Partitioning a decade of evapotranspiration and carbon dioxide fluxes at a forested Ameriflux eddy-covariance site in southern Indiana

Abstract:

The eddy covariance (EC) technique measures fluxes of water vapor and carbon dioxide between ecosystems and the atmosphere. Net ecosystem exchange of CO₂ (NEE) is the balance between ecosystem respiration (ER) and gross primary production (GPP), and evapotranspiration (ET) is the sum of transpiration and evaporation. In order to relate these measurements to physical and ecological processes, it is often necessary to partition the fluxes into their components. While established techniques for partitioning NEE are widely used in the EC community, partitioning of ET remains a challenge. A recently developed partitioning procedure uses assumed correlations between stomatal (photosynthesis and transpiration) and non-stomatal (respiration and evaporation) sources of water vapor and CO₂ to simultaneously partition NEE and ET into their respective components. Because the technique uses the same high-frequency measurements as EC, it is easily applicable to existing EC datasets, provided ecosystem-scale water use efficiency can be specified. We applied the method to a ten-year record of EC fluxes at the Morgan Monroe State Forest (MMSF) Ameriflux site, using estimates of water use efficiency from recent leaf-level gas exchange measurements. The technique has been tested in agricultural systems, but has not previously been evaluated in forests. ER and GPP from the correlation-based procedure qualitatively matched estimates from a more traditional partitioning method based on fitting nighttime NEE to a function of temperature and calculating GPP as the residual during the day, although the magnitudes of GPP and ER from the correlation-based technique were higher than those from the traditional technique. Partitioned respiration and evaporation were also consistent with sub-canopy flux measurements. Transpiration accounted for the majority of ET during the growing season, but had a strong seasonal cycle. Both evaporation and transpiration declined during periods of low soil moisture, but transpiration had a stronger response, with an especially pronounced decline during a severe midseason drought in 2012. Our results show that this technique can effectively partition fluxes over forest canopies, and could help to enable routine ET partitioning of EC measurements for eco-hydrological studies.