Observed divergence in the trends of temperature controls on Chinese ecosystem water use efficiency

Abstract

How have plants addressed the trade-off between carbon gain and water loss in this warming world? Ecosystem water-use efficiency (WUE), defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET), is a key indicator of the carbon–water relationship. WUE is expected to change due to climate change, yet the extent to which GPP or ET affects WUE changes remains unclear. Moreover, the potential time-varying variations in the WUE responses to recent warming have been overlooked. In this study, we assessed the relative contributions of GPP and ET to WUE using variance decomposition and investigated trends in temperature controls on WUE using moving windows and partial correlation analysis. Our results include: (1) the national multi-year average WUE in China increased significantly at a rate of 0.0174 gC·kg-1H2O·a-1, with 86.88% of the study area exhibiting increased trends. Forest ecosystems, except for ENF, had relatively higher WUE, and the highest value was observed in the deciduous broad-leaf forest (3.77 gC·kg-1H2O), while grassland ecosystems had the lowest WUE of only 1.05 gC·kg-1H2O. (2) GPP, rather than ET, primary drove WUE variations in most areas, except in Northeast China and Southwest China. GPP always contributed more to WUE than ET in each land cover type. In forests, ET contributed more to WUE than in other land cover types. (3) WUE exhibited significantly positive correlations with temperature and radiation, with a positive correlation with temperature in 63.73% (14.83% significant in level of p < 0.05 and the same as below) of the study area and positive correlation with radiation in 74.12% (20.71% significant) of the study area. The control of precipitation on the WUE was complex, with a positive correlation with precipitation in 54.29% of the study area (9.38% significant). (4) The correlation coefficients with temperature exhibited an increasing trend in 52.26% of the study areas and a decreasing trend in 47.74% of the study areas, with a notable divergence in the spatial distribution of the trend of the temperature controls on WUE. Warming is projected to enhance ecosystem functioning in northern regions but may have adverse effects in the south. These findings shed light on the dynamic response of ecosystem functioning to ongoing warming, and would improve our understanding of the terrestrial carbon–water cycle.