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Harvard Forest Symposium Abstract 2007

  • Title: Early Vegetation Responses in the Simes Hemlock Removal Experiment
  • Primary Author: Audrey Barker Plotkin (Harvard Forest)
  • Additional Authors: Aaron Ellison (Harvard University); David Orwig (Harvard Forest); Brynne Simmons (Johnson C Smith University)
  • Abstract:

    Eastern hemlock (Tsuga canadensis) is declining regionally, with the spread of the invasive pest, hemlock woolly adelgid (Adelges tsugae). Hemlock is removed from the forest by insect-caused mortality, and by increased hemlock logging activity as human management decisions are influenced by the perceived threat of the insect. To experimentally assess the ecosystem effects of hemlock removal on the forest, we designed an experiment that contrasts both the direct (death of standing hemlock) and indirect (moderate to heavy logging of hemlock forests) effects of the adelgid. In 2003, we established two replicates of the following four treatments in hemlock dominated stands at the Simes Tract of Harvard Forest: hemlock girdling (hemlocks of all sizes girdled), commercial logging (all merchantable hemlock and selected other trees cut and removed), untreated hemlock (control) and untreated hardwood forest. In Winter/Spring 2005, the treatments were implemented, so we now have two seasons of pre-treatment data and two seasons of post-treatment data. In the girdled plots, more than half of the hemlocks were dead by July 2006 (Figure, top left panel), 14 months after girdling. Most remaining hemlocks had low vigor at these sites. In the logged plots, there was a notable sprouting response from the hardwood trees that were cut (Figure, bottom left panel).



    Eastern hemlock is a foundation species (Ellison et al. 2005?), controlling ecosystem processes in the forests it dominates. For example, the deep shade cast by hemlock forest allows few tree seedlings and understory plants to establish. Therefore, we are quite interested to examine the understory response as light and nutrient cycling rates increase. By the end of the first growing season after logging, we noted the open-site colonists Rubus sp. and Erichtites hyracifolia. By midsummer 2006, these species were beginning to colonize one of the girdled plots, and other open-site specialists were emerging in the logged plots, including Betula papyrifera, Prunus pensylvanica, Rhus copallina, Aralia hispida and more Carex sp. In the logged sites, we noted abundant seedlings of black birch (Betula lenta), the tree species that is most abundant in adelgid-killed and logged hemlock sites in Connecticut and Massachusetts. However, seedling density and herb/shrub cover have not yet increased enough to produce major trends across the 10, 1m2 plots we have been monitoring in each site (Figures, top and bottom right panels). We predict that understory cover and seedling density will continue to increase over the next few years in the logged plots, and increase more slowly and with a smaller maximum in the girdled plots.



    This study has a contrasting counterpart at the Arnold Arboretum, a heavily managed urban site. Unlike the modest vegetation responses we have thus far observed at the Simes site, the Arboretum site showed dramatic increases in herb and shrub cover just one year after logging hemlock (see Lux et al., this volume). These studies should help inform ecological theory of foundation species and management strategies in response to hemlock decline.

  • Research Category: Invasive Plants, Pests & Pathogens
    Large Experiments and Permanent Plot Studies

  • Figures:
  • C:dataSimes_hemlocksimes-abp-symp07.pdf