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Harvard Forest Symposium Abstract 2006

  • Title: Accelerating Carbon Uptake Rates at the Harvard Forest Environmental Measurement Site
  • Primary Author: J. William Munger (Harvard University)
  • Additional Authors: Carol Barford (University of Wisconsin -- Madison); John Budney (Harvard University); Daniel Curran (Harvard University); Elizabeth Hammond Pyle (Harvard University); Kathryn McKain (Harvard University)
  • Abstract:

    After 14 years of eddy-covariance CO2 flux measurements and ground-based sampling to quantify biomass at the Harvard Forest, we are able to identify a trend of accelerating net ecosystem exchange (NEE). On average, both uptake (gross ecosystem exchange – GEE) and ecosystem respiration (Re) have been increasing by about 2% per year. We have also measured aboveground live biomass on a set of plots near the EMS tower since 1993 and quantified coarse woody debris (CWD) in 2000 and 2003. Incremental carbon budgets determined from biomass inventories agree with the eddy-covariance measurements and show a similar trend of increasing carbon accumulation in the forest. We hypothesize that increasing rates of NEE at this site could be due to long-term ecosystem dynamics and succession, accumulation of nitrogen, or a transient response as the forest recovers from a disturbance. Uptake has been dominated primarily by growth of live biomass and secondarily by accumulation of dead wood on the ground. Red oaks account for the majority of the biomass increase, while all other species have been nearly static in their growth or slightly declining. Increased photosynthetic capacity of these trees may partially account for the increasing GEE. The observed increase in uptake, most notably since 2001, may be the result of a disturbance event in 1998. We do not expect this trend of increasing carbon uptake to continue indefinitely. As LAI increases, new vegetation will become light limited, and GEE will approach a maximum. Respiration will increase proportionally with biomass and subsequently grow closer in magnitude to GEE. Future work to investigate the mechanisms behind the increased uptake includes foliar nitrogen sampling to look for changes in nitrogen availability and utilization, and a comprehensive CWD survey to improve our estimate of the contribution of dead wood to the forest carbon budget.


  • Research Category: Forest-Atmosphere Exchange