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Harvard Forest Symposium Abstract 2008

  • Title: Trends in carbon uptake at the HFEMS tower siteTrends in carbon uptake at the HFEMS tower site
  • Primary Author: J. William Munger (Harvard University)
  • Additional Authors: John Budney (Harvard University); Daniel Curran (Harvard University); Elizabeth Hammond Pyle (Harvard University); Kathryn McKain (Harvard University); Steven Wofsy (Harvard University)
  • Abstract:

    Terrestrial ecosystems, especially forests, are credited for absorbing 20-30% of the CO2 emitted by fossil fuel combustion that would otherwise contribute to rising CO2 concentrations in the atmosphere. Understanding the mechanisms that control this uptake, and predicting how it will change in the future are critical research questions that drive research at the HFEMS tower. Over the past seventeen years measurements of atmospheric exchange of CO2 by eddy-covariance and ground-based sampling of forest biomass have shown a consistent carbon uptake by the forest surrounding the tower. Surprisingly, the carbon uptake rate by this 80-100 year old stand has been increasing. (Figure 1) These trends of increasing uptake were recently reported in Urbanski et al.(2007). Uptake has been dominated primarily by growth of live biomass and secondarily by accumulation of dead wood (CWD). Red oaks account for the majority of the biomass increase, while all other species have been nearly static in their growth or slightly declining. Variations in woody biomass increment typically lag behind the variations in NEE observed by eddy-flux measurements. The current focus of work at HFEMS is to better understand what accounts for the increasing carbon uptake. Potential mechanisms include, increasing photosynthetic efficiency due to nitrogen fertilization, increasing leaf area (LAI), or shifts in species composition toward more red oak. Recent initiatives include systematic measurements to distinguish among these factors. Annual sampling of leaf nitrogen and detailed LAI measurements began in 1998, and have been repeated annually starting in 2006. Litter samples are sorted to quantify the contribution of each species to the canopy, and show the same dominance by red oak as the biomass inventory. Preliminary results from the foliar-N measurements show that mean N concentrations in green foliage have increased significantly since 1998, which had unusually low annual carbon uptake, though a direct linkage between high foliar N and high growth has not yet been demonstrated for individual trees. Likewise leaf area has increased since 1998. Additional years will be needed to detect trends that exceed the inherent variability in these factors.

    Urbanski, S., C. Barford, S. Wofsy, C. Kucharik, E. Pyle, J. Budney, K. McKain, D. Fitzjarrald, M. Czikowsky, J. W. Munger, Factors Controlling CO2 Exchange on time scales from hourly to decadal at Harvard Forest, J. Geophys. Res.- Biogeosciences, 112, doi:10.1029/2006JG000293, 2007.

    Figure 1 Annual Net Ecosystem carbon uptake by the forest observed at the HFEMS tower is indicated by vertical bars. Positive numbers indicate forest uptake. The solid red line shows the least squares trend line and dashed red lines indicate a 90% confidence interval for the trend, based on a Monte Carlo approach using the uncertainty on the fitting coefficients. Annual above-ground woody biomass increment determined from diameter measurements of all trees >10 cm dbh on 33 inventory plots adjacent to the tower is shown by the brown line. AGWI in 1994-1997 is given as constant based on biomass measurements in 1993 and 1998.

    Figure 2 Annual leaf litter inputs measured on 33 plots adjacent to the Harvard Forest EMS tower is shown by the black line. Vertical segments show the 95% confidence interval based on sampling variability. The green line shows average value of peak growing season LAI on the 33 plots, with 95% confidence interval. Both litter and LAI measurements are continuing.

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

  • Figures:
  • Munger_et_al_fig1.pdf