A shift in the dominant role of atmospheric vapor pressure deficit and soil moisture on vegetation greening in China

Abstract

Although evidence of ecosystem productivity to vapor pressure deficit (VPD) and soil moisture (SM) is abundant, their respective effects remain unclear due to tightly land-atmosphere interactions. In China, the spatial heterogeneity in eco-hydrosphere has further impeded our understanding of ecosystem productivity in response to climate change. This study aimed to assess ecosystem productivity in China during 2001 to 2016 using solar-induced chlorophyll fluorescence (SIF). SIF corresponded well with gross primary productivity (GPP) and was strongly related to VPD and SM. The land-atmosphere interaction mechanism was more notable near the agro-pastoral ecotone, which was reflected in the stronger SM-VPD coupling effect on vegetation and more significant effect of VPD suppressing SIF and SM promoting SIF. We decoupled SM and VPD by equal-width binning of percentiles at a monthly scale. SIF tended to increase and decrease with increasing VPD in SM bins, accounting for 47.59% and 52.41% of the total vegetated area, respectively, whereas photosynthesis tended to increase and decrease with increasing SM in VPD bins, accounting for 62.18% and 37.82% of the total vegetated area, respectively. During 2001 to 2016, VPD and SM had a dominant effect on SIF in 52.31% and 47.69% of China, respectively, which occurred along a latitudinal gradient. However, since 2007, which was identified as the point of accelerated vegetation greening in China, the dominant factor of vegetation productivity has shifted from VPD to SM in terms of respective contributions and dominated areas. Our results emphasize an increasingly important role of SM in China, though the additive effects of VPD cannot be negligible. We conclude that the clarification of the shifting roles of SM and VPD in terrestrial ecosystem allows for more accurate prediction of vegetation productivity at a regional scale and provides greater insights into interactions between vegetation and climate.