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Harvard Forest REU Symposium Abstract 2019

  • Title: Quantifying and Visualizing the Recent Decline in Hemlock Woolly Adelgid Population
  • Author: Nathan Oalican (Harvard University)
  • Abstract:

    The persistence of eastern hemlock as a major constituent of eastern North American forests is currently threatened by hemlock woolly adelgid (HWA), an invasive sap-feeding insect native to East Asia. HWA infestation results in complete or near complete mortality of affected hemlock stands. However, fluctuations in HWA density may affect hemlock survival. This study assesses the current population of HWA through a rigorous census of over 3,000 hemlock trees in nine plots at the Harvard Forest in Petersham, Massachusetts. In addition, this study utilizes ground-based hemispherical photography and aerial imaging from Landsat 8 to determine if plot-level findings can be scaled to a regional level. The census found that HWA is present in significantly fewer trees in 2019 than in 2014. The decline in HWA coincides with the spread of elongate hemlock scale, an invasive insect which similarly feeds on hemlock needles and may compete with HWA for needle space. As part of the census, hemlocks were visually assessed for vigor. Between 2014 and 2019, average change in tree vigor was statistically insignificant. However, during this time period the most frequent category of tree vigor measurements improved from 25-50% foliage loss to less than 25% foliage loss. This suggests that over the time period, Hemlock mortality due in-part to HWA infestation still occurred, though there was a widespread foliar recovery for mildly afflicted hemlocks in response to decreased adelgid presence.

  • Research Category: Invasive Plants, Pests & Pathogens; Group Projects