Vegetation greening intensified soil drying in some semi-arid and arid areas of the world

Vegetation greening and soil drying, which are simultaneously observed, have brought controversy about whether vegetation greening leads to soil drying or wetting. Relevant conclusions from previous small-scale research are uncertain worldwide, and the indirect effects of climate via vegetation change on soil moisture (SM) are poorly understood. Here, to explore the effects of vegetation greening on SM change, the patterns of SM trends (wetting to more wetting (W to W), wetting to drying (W to D), drying to wetting (D to W) and drying to more drying (D to D)) were identified in the global vegetation greening (GVG) areas from 1982 to 2015 by using the turning years in the quantity of vegetation greenness and then validated by structural equation model (SEM) and 400 ground stations. The main results are as follows: 1) In the study period, 65.87% (33.57%, Sig.) of the GVG areas featured soil drying including the southeast of the United States, Africa north of the equator, the inland of Europe, the south of China and the inland of Australia, which were mainly from grasslands, barren, savannas, open shrublands, woody savannas and croplands. 2) Vegetation significantly and negatively influenced the average SM of global greening and drying areas, and their climate was arider than that in global greening and wetting areas. 3) Soil in over half of the GVG areas was toward drying after vegetation greenness increased but only the D to D pattern was mainly distributed in drylands (Semi-arid, Arid and Hyper-arid areas) such as the Loess Plateau of China and eastern Australia. The W to W and W to D patterns were remarkably observed in the in-situ SM. 4) SEM showed that the direct effects of vegetation increase on W to W and D to D patterns were stronger than those of temperature and precipitation, especially that the indirect effect of temperature on D to D pattern via promoting vegetation greening outweighed the direct effect of temperature. Overall, the increase in vegetation mainly caused by climate warming has exacerbated the tendency of soil drying in some drylands. Therefore, vegetation restoration or forestry management requires the consideration of local SM-carrying capacity for plants, especially in Semi-arid and Arid ecosystems.