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Harvard Forest Symposium Abstract 2005

  • Title: Landscape level analyses of hemlock woolly adelgid outbreaks in Massachusetts
  • Primary Author: David Orwig (Harvard Forest)
  • Additional Authors: Megan Manner (Duke University); Donald Niebyl (Great Basin Institute); Nick Povak (University of Wisconsin -- Madison)
  • Abstract:

    Over the last 18 years, hemlock woolly adelgid (HWA) (Adelges tsugae), an introduced aphid-like insect from Asia, has expanded its range from southern Connecticut to northern Massachusetts, causing widespread mortality and decline of Tsuga canadensis (eastern hemlock). Building on similar analyses in Connecticut, we have mapped the distribution of all T. canadensis stands (> 3 ha) prior to HWA infestation in a 4000 km2 transect through Massachusetts to characterize the temporal and spatial patterns of damage generated by HWA since the time of its arrival into the area in 1989. Over 5000 stands with > 10% hemlock were mapped, representing over 86,000 ha, or 21% of the study area. We have completed a three-year sampling effort, collecting data on forest structure, composition, and crown vigor, site and edaphic characteristics, potential replacement species, and the spatial pattern of HWA and associated canopy damage from 123 stands. Eighty-two stands contained > 50% hemlock while 41 stands contained < 50% hemlock (Figure 1). Overstory T. canadensis importance ranged from 24 – 92% and total stand densities varied from 225 - 2025 ha-1. Stand size varied from 7 to 317 ha in size and almost 60% of stands were found on southern, southwestern, or western aspects with slopes of 20 - 30%. HWA was found in close proximity (a few km) to VT, suggesting it is continuing to migrate unimpeded at the northern extent of its current range.


    HWA was present in 40% of the sampled stands, although overstory hemlock mortality levels are still very low in most stands. Only 5 stands had greater than 20% overstory hemlock mortality and 2 forests located along the southern border had overstory losses > 50%. Over half of the stands visited had experienced some level of hemlock logging in the last 10 years, although very few were harvested due to HWA-induced mortality. This information has been incorporated into a GIS analysis of the landscape-level, biological, edaphic, and historical factors associated with the patterns of HWA-induced hemlock damage. Potential replacement species already present in the canopy of many T. canadensis forests include Quercus rubra (red oak), Pinus strobus (white pine), Betula lenta (black birch), and several Acer (maple) species. Megan Manner will use data from this study and the previous study in Connecticut along with graph theory and GIS for her Master’s thesis at Duke University examining the connectivity of hemlock stands across portions of southern New England.


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