The role of timing in drought and defoliation-related tree mortality in temperate mixed-hardwood forests

Danielle Tanzer, Keenan J. Rivers, Chandi Witharana and Robert Fahey, Natural Resources and the Environment, University of Connecticut, Storrs, CT, Robert Bagchi and James G. Mickley, Ecology & Evolutionary Biology, University of Connecticut, Storrs, CT, Audrey A. Barker Plotkin, Department of Environmental Conservation, University of Massachusetts, Amherst, MA, James D. Hurd Jr, Department of Natural Resources and the Environment, University of Connecticut, Storrs, CT

Background/Question/Methods
With climate change, forest disturbances are expected to increase in frequency and severity, which may lead to an increase in disturbance interactions. Interactions can intensify effects on vegetation response and tree mortality. Relative timing of multiple disturbances may vary across a landscape and the influence of these different temporal patterns on disturbance interaction outcomes is not well understood. Southern New England experienced widespread tree mortality related to severe drought in 2016 and gypsy moth defoliation from 2016 to 2018. This study characterizes the temporal patterns of gypsy moth defoliation and assesses how defoliation severity and timing (relative to drought) influenced vegetation response and tree mortality outcomes. Field-based assessments were conducted for 37 sites throughout eastern Connecticut and central Massachusetts. Assessments included surveys of all live and dead canopy trees, including both preferred and non-preferred gypsy moth host species, field-based assessment of gypsy moth defoliation, and surveys of gypsy moth population based on caterpillar and egg mass counts. For each study site, time series analysis of remote sensing data was used to determine annual and within-season patterns of defoliation and refoliation, and both time series analysis and high-resolution imagery were used to assess the timing and extent of tree mortality.
Results/Conclusions
Based on time series analysis, field study sites were placed into categories relating temporal patterns of gypsy moth defoliation – none (4 sites), 2017 only (15 sites), 2018 only (3 sites), 2016 and 2017 (2 sites), 2017 and 2018 (7 sites), and all 3 years (6 sites). Within-season patterns of defoliation and refoliation were assessed for all sites that experienced defoliation. Preliminary results indicate that 84% of study sites experienced tree mortality with overall mortality rates ranging from 3-70% and oak mortality rates ranging from 11-90%. Analysis of variance found that both overall and oak mortality rates varied significantly among temporal defoliation patterns (p = 0.01 for overall mortality and p < 0.001 for oak mortality), with rates increasing significantly with the number of years a site was defoliated. Results provide insight on how the timing of interacting forest disturbances influences vegetation response and tree mortality outcomes and will support future prediction and monitoring efforts related to disturbance interaction.