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Harvard Forest Symposium Abstract 2006

• Title: Ecosystem Responses to Forest Cutting After HWA Infestation: New insights from an urban site
• Primary Author: Heidi Lux (Harvard Forest)
• Additional Authors: Peter Del Tredici (100th EN Co); David Orwig (Harvard Forest)
• Abstract:

  Forests in the Massachusetts and Connecticut are increasingly impacted by the invasion of the hemlock woolly adelgid (Adelges tsugae) both through direct effects of mortality from infestation and from salvage logging. Over the past five years the impact of salvage logging in these systems has been studied by our group, and in some cases, it has been clear that logging of damaged stands has a greater impact on ecosystem function than the effects of deterioration due to adelgid driven mortality alone.
  
  
  
  
  
  In 2003, we began to study post-adelgid logging and ecosystem function in a different setting; the urban Arnold Arboretum. Hemlock Hill at Harvard University’s Arnold Arboretum is located in Jamaica Plain and has long been considered a remnant of the forest primeval in the heart of Boston. Hemlock woolly adelgid (HWA) was discovered on downed branches after a late winter storm in 1997, and had likely infested trees there for years before. Since this is a heavily used portion of the Arboretum, which is part of the Boston Parks System, the decision has been made to remove many of the dead and dying trees to reduce risks posed to the general public by falling limbs, and combine this necessary removal with an ecosystem experiment. Over 1800 hemlock trees were tagged, measured for diameter, assigned a crown health rating, and mapped with GPS coordinates. During 2004, six 15 x 15 m plots were fenced off, and baseline data on soil nutrient cycling, microclimate, and vegetation information were collected. Hemlocks were removed from 4 plots in February of 2005, while 2 remain untouched as control plots. Slash was chipped and left on site in the Chipped treatment plots, or removed from the Logged (only) treatment. The logging operation was done by crane and extremely low-impact. This series of treatments coincides with those at the Harvard Forest’s Hemlock Removal Experiment at the Simes Tract in Petersham, where we are collecting analogous data and making comparisons.
  
  
  
  
  
  To date we have completed one year of pre-treatment and one year of post-treatment measurements. In general, both nitrogen availability and nitrogen cycling rates are very high at the arboretum, particularly nitrate. This was true even before treatments began, with availability (Figure 1) and rates from 3 to 20 times higher than we have observed at our more rural study sites. Nutrient cycling responses are likely to manifest more clearly this growing season and next, based on what we have observed at logged, infested sites in Connecticut where the mobilization of nutrients peaks in the second and third years post-cut. Vegetation responses to cutting at the arboretum have been quick and dramatic, with shoulder-high cover of up to 50% in the summer of 2005, just months after cutting (Figures 2 and 3). While we expected a proliferation of weedy and shrubby species, vegetation at other logged sites has taken a year or two to establish significantly.
  
  
  
  
  
  The next two seasons of study on Hemlock Hill are likely to be a time of continued dramatic change, and as it unfolds, we will continue to compare it to our other sites, particularly in Massachusetts.
  
  
  

• Research Category: Invasive Plants, Pests & Pathogens