contributes to a more comprehensive understanding of the scaling of these relationships among individual trees, forest stands, and the regional scale. Better understanding interactions among nitrogen, defoliation due to gypsy moth caterpillars, and tree recovery could help inform predictions of the impact and recovery trajectory of northeastern U.S. forests in response to future insect outbreaks.