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Harvard Forest Symposium Abstract 2016

• Title: What controls the net forest-atmosphere exchange of carbonyl sulfide? Results from 2 years of eddy flux measurements
• Primary Author: Richard Wehr (University of Arizona)
• Additional Authors: Roisin Commane (Harvard Research Group); J. William Munger (Harvard University); Scott Saleska (University of Arizona); Steven Wofsy (Harvard University)
• Abstract:

  Carbonyl sulfide (OCS) is currently a focus of ground- and satellite-based measurements as well as of model development, owing mainly to its potential use as a large-scale proxy for gross primary production (GPP). OCS is known to be taken up by leaves and either taken up or emitted by soils, depending on the circumstances. Because OCS is destroyed by the enzyme carbonic anhydrase within the leaf rather than by any light-dependent reaction, the leaf uptake is expected to be related to the conductance of the diffusive pathway into the leaf (stomata + mesophyll + leaf boundary air layer) rather than to GPP directly, though GPP and the diffusive conductance are often strongly correlated.
  
  
  
  Here we use 2 years of eddy covariance measurements of the net ecosystem-atmosphere exchange of OCS, along with measurements of the vertical profile of OCS within the forest, to investigate the controls on ecosystem-scale OCS uptake and emission. Using subcanopy flux-gradient approach, we are able to partition the net flux into ground and canopy components. We find that ecosystem OCS uptake consisted of uptake by leaf stomata, canopy surface water, and soil. The uptake by soil was small but significant, stable, and always positive. Following rains, the uptake by canopy surface water (previously unrecognized) exceeded the uptake by soil. Patterns in the uptake by leaf stomata nearly matched simple predictions based on the OCS concentration in the canopy airspace and on the diffusive conductances of the leaf. This near match suggests that mesophyll conductance does not exhibit large, rapid variations in response to light, humidity, or other environmental variables, contrary to some suggestions in the literature. However, observed stomatal uptake did exceed predictions in the latter half of the night and in the morning, which are the times when our estimates of stomatal conductance are least constrained. OCS might therefore provide improved estimates of stomatal behavior.
  
  

• Research Category: Forest-Atmosphere Exchange