Woody plant encroachment enhanced global vegetation greening and ecosystem water‐use efficiency

Abstract

AbstractAimEncroachment of woody plants into grasslands and savannas (WPE) has been observed world‐wide. However, the general ecohydrological effects of this striking change in land cover are uncertain owing to divergent results in various areas and unknown global spatial distribution. Here, we reveal the patterns and dynamics of WPE and its effects on leaf area index (LAI), gross primary production (GPP), components of evapotranspiration (ET) and ecosystem water‐use efficiency (EWUE).LocationGlobal.Time periodContemporary.Major taxa studiedWoody plants.MethodsWe used remote sensing to identify the distribution of WPE in 2002–2018, validated at 442 WPE sites, and analysed the pattern of WPE across geographical gradients. The multi‐time‐scale impacts of WPE were revealed through pairwise comparison. Differences among biomes/climate zones were compared by the Kruskal–Wallis test. The relationship of WPE to vegetation greening and the effects of WPE on EWUE were explored.ResultsGlobal WPE expanded persistently from 2002 to 2018, but the rate of increase decreased after 2010; spatially, the average rate of change was .3%/year. High values of WPE in 2018 occurred in arid and semi‐arid regions, with the peak WPE in the multi‐annual mean precipitation of 350–400 mm. Pairwise comparison showed that WPE increased the LAI, GPP, ET and ratio of transpiration to ET, with the strongest effects in summer, and enhanced annual EWUE. Both global pixel‐ and site‐level WPE mainly showed vegetation greening. The above findings varied among bioclimatic conditions; particularly in semi‐arid areas, WPE was positively correlated with vegetation greening and remarkably improved EWUE.Main conclusionsThe ongoing WPE contributed to vegetation greening and elevated vegetation productivity by increasing the LAI and partitioning more water into transpiration; these findings indicate that the WPE process should be incorporated into the carbon cycle and ecohydrology models. However, attention should also be focused on controlling adverse consequences of WPE in arid areas.