02827nas a2200337   4500008004100000022001300041245011600054210006900170260001600239300001400255490001200269520176800281653003502049653002002084653003002104653002302134653001502157653002902172100001902201700002002220700002302240700002102263700002002284700001702304700002602321700002002347700002002367700001702387700001902404856006602423       2015                        eng d  a0168192300aBiophysical controls on carbon and water vapor fluxes across a grassland climatic gradient in the United States0 aBiophysical controls on carbon and water vapor fluxes across a g  cJan-12-2015  a293 - 3050 v214-2153 a<p>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&ndash;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 &lt;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 &plusmn; 0.55 to 2.52 &plusmn; 0.52 g C mm&minus;1 ET) 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.</p>10aEcosystem water use efficiency10aEddy covariance10aEnhanced vegetation index10aEvapotranspiration10aGrasslands10aGross primary production1 aWagle, Pradeep1 aXiao, Xiangming1 aScott, Russell, L.1 aKolb, Thomas, E.1 aCook, David, R.1 aBrunsell, N.1 aBaldocchi, Dennis, D.1 aBasara, Jeffrey1 aMatamala, Roser1 aZhou, Yuting1 aBajgain, Rajen  uhttp://linkinghub.elsevier.com/retrieve/pii/S0168192315007005