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Harvard Forest Symposium Abstract 2015

• Title: Ten years of forest development in the Hemlock Removal Experiment
• Primary Author: Audrey Barker Plotkin (Harvard Forest)
• Additional Authors: Allyson Degrassi (University of Vermont (UVM)); Aaron Ellison (Harvard University); Simone Johnson (Lincoln University (Missouri)); Alison Ochs (Mount Holyoke College); Ariel Reis (Not Specified); Jessica Robinson (Knox College); Ahmed Siddig (University of Khartoum); Claudia Villar-Leeman (Bowdoin College); Joel van de Sande (University of California - Santa Cruz)
• Abstract:

  Eastern hemlock (Tsuga canadensis) is declining regionally, concomitantly with the spread and irruption of the nonnative insect, the hemlock woolly adelgid (Adelges tsugae; HWA). Hemlock is killed by the insects as well as 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 forest structure and function, we designed an experiment that contrasts death of standing hemlock with moderate to heavy logging of hemlock forests. In 2003, we established two replicates of the following four treatments at the Simes Tract of Harvard Forest: hemlock girdling (hemlocks of all sizes girdled; simulates many of the effects of insect mortality), commercial logging (all merchantable hemlock and selected other trees cut and removed), untreated hemlock (control) and untreated hardwood forest. Large plots (0.8 ha) allow treatment effects to be detected at the stand scale, and accommodate a large suite of responses. In Winter/Spring 2005, the treatments were implemented, so we now have 10 seasons of post-treatment data.
  
  
  
  Tree censuses conducted in 2004 (before girdling and logging), 2009, and 2014 showed similar trends in forest structure for the two manipulated treatments, and for the two unmanipulated treatments. Basal area dropped about 70% in logged and girdled plots between 2004 and 2009, and made a modest recovery between 2009 and 2014 (Figure 1 left). Meanwhile, the unmanipulated hemlock and hardwood plots made modest gains in basal area and slightly declined in stem density (Figure 1 right), typical of young-to-middle-aged forests of our region.
  
  
  
  Robust tree regeneration occurred in all manipulated plots (Figure 2 left); by 2013, density of tree saplings (individual trees > 1.3 m tall but < 5 cm dbh) was between 10,000-20,000 stems per hectare (ie, an average of one to two saplings per m2). The sapling cohort was overwhelmingly dominated by black birch (Betula lenta, Figure 2 right). This novel forest type has also been observed in many post-adelgid stands in Connecticut and Massachusetts. A few of these saplings grew into the tree-sized class (dbh > 5cm) by 2014, especially in the logged plots.
  
  
  
  In a long-term experiment, the experimental design may need adjustment over time. Notably in this study, the hemlock control is transforming into an adelgid-infested treatment, as HWA colonizes the site. In 2009, HWA was found on 28% of hemlocks examined; in the 2014 survey, HWA was found on 65% of hemlocks examined. Break-point analysis of light reaching the forest understory indicated a break between 2009 and 2010 (Figure 3), which we are designating a break between “Phase I” and “Phase II” of the study. In the first phase of the experiment (2003-2009), we focused on contrasts between the manipulations and the hemlock control. In this next phase of the experiment (2010- ), we are interested in contrasting the girdling treatment simulation of the physical aspects of adelgid infestation with the full effects of hemlock decline and death caused by HWA now unfolding in the hemlock control plots.
  
  

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


• Figures:
ABP-symp15-figures.pdf