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Harvard Forest Symposium Abstract 2009

  • Title: Long-term forest dynamics resulting from chronic hemlock woolly adelgid infestation
  • Primary Author: David Orwig (Harvard Forest)
  • Additional Authors: Relena Ribbons (University of Copenhagen)
  • Abstract:

    Eastern hemlock (Tsuga canadensis) stands across the eastern United States have experienced widespread decline and mortality due to the introduced hemlock woolly adelgid (Adelges tsugae; HWA). We investigated the overstory and understory vegetation dynamics associated with chronic HWA infestation over a 13-year period. From 1995-1998, forest structure, composition, health, and physical site characteristics were examined in permanent plots established at eleven sites throughout central Connecticut. Plots were re-sampled at 4 to 5 year intervals through 2008 for overstory dbh, foliar loss, crown classification, T. canadensis mortality levels, and understory vegetation dynamics. Overstory mortality was variable across sites, originally ranging from 5-98%; by 2008 seven sites had mortality levels > 80%. Interestingly, three sites in the central and northern extent of the study area had lower mortality levels of only 17 to 38%. Progressive hemlock mortality has resulted in a shift in forest structure, to forests dominated by Betula lenta (black birch), Acer rubrum (red maple), and Fagus grandifolia (American beech). Black birch sapling density increased from 125-250 ha-1 in the mid-1990s to >4,000 ha-1 at some sites by 2008. In heavily damaged stands, black birch has recently recruited into the tree size category (>8cm dbh). Seedling layer vegetation mirrors saplings in composition, and the relative density of red maple seedlings has increased from 10% to > 50% at several sites. The recent migration of the co-occurring invasive elongate hemlock scale (Fiorinia externa), adds an additional layer of complexity, potentially slowing or increasing the decline of hemlock forest health in HWA infested stands. Contrary to initial expectations, healthy hemlocks remain at some sites, despite 15 years of adelgid infestation. Future work will examine the relationships among site characteristics and co-occurring pest interactions.

  • Research Category: Invasive Plants, Pests & Pathogens