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Harvard Forest Symposium Abstract 2019

  • Title: Do Insect Outbreaks Alter the Climate Sensitivity of Surviving Trees in Temperate Forests?
  • Primary Author: Tessa Mandra (Harvard Forest)
  • Additional Authors: Ruben Delgado (Harvard Forest); David Orwig (Harvard Forest); Yude Pan (USDA Forest Service); Neil Pederson (Harvard Forest); Tim Rademacher (Harvard University)
  • Abstract:

    Hemlock Woolly Adelgid (HWA) and other invasive species outbreaks alter the structure and composition of forests. HWA has been spreading across the Northeastern U.S., severely reducing eastern hemlock populations in many cases. These outbreaks provide a distinct disturbance that allows for the study of how sensitive surviving trees are to climate before and after HWA introduction. Silvicultural studies indicate that lower forest densities reduce the vulnerability of surviving trees to severe drought. An unpublished tree-ring based study in the Southern Appalachians suggests that the loss of American chestnut changed the climatic response of surviving trees in various ways. To examine how the death of eastern hemlock trees killed by HWA may have affected the climate response of surviving trees, we installed permanent plots in 2017 and 2018 in an old-growth forest in northwest New Jersey and mapped individual trees, including eastern hemlock, eastern white pine, chestnut oak, northern red oak, black birch, red maple, and other minor species. An initial tree-ring sampling of canopy trees in 2003 indicated mortality of some eastern hemlock trees had begun to accelerate the growth rates of chestnut oak. By 2018 eastern hemlock mortality was heavy (see picture), though many individuals were still alive. With a database of ~42,500 annual tree growth measurements, we will use Kalman Filter and bootstrapping techniques to investigate whether the climatic response of surviving trees has changed. We expect that: 1) surviving trees will become less sensitive to June drought and 2) the positive correlation between eastern hemlock and March temperatures will remained unchanged. If these hypotheses hold, it indicates that the loss of eastern hemlock in the Northeastern U.S. could, interestingly, make surviving forests less vulnerable to climatic change in the near term. The 5-year drought in southern New England and New York State, from 2012-2016, will be a good test of this hypothesis. Once this work is complete, we will continue studying this question of changing climate sensitivity in natural forests of the Northeastern U.S. for other disturbances including gypsy moth outbreak, the 1938 hurricane, and timber harvesting.

  • Research Category: Biodiversity Studies; Conservation and Management; Historical and Retrospective Studies; Invasive Plants, Pests & Pathogens; Large Experiments and Permanent Plot Studies; Physiological Ecology, Population Dynamics, and Species Interactions

  • Figures:
  • UttertownBlackBirchHeavyRegenNov2018.jpg