Defoliation frequency outweighs timing as a driver of tree mortality related to drought-defoliation interaction

Document Type

Article

Publication Date

9-15-2025

Department

College of Forest Resources and Environmental Science

Abstract

Climate change is expected to increase the frequency and severity of forest disturbance, making it more likely that multiple forms of disturbance will occur simultaneously. The resulting disturbance interactions could have compounding effects on tree mortality. Between 2016 and 2018, southern New England experienced both severe drought and spongy moth defoliation resulting in widespread tree mortality. The frequency and timing of defoliation varied substantially across the region, while drought conditions were relatively uniform. Our goal was to assess the influence of variable defoliation frequency and timing (relative to the preceding drought) on variation in tree mortality rates using a combination of remotely sensed data on defoliation patterns and field data on forest stand characteristics and tree mortality. Stand-level tree basal area mortality associated with the drought-defoliation interaction averaged 19 % across all study sites, with a maximum level of 50 %. Based on our analysis, mortality rates were most strongly explained by combinations of defoliation severity, frequency, and stand characteristics (R2 = 0.52–0.60). Sites experiencing only drought but no defoliation or drought and a single year of defoliation had comparably low mortality rates, while sites experiencing drought and multi-year defoliation had higher mortality rates. Our research suggests that defoliation frequency was an important factor in predicting tree mortality, while defoliation timing and drought itself had less importance in this interaction.

Publication Title

Forest Ecology and Management

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