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Harvard Forest Symposium Abstract 2015

  • Title: Ecosystem-Scale Photosynthesis and Respiration at the Harvard Forest Determined by Three Years of Isotopic Eddy Covariance
  • 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 photosynthesis and respiration of individual leaves and soil plots have been much measured, but the photosynthesis and daytime respiration of a whole forest of interacting organisms cannot be measured directly. Most of what we know about ecosystem-scale photosynthesis (GPP) and daytime ecosystem respiration (DER) comes from tower-based eddy covariance measurements of the net ecosystem-atmosphere CO2 exchange (NEE), which is the small difference between GPP and DER (i.e. NEE = DER – GPP). NEE is routinely partitioned into GPP and DER based on one or another hypothesis about their responses to light, water, and/or temperature in the environment, but those hypotheses have not been testable.

    Here we partition three growing seasons (2011-2013) of NEE based on eddy covariance measurements of its 13C isotopic composition, to provide a first test of the hypotheses routinely used to estimate GPP and DER. Our method takes advantage of the fact that photosynthesis and respiration usually have distinct isotopic signatures, because the photosynthetic signature varies hourly (e.g. with light availability) while the respiratory signature is governed mostly by soil substrate composition and so varies only daily or weekly.

    We find that isotopically estimated GPP and DER roughly agree with standard estimates except in the first half of the growing season, after leaf expansion (June-July). In that period, isotopically estimated DER is about half nighttime ecosystem respiration (i.e. nighttime NEE), and is therefore lower than standard estimates of DER, which are based on the relationship between nighttime NEE and temperature. The likely explanation for this discrepancy is that canopy respiration—which constitutes about half of nighttime ecosystem respiration in June-July but is minimal in August-September (Giasson et al, Ecosphere, vol. 4 no. 11, 2013)—is strongly inhibited by sunlight during the day. Such inhibition is known to occur in many species at the leaf level (Heskel et al, Ecosphere, vol. 4 no. 8, 2013).

    The higher standard estimates of DER in June-July correspond to higher standard estimates of GPP. Whereas isotopically estimated GPP responds consistently to light over the growing season, standard estimates of GPP (and of light-use efficiency and water-use efficiency) show an anomalous peak in June-July that is not supported by published leaf-level measurements (Bassow and Bazaaz, Ecology, vol. 79 no. 8, 1998). Isotopic partitioning thus suggests a new picture of the behavior of ecosystem-scale photosynthesis and respiration, one that contradicts the standard hypotheses but is consistent with expectations based on leaf-level measurements.

  • Research Category: Forest-Atmosphere Exchange