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Harvard Forest Symposium Abstract 2012

• Title: Net Carbon exchange and biomass accumulation as a function of species composition and stand age at the Harvard Forest
• Primary Author: J. William Munger (Harvard University)
• Abstract:

  Forests in New England include a mixture of deciduous and coniferous species and a range of disturbance history from intensive agriculture abandoned in the late 1800s to permanent woodlots that were managed for forest products but never cleared. Three flux tower sites established at the Harvard Forest Long-Term Ecological Research (LTER) site capture the range of forest types that typify central Massachusetts. By making continued long-term observations at these sites we can detect differences in climate response and successional trajectories.
  
  
  
  
  
  The Environmental Measurement Site (EMS) is in a stand dominated by red oak and red maple, with the oldest trees established prior to 1895 on abandoned pasture. Scattered pines and patches of hemlock make up as much as 44% of the basal area in the area northwest of the tower and are present as small saplings in the understory throughout the site. The Hemlock (HEM) tower is sited adjacent to a nearly pure stand of eastern hemlock with some individuals up to 230 years old. The site has not been affected by any stand-clearing disturbance since European settlement. A third tower, Little Prospect Hill (LPH), is in a stand also dominated by red oak and red maple on abandoned pasture that is regenerating from a 1957 fire and has fewer conifers. Towers at each site are instrumented for eddy-covariance measurements of CO2, H2O, energy, and meteorological variables. Permanent vegetation plots are associated with each tower site to quantify changes in above-ground biomass and woody debris.
  
  
  
  
  
  Comparing the hourly CO2 fluxes from the three towers highlights the similarities and differences in canopy physiology in the three stands (Figure 1). The peak CO2 uptake rates (largest absolute value negative fluxes) in mid summer are greater for the two deciduous-dominated sites. The older EMS site slightly exceeds the younger site, LPH. The reduced magnitude of CO2 uptake during summer months at HEM is offset by a much longer growing season that adds 1.5 – 2 month in the spring and fall compared to the active season for deciduous stands. The scattered conifer patches and subcanopy hemlocks, which make up less than 10% of overall biomass at the EMS site, contribute to CO2 uptake rates that are about 30% of the corresponding rates in the hemlock stand in early spring.
  
  
  
  
  
  Biomass accumulation in above-ground wood has been remarkably similar for hemlock and deciduous-dominated stands over the last decade (Figure 2). Even though the hemlock stands are older, they are still actively accumulating woody biomass. Observations of carbon fluxes and biomass at HEM establish baseline conditions to compare against as this site is infected by Hemlock Wooly Adelgid (HWA), an insect pest that attacks eastern hemlock.
  
  
  

• Research Category: Forest-Atmosphere Exchange
  Large Experiments and Permanent Plot Studies


• Figures:
Figures 1 & 2.pdf