Close Encounters

Connecting soil nitrogen and gypsy moth herbivory in a US oak forest

Can a better understanding of ecosystem nitrogen cycling allow us to more accurately predict invasive insect outbreaks?

The gypsy moth (Lymantria dispar) is an invasive forest insect that was introduced to the USA in the late 1860s. Native to mainland Europe, it can have a devastating impact on woodland ecosystems. The gypsy moth and its larvae are known to consume the leaves of over 500 tree species, and it is listed as one of the 100 most destructive invasive species worldwide by the IUCN Global Invasive Species Database. Periodically, the gypsy moth exhibits large irruptions during which populations increase by several orders of magnitude. During a population irruption, L. dispar caterpillars consume large quantities of leaves, and extensive leaf herbivory can lead to widespread defoliation of entire forest stands. Nitrogen (N) is a critical nutrient for both caterpillar and tree growth so understanding the connections between ecosystem nitrogen cycling and L. dispar defoliation could provide better predictions of disturbance impacts at a range of spatial scales.

Lymantria dispar caused intense defoliation of mature trees during a severe, multi-year defoliation in 2015-2018. Image credit: E. Conrad-Rooney.

In their new work published in AoBP, Conrad-Rooney et al. investigate a severe, multi-year gypsy moth population irruption that caused intense leaf herbivory and tree defoliation in a temperate New England oak forest. They found that trees and forest stands with lower soil nitrogen concentrations experienced more defoliation than those with higher soil nitrogen concentrations. The inorganic nitrogen content of the soil solution was strongly positively correlated with defoliation intensity and the number of sequential years of defoliation. This could suggest that larger nitrogen pools might promote the resistance of oak trees to defoliation. Based on the strong relationships found between ecosystem N cycling and defoliation following a severe, multi-year invasive insect irruption, the authors highlight the importance of investigating the long-term effects of severe and recurring ecosystem disturbances. Considering that these types of disturbances are likely to increase in frequency and intensity, they conclude that long-term monitoring efforts are critical to understanding ecosystem feedbacks and potential thresholds.

Researcher highlight

Emma Conrad-Rooney is a recent graduate of Wellesley College, Massachusetts, USA where she studied biological sciences and conducted forest ecology research with the Matthes EcoLab. While an REU student at Harvard Forest, Emma completed field work for her senior honors thesis on gypsy moth caterpillar defoliation and forest nitrogen cycling. She now works as the lab manager for the Templer Lab at Boston University and plans to pursue a PhD in forest biogeochemistry. Emma hopes to conduct policy-relevant research on how forest ecosystems are affected by disturbances including climate change, insect outbreaks, and urbanization.