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Title: Biophysical controls on carbon and water vapor fluxes across a grassland climatic gradient in the United States

Author
item WAGLE, P. - University Of Oklahoma
item XIAO, X. - University Of Oklahoma
item Scott, Russell - Russ
item KOLB, T.E. - Northern Arizona University
item COOK, D.R. - Argonne National Laboratory
item BRUNSELL, N. - University Of Kansas
item BALDOCCHI, D.D. - University Of California
item BASARA, J. - University Of Oklahoma
item MATAMALA, R. - Argonne National Laboratory
item ZHOU, Y. - University Of Oklahoma
item BAJGAIN, R. - University Of Oklahoma

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/2015
Publication Date: 12/15/2015
Publication URL: https://handle.nal.usda.gov/10113/5354172
Citation: Wagle, P., Xiao, X., Scott, R.L., Kolb, T., Cook, D., Brunsell, N., Baldocchi, D., Basara, J., Matamala, R., Zhou, Y., Bajgain, R. 2015. Biophysical controls on carbon and water vapor fluxes across a grassland climatic gradient in the United States. Agricultural and Forest Meteorology. 214-215:293-305. https://doi.org/10.1016/j.agrformet.2015.08.265.
DOI: https://doi.org/10.1016/j.agrformet.2015.08.265

Interpretive Summary: Grasslands cover approximately 40% of the world’s land surface and play an important role in the global carbon and water budgets. Accurately describing the dynamics and underlying mechanisms of exchange of carbon dioxide and water vapor fluxes between grasslands and the atmosphere is crucial to predict the how grasslands will respond to climate change. This study examined these grassland-atmosphere interactions using measurements of carbon dioxide and water vapor fluxes and satellite-derived vegetation properties at 12 grassland sites in the United States. Precipitation exerted a strong control on grassland uptake and release of carbon dioxide and on the magnitude of the evaporation that returned precipitation back to the atmosphere. Most grasslands were carbon sinks under average conditions. However, all grasslands across the US, spanning several different climates, could shift from a carbon sink to a carbon source under climatic extremes (e.g., drought). We found a close coupling between grassland productivity and evaporation at each site, but grasslands in wetter climates had more productivity for a given amount of evaporation. Our results also demonstrate the potential use of the satellite metrics for a better understanding and mapping of grassland growth and evaporation.

Technical Abstract: Understanding of the underlying causes of spatial variation in exchange of carbon and water vapor fluxes between grasslands and the atmosphere is crucial for accurate estimates of regional and global carbon and water budgets, and for predicting the impact of climate change on biosphere–atmosphere feedbacks of grasslands. We used ground-based eddy flux and meteorological data, and the Moderate Resolution Imaging Spectroradiometer (MODIS) enhanced vegetation index (EVI) from 12 grasslands across the United States to examine the spatial variability in carbon and water vapor fluxes and to evaluate the biophysical controls on the spatial patterns of fluxes. Precipitation was strongly associated with spatial and temporal variability in carbon and water vapor fluxes and vegetation productivity. Grasslands with annual average precipitation <600 mm generally had neutral annual carbon balance or emitted small amount of carbon to the atmosphere. Despite strong coupling between gross primary production (GPP)and evapotranspiration (ET) across study sites, GPP showed larger spatial variation than ET, and EVI had a greater effect on GPP than on ET. Consequently, large spatial variation in ecosystem water use efficiency (EWUE = annual GPP/ET; varying from 0.67 ± 0.55 to 2.52 ± 0.52 g C mm-1ET) was observed. Greater reduction in GPP than ET at high air temperature and vapor pressure deficit caused a reduction in EWUE in dry years, indicating a response which is opposite than what has been reported for forests. Our results show that spatial and temporal variations in ecosystem carbon uptake, ET, and water use efficiency of grasslands were strongly associated with canopy greenness and coverage, as indicated by EVI.