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Harvard Forest Symposium Abstract 2013

• Title: Partitioning CO2 fluxes with isotopologue measurements and modeling to understand mechanisms of forest carbon sequestration
• Primary Author: Richard Wehr (University of Arizona)
• Additional Authors: Rose Abramoff (Boston University); Eric Davidson (University of Maryland - Center for Environmental Science); Adrien Finzi (Boston University); Marc-Andre Giasson (Boston University); J. McManus (Aerodyne Research Inc.); Paul Moorcroft (Harvard University); J. William Munger (Harvard University); David Nelson (University of Maryland Center for Environmental Science); Scott Saleska (University of Arizona); Kathleen Savage (Woods Hole Research Center); Daniel Scott (Harvard University); Steven Wofsy (Harvard University); Mark Zahniser (Aerodyne Research Inc.)
• Abstract:

  Ecosystem models differ widely in their predictions of how forest carbon dynamics will interact with a changing climate, and the uncertainty concerning that interaction is one of the largest sources of uncertainty in predictions of future climate. We are investigating the mechanisms controlling carbon sequestration at the Harvard Forest by combining newly developed instrumentation for automated, in situ CO2 isotopologue measurements with novel experimental manipulations of belowground processes, as well as with traditional methods like soil chambers, plot trenching, and minirhizotrons. The resulting data are being integrated in—and used to refine—the Ecosystem Demography 2 (ED2) model.
  
  
  
  Here we focus on our in situ isotopic analyses. We measured the stable carbon isotopic composition of soil-respired CO2 (δSR) and of the net ecosystem-atmosphere CO2 exchange (δNEE). The soil respiration measurements indicate that spatial variation in δSR can be 1 ‰ over distances as short as 2 m. These spatial differences are preserved as δSR varies in time. Short-term temporal variation occurs in response to rain, which can lower δSR by 1-2 ‰ for a day or two; any diel cycle in δSR must be much less than 1 ‰. We trenched one of our two isotopic chamber plots in late 2012 and will be investigating the role of roots in 2013.
  
  
  
  We used the δNEE measurements to partition net ecosystem-atmosphere exchange (NEE) into ecosystem-scale photosynthesis and respiration, a longstanding goal in carbon cycle science. Standard approaches assume the behavior of respiration and/or photosynthesis a priori; for example, by using nighttime data to set respiration as a function of temperature. Isotopic flux partitioning (IFP) is an alternative method that identifies the distinct isotopic signatures of photosynthesis and respiration in NEE. IFP does not assume the behavior of photosynthesis or respiration (though it does make some assumptions concerning the isotopic fractionation due to photosynthesis), and is the first method for measuring these gross fluxes at the Harvard Forest. IFP indicates that non-foliar ecosystem respiration responds more strongly to sunlight than to temperature or moisture at the Harvard Forest, possibly via the influence of canopy photosynthetic activity on root respiration. Both lagged (half day) and rapid (1-2 hr) responses to sunlight are suggested; the latter is surprising but not unprecedented in the literature. It is therefore probable that conventional partitioning methods misestimate gross ecosystem photosynthesis and respiration.
  
  
  
  These results will be further explored by comparison to diel patterns of soil respiration (from chambers), to seasonal patterns of root growth (via mini-rhizotrons), and to a new version of the ED2 model that simulates carbon isotopes. Together, these studies will explore the implications of ecosystem carbon allocation mechanisms for overall ecosystem carbon sequestration and feedbacks to climate.
  
  

• Research Category: Forest-Atmosphere Exchange