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Harvard Forest Symposium Abstract 2006

• Title: Vegetation driven decomposition changes resulting from hemlock woolly adelgid
• Primary Author: Richard Cobb (Southern Maine Community College)
• Additional Authors: David Orwig (Harvard Forest)
• Abstract:

  Understanding ecosystem dynamics of hemlock woolly adelgid (Adelges tsugae – HWA) infested forests has value for policy makers, watershed managers, and conservationists considering salvage harvest or seeking to reestablish ecosystem processes in impacted stands. However, developing management plans that ameliorate impacts are difficult because of little empirical data describing herbivory-decomposition relationships. We propose that changes in decomposition and N cycling in eastern hemlock (Tsuga canadensis) forests infested with HWA are a function of gradual, additive factors that change over the course of stand decline. Previous study has shown that HWA hervibory alters decomposition directly through changes in litter and throughfall chemistry (Cobb et al. in press; Stadler et al. in press). As stands decline indirect changes including inputs of higher quality litter and altered microclimate following establishment of black birch (Betula lenta) further accelerate decomposition. These latter changes occur more slowly and have a greater duration compared to direct changes.
  
  
  
  
  
  In order to better understand indirect consequences of HWA infestation in these forests we compared decomposition of senescent hemlock, black birch, and hemlock-black birch litter mixes across a gradient of herbivory and vegetation change caused by HWA (Figure 1). Black birch litter N levels are three times greater compared to hemlock whereas litter lignin levels were similar. Stand types included uninfested controls (controls), heavily HWA-infested low-mortality (high-low) hemlock forests, and heavily HWA-infested high-mortality (high-high) forests. High-high sites have canopies dominated by black birch and remaining hemlock trees are heavily infested by HWA. Litter and stand type had significant effects on litter mass loss. Black birch decomposed most rapidly followed by mixed litter and then hemlock. A one-to-one, linear relationship was found between observed rates of mixed litter mass loss and estimated rates based on the values of each litter type decomposing alone. High-high sites showed the greatest rates of mass loss for all litter types compared to uninfested and high infestation-low mortality sites.
  
  
  
  
  
  Vegetation changes are the dominant factor affecting long-term rates of decomposition in declining eastern hemlock stands. Black birch forests have higher quality litter and different temperature, moisture, and light regimes. Decomposition changes in HWA infested stands are an important mechanism contributing to loss of forest floor mass and increased N cycling in these forests. This study demonstrates that decomposition changes occur gradually as the stands decline and further support a growing body of evidence that show ecosystem changes in HWA infested stands are gradual and less severe than in many salvage-logged hemlock forests. Conserving the structural and functional diversity contributed by eastern hemlock will rely on protection of remaining forests, effective HWA control efforts, and establishment of species functionally similar to hemlock in impacted stands.
  
  
  
  
  
  Literature cited
  
  
  Stadler, B., T. Muller, and D. Orwig. 2006. The ecology of energy and nutrient fluxes in hemlock forests invaded by the hemlock woolly adelgid. Ecology. In press.
  
  
  
  
  
  Cobb. R., D.A. Orwig, and S. Currie. 2006. Decomposition of green foliage in eastern hemlock forests of southern New England impacted by hemlock woolly adelgid populations. Canadian Journal of Forest Research. In press.
  
  
  

• Research Category: Invasive Plants, Pests & Pathogens