Abstract
Global vegetation greenness has already been confirmed to increase substantially since the early 1980s, and this phenomenon is extremely heterogeneous in both time and space. The Loess Plateau (LP) serves as an ecological barrier of northern China, where the ecological environment is characterized by exceptional fragility and sensitivity. However, the spatial pattern of vegetation dynamics in both greenness and productivity and their relationship with vegetation ecosystem water-use efficiency (WUE) are still poorly documented, especially in recent 20 years. In this study, the leaf area index (LAI) and gross primary productivity (GPP) were employed to characterize vegetation canopy leaf area and productivity, respectively. The time series data of ecosystem WUE in the LP were obtained by the ratio of GPP and ET during the period of 2000–2018, the dynamic trend of vegetation greenness and productivity in the LP was detected by using a linear fitting and nonlinear method, then, the extent to which the sustained greening in the LP revealed an increase of ecosystem WUE in recent 20 years was quantified. The results showed that sustained vegetation greening and increased GPP elucidated a great improvement in ecosystem WUE in the LP during 2000–2018, in which the sustained greening potentially reveals an increase of ecosystem WUE in >69.33% regions of the LP in recent 20 years. >93% regions of the LP have been experiencing a vegetation greening progress derived by both the annually averaged LAI (LAIAN) and growing season (April to October) maximum LAI (LAIGS). Furthermore, precipitation and soil moisture positively contributed to an interannual increase in ecosystem WUE; however, increasing temperature greatly resulted in a decrease in ecosystem WUE in the LP in recent 20 years. Importantly, this work also implied that ecological water is significantly consumed by the increasing vegetation photosynthesis derived from the significant growth of ET in the LP since the 2000s. These findings implied that the increasing ecological water consumption should be incorporated into indigenized ecosystem process models.
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