Trade-off between carbon sequestration and water loss for vegetation greening in China

Land use management of forests and croplands mainly drives the vegetation greening in China. Vegetation greening strongly modulates the trade-off between carbon sequestration via photosynthesis and water loss from evapotranspiration (ET) at the terrestrial ecosystem (representing by ecosystem water use efficiency, WUE). The function of vegetation greening in terrestrial carbon sequestration is well known, but the impacts of water loss from ET caused by vegetation greening on WUE are often neglected. Here, the GIS-based Priestley-Taylor Jet Propulsion Laboratory model was established to evaluate ET in China from 2001 to 2015, incorporating vege-tation dynamics as a key component. To quantify the net effect of the ET caused by vegetation greening on WUE, we compared two different simulation scenarios: actual vegetation greening scenario and simulated without vegetation greening scenario. The results show that forests and croplands mainly contribute to the growth in GPP and NPP in China with annual rates of 2.53 gC.m(-2) yr(-2) and 1.59 gC.m(-2) yr(-2) from 2001 to 2015, respectively. With the increase of terrestrial carbon sequestration, the ET under actual vegetation greening scenario was generated 6.78 mm.yr(-1) more than that under simulated without vegetation greening scenario. But as a result of the negative impacts of vegetation physiological effect (elevated CO2 concentration and the decreased VPD) on ET, values of ET under two different scenarios all exhibited a decline trend from 2001 to 2015 with rates of -2.04% and -3.63%, respectively. Consequently, although the WUE under two different scenarios exhibited increased trends (6.44%, actual vegetation dynamics scenario; 10.74%, simulated without vegetation greening scenario), the ET caused by vegetation greening led to an obvious divergence between the WUE under two different scenarios. For better understanding the impacts of human activities on carbon and water cycles at the terrestrial ecosystem, it is necessary to take the water loss from ET caused by vegetation greening into consideration, which is crucial for enhancing the sustainability of future vegetation-related projects.