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Harvard Forest REU Symposium Abstract 2012

• Title: The Impact of Co-occurring Invasive Insects on Eastern Hemlock Branch Growth and Chemistry
• Author: Vincent Waquiu (Southwestern Indian Polytechnic Institute)
• Abstract:

  Throughout most of eastern New England, eastern hemlock (Tsuga canadensis) has been experiencing widespread decline due to the invasion of two invasive insects, the hemlock woolly adelgid (HWA; Adelges tsugae) and the elongate hemlock scale (EHS; Fiorinia externa). Hemlock is a foundation species that creates distinctive biogeochemical, habitat, and microclimatic conditions. The HWA causes foliar damage, crown loss and mortality, while the EHS usually causes needle discoloration but rarely leads to hemlock mortality. Our objective was to investigate how the presence of these two invasive insects affects twig, needle, and fallen litter chemistry (nitrogen) and new branch growth across sites varying in insect densities: high HWA, high EHS, HWA and EHS, and no infestation (control). Three hemlock stands of each treatment were selected from a large set of stands examined across southern New England over the last 15 years. Within each site we examined 3 small hemlock trees to conduct foliar and litter analysis, and insect density, and branch growth determination. Based on earlier studies, we hypothesized that insect feeding would lead to elevated nitrogen content in twigs and needles compared to uninfested trees.
  
  Results suggest that uninfested trees had significantly higher new growth (> 3cm) compared to trees with both HWA and EHS (1.5 cm). The percentages of branches that produced new growth was significantly lower (40-50%) on HWA infested trees compared to EHS infested and uninfested trees (75 and 80%, respectively). Branch growth and percent of branches producing new growth declined with increasing HWA density but not EHS density. Foliar nitrogen was significantly higher in infested compared to uninfested trees, but did not differ among insect treatments. Twigs that were fed on by HWA alone or in combination with had significantly higher nitrogen content than EHS-infested or uninfested trees. Nitrogen content of litter collected from the forest floor did not differ among treatment types but contained higher N content than live, intact needles. Carbon content of twigs, needles, and litter was not significantly different among insect treatment; lastly, the nitrogen content of the insect by-products differed greatly, as EHS wax covers contained just over 6% nitrogen and HWA wool just under 2%. These results warrant future work on how these byproducts influence subsequent decomposition. Continued feeding by both insects in these forests will likely lead to continued hemlock deterioration and associated twig and needle chemical changes, along with altered ecosystem function.
  
  

• Research Category: Invasive Plants, Pests & Pathogens