Carbon dioxide and water vapor exchange in a warm temperate grassland

  • Authors: Novick, K.A.; Stoy, P.C.; Katul, G.G.; Ellsworth, D.S.; Siqueira, M.B.S.; Juang, J.; Oren, R.
  • Publication Year: 2004
  • Publication Series: Scientific Journal (JRNL)
  • Source: Oecologia 138: 259-274


Grasslands cover about 40% of the ice-free global terrestrial surface, but their contribution to local and regional water and carbon fluxes and sensitivity to climatic perturbations such as drought remains uncertain. Here, we assess the direction and magnitude of net ecosystem carbon exchange (NEE) and it components, ecosystem carbon assimilation (Ac and ecosystem respiration (FE), in a southeastern United States grassland ecosystem subject to periodic drought and harvest using a combination of eddy-covariance measurements and model calculations. We modeled Ac and evapotranspiration (ET) using a big-leaf canopy scheme in conjunction with ecophysiological and radiative transfer principles, and applied the model to assess the sensitivity of NEE and ET to soil moisture dynamics and rapid excusions in leaf area index (LAI) following grass harvesting. Model results closely match eddy-covariance flux estimations on daily, and longer, time steps. Both model calculations and eddy-covariance estimates suggest that the grassland became a net source of carbon to the atmosphere immediately following the harvest, but a rapid recovery in LAI maintained a marginal carbon sink during summer. However, when itegrated over the year, this grassland ecosystem was a net C source (97 g C m-2 a-1) due to a minor imbalance between large Ac (-1.202 g C m-2 a-1 and RE (1,299 g C m-2 a-1) fluxes. Mild drought conditions during the measurement period resulted in many instances of low soil moisture (θ<0.2 m3m-3), which influenced Ac and thereby NEE by decreasing stomatal conductance. For this experiment, low θ had minor impact on RE. Thus, stomatal limitations to Ac were teh primary reason that this grassland was a net C source. In the absence of soil moisture limitations, model calculations suggest a net C sink of -65 g C m-2a-1 assuming the LAI dynamics and physiological properties are unaltered. These results, and the results of other studies, suggest that perturbations to the hydrologic cycle are key determinants of C cycling in grassland ecosystems.

  • Citation: Novick, K.A.; Stoy, P.C.; Katul, G.G.; Ellsworth, D.S.; Siqueira, M.B.S.; Juang, J.; Oren, R. 2004. Carbon dioxide and water vapor exchange in a warm temperate grassland. Oecologia 138: 259-274
  • Keywords: Net ecosystem exchange, ecosystem modeling, evapotranspiration, eddy-covariance, grassland ecosystems
  • Posted Date: April 1, 1980
  • Modified Date: October 28, 2013
  • Print Publications Are No Longer Available

    In an ongoing effort to be fiscally responsible, the Southern Research Station (SRS) will no longer produce and distribute hard copies of our publications. Many SRS publications are available at cost via the Government Printing Office (GPO). Electronic versions of publications may be downloaded, printed, and distributed.

    Publication Notes

    • This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.
    • Our online publications are scanned and captured using Adobe Acrobat. During the capture process some typographical errors may occur. Please contact the SRS webmaster if you notice any errors which make this publication unusable.
    • To view this article, download the latest version of Adobe Acrobat Reader.