Grazing Effects on Water Use Efficiency on a Mongolian Desert Steppe

Abstract:

Ecosystem-level water use efficiency (WUE), defined as the ratio of gross primary production (GPP) to evapotranspiration (ET), was assessed by continuous and simultaneous direct eddy-covariance (EC) measurements of carbon and water fluxes on adjacent pastures of grazed (DS) and ungrazed steppes (FS) in the Mongolia Plateau for a two-year period from 2010 to 2012. We found that the WUE was well positively linear correlated (r2=0.90) with the GEP both in the DS and FS. Due to our desert steppe was very sensitive to the precipitation, WUE was co-varied with the precipitation. WUE increased with the GEP increase under good water conditions, when the GEP reached its maximal value (DS: 3 g C m^-2, FS: 2 g C m^-2), the WUE was suppressed and kept a stable value during the peak growing season. Both GEP and WUE was near zero when the soil moisture was lower. We also found that the WUE was negatively correlated with ET. The WUE was higher in GS than that in FS. The mean seasonal WUE was 0.93 in GS and 0.54 g C kg^-1 H₂O in FS, with a peak monthly WUE of 1.32 in GS and 0.73 g C kg^-1 H₂O in FS, respectively. The difference between GS and FS mainly caused by that the ET was changed with the GEP during the entire growing season. This suggests the importance of both plant population dynamics and water statues should be considered in WUE studies.