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Harvard Forest Symposium Abstract 2013

  • Title: Characterizing 19 years of Harvard Forest carbon exchange data and analysis of the factors responsible for long-term trends and interannual variation
  • Primary Author: Jakob Lindaas (Harvard College (Harvard University))
  • Additional Authors: J. William Munger (Harvard University); Steven Wofsy (Harvard University)
  • Abstract:

    The Harvard Forest EMS site has been a net sink of CO2 since eddy flux covariance measurements began in 1992. Retrospective analysis of 19 years of this carbon flux data, 1992-2010, seeks to determine the contributions of environmental factors to the observed trends and anomalies in Net Ecosystem Exchange. Annual NEE has nearly tripled over the time period, and the overall mean annual NEE (flux of CO2 from the atmosphere to the surface) is -3.07 Mg C ha-1 yr-1 with a range from -1.02 Mg C ha-1 yr-1 in 1998 to -6.44 Mg C ha-1 yr-1 in 2008. The environmental variables of temperature, precipitation, PAR (photosynthetically active radiation), and soil temperature have considerable variability over this period but show no significant trends. Anomalies in NEE influenced by disturbances such as late frosts and drought can persist through the remainder of growing season or longer. Increasing growing season length correlates strongly with the trend in increasing CO2 uptake, with both the spring start date moving earlier and the fall end date moving later. Additionally the mean ecosystem response to temperature and PAR has changed over the 19 years, with the majority of the change happening during the growing season. Evaluation of this change points to changing physiology and ecosystem functioning driving the long-term trend in increased uptake. Neither the increased growing seasons nor the weather variations alone can account for the variation and trend in NEE. Increasing canopy efficiency suggests there has been an enhancement in foliar nitrogen or changing canopy composition, with possible small contributions from CO2 fertilization and water stress alleviation, however the record of foliar N in the tower footprint is too sparse to identify a trend yet. Annual observations of foliar N are continuing.



    (Based on Senior Thesis presented by Jakob Lindaas)

  • Research Category: Forest-Atmosphere Exchange