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Harvard Forest Symposium Abstract 2015

  • Title: Long-Term Carbon Budget Observations at the Harvard Forest hardwood and hemlock flux tower sites
  • Primary Author: J. William Munger (Harvard University)
  • Additional Authors: David Foster (Harvard University); Evan Goldman (Harvard University); Liza Nicoll (Harvard Forest); David Orwig (Harvard Forest); Mark VanScoy (Harvard Forest); Steven Wofsy (Harvard University)
  • Abstract:

    Forests influence the budgets of greenhouse gases, and understanding how they will respond to environmental change is critical to accurately predicting future GHG trends. The time scale for climate change is long and forest growth is slow, thus very long measurement periods are required to observe meaningful forest response. At short time scales, physiology and responses to environmental conditions dominate the rates of carbon exchange. But ultimately carbon balances depend on the fate (allocation) of fixed carbon and the residence time of material that is formed from fixed carbon. Over the past 25 years measurements at the Harvard Forest site in central Massachusetts have focused on carbon budget processes that span the hourly to decadal scale. A pair of eddy flux towers within a mixed forest stand dominated by red oak and red maple and a hemlock dominated stand where CO2, observe H2O and energy fluxes together with meteorological conditions. Records of land use and disturbance and vegetation plot data extend back to 1907. Additional plots have been established adjacent to the flux towers and remeasured annually since 1998. The combined suite of measurements merges observations of instantaneous ecosystem responses to environmental forcing with details of vegetation dynamics and forest growth that represent the emergent properties relevant to long-term ecosystem change. Fluxes show a clear difference in uptake capacity and phenological patterns between the stands. Hardwood dominated stands have peak uptake during a relative short season through mid-summer. The Hemlock stand has a longer season from spring through fall, but the maximum uptake rate is never as high as observed at the hardwood stand (Figure 1). Decadal trends at the EMS tower show increasing carbon uptake from 2000-2008, that could not be explained by response to environmental variability (Figure 2). Net carbon uptake efficiency at EMS indicated by peak NEE under optimum light conditions, has increase as well, along with extension of the active growing season. However, significant excursions in the trend highlight the sensitivity of forest carbon stocks. The pulse of high annual carbon uptake (peak 6 Mg-C ha-1y-1 in 2008) from 2000-2008 is only partially matched by carbon stored in woody biomass, leaving a large fraction of carbon to have accumulated in litter and fine roots in the forest floor, which has as much carbon as the above-ground woody biomass, but shorter turnover time. Despite differences in their short-term carbon uptake patterns, both stands are accumulating biomass; adding roughly 20 Mg-C ha-1 as woody biomass in trees >10cm dbh since 1990.

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

  • Figures:
    • LTER_2015_Fig1.pdf
    LTER_2015_Fig2.pdf