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Harvard Forest Symposium Abstract 2014

  • Title: New insights into controls on temperate forest photosynthesis and respiration provided by stable carbon isotope flux measurements
  • Primary Author: Richard Wehr (University of Arizona)
  • Additional Authors: J. McManus (Aerodyne Research Inc.); J. William Munger (Harvard University); David Nelson (University of Maryland Center for Environmental Science); Scott Saleska (University of Arizona); Steven Wofsy (Harvard University); Mark Zahniser (Aerodyne Research Inc.)
  • Abstract:

    The net ecosystem-atmosphere exchange flux of CO2 (NEE) is routinely measured by eddy covariance at over 500 tower sites around the world. NEE is the balance of ecosystem-scale photosynthesis and respiration, and all analyses of NEE necessarily involve partitioning it into these somewhat independent and somewhat coupled components. There being no means to measure each component directly during the day, partitioning has relied on prescribed responses of photosynthesis and/or (daytime) respiration to light, water, and temperature—responses that have gone without direct validation since continuous eddy covariance measurements began over 20 years ago.

    Isotopic flux partitioning (IFP) is an alternative that avoids such prescriptions, instead identifying the photosynthetic and respiratory components of NEE by their distinct stable isotopic signatures. The ratio of 13C to 12C differs between photosynthesized and respired carbon because there is a strong isotopic fractionation by photosynthesis that varies on timescales shorter than the mean age of the substrate for respiration. We have used IFP at the Harvard Forest to determine the relationships of forest ecosystem-scale photosynthesis (GPP) and daytime respiration (Reco) to light, water, and temperature at hourly to seasonal timescales without a priori constraints. We have found that the response of Reco to temperature effectively inverts at high temperatures and low soil water contents (Figure 1), leading standard partitioning to substantially overestimate Reco, GPP, and water use efficiency during hot, dry periods. Moreover, we have found that photosynthesis becomes more efficient going into the fall, for reasons that remain unclear. Otherwise, GPP responds consistently to light, diffuse light fraction, and leaf-air vapor pressure differential, but does not decline in direct response to (moderately) high temperature. These results elucidate the controls on forest ecosystem productivity, with consequences for predicting the evolution of the coupled biosphere-climate system.

  • Research Category: Forest-Atmosphere Exchange

  • Figures:
  • Fig 1 for Wehr Abstract.jpg