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Harvard Forest Symposium Abstract 2005

  • Title: The ecology of energy and nutrient fluxes in hemlock forests invaded by hemlock woolly adelgid
  • Primary Author: Bernhard Stadler (Bayreuth Institute for Terrestrial Ecosystem Research)
  • Additional Authors: Thomas Muller (Centre for Agricultural Landscape and Land Use Research - Muncheberg); David Orwig (Harvard Forest)
  • Abstract:

    In 2004, we analyzed the effects of HWA at an early infestation stage on vertical energy and nutrient fluxes from the canopy to the forest floor. Field experiments including throughfall collection and litter lysimeters were complemented with a microcosm litter leaching experiment in the laboratory examining the effects of HWA affected and unaffected throughfall on litter type, leachate and litter chemistry. Early in the season adelgid infestation caused higher DOC (+24.6%), DON (+28.5%) and K (+39.3%) fluxes and lower inorganic nitrogen fluxes (-39.8%) in throughfall beneath infested compared to uninfested trees. The same effects were shown in the litter solutions collected nearby but differences were often less pronounced, probably due to the higher buffering capacity of litter. Increased particulate organic matter fluxes (TOC: +50%; N: +30.1%), however, indicate the importance of trophic aspects and better growth conditions for microbes on HWA infested trees. Needle litter collected beneath uninfested hemlock had significantly lower N concentrations compared to needles collected beneath infested trees, while no difference in N concentrations was found in birch litter. Bacteria were significantly more abundant on hemlock and birch litter beneath infested trees, while yeasts and filamentous fungi showed no consistent response to nutrient input via HWA throughfall.


    The litter leaching experiment showed that less DOC was leaching from birch than from hemlock needles when exposed to HWA throughfall. Overall, DON concentrations in leachates were higher from birch than from hemlock litter but the addition of HWA affected throughfall reduced DON leaching relative to that from hemlock litter. NH4-N leaching was low from both litter types but the overall nitrogen export from microcosms was significantly higher from birch litter. Thus, throughfall affected by HWA leads to qualitative and quantitative differences in nitrogen export from the litter layer. The N concentration of hemlock litter did not change with time but the N concentration in birch litter increased significantly during the course of the experiment and more so when HWA affected throughfall was applied. Combined with our previous findings on nutrient cycling in the canopy of medium and heavy infested eastern hemlock we suggest a conceptual model for the temporal and vertical transition of energy and nutrient fluxes relative to progressing HWA infestation from a pure hemlock to a birch/maple dominated forest. There is no simple conversion of increasing rates of HWA infestation into nutrient fluxes because progressive needle loss and changes in needle chemistry are likely to produce a humped-shaped DOC curve, while nitrogen fluxes initially decrease with increasing infestation but rise eventually with decline in hemlock vitality and immigration of hardwood species into a hemlock stand.


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