Afforestation changes the trade-off between soil moisture and plant species diversity in different vegetation zones on the Loess Plateau

• Trade-offs between SMC and species diversity are lowest for natural grasslands. • Species diversity of afforested land in the steppe zone is at the expense of SMC. • The SMC of afforestation lands in the forest zone can maintain existing species diversity. • The trade-offs between SMC and species diversity are balanced for the forest steppe zone. Soil moisture and plant species diversity are key factors determining the structure, function and stability of terrestrial ecosystems in arid and semiarid environments. However, the relationship between species diversity and soil moisture of different vegetation restoration measures over a precipitation gradient has not been thoroughly explored, inhibiting decision-makers’ ability to formulate vegetation restoration policies. In this study, we examined the relationship between the soil moisture content (SMC) and species diversity of two vegetation restoration measures (afforested lands and natural restoration grassland) in different vegetation zones (steppe, forest-steppe and forest) on the Loess Plateau. The results showed that the species diversity of afforested lands was ranked as forest zone < forest-steppe zone < steppe zone, which contrasts with natural grasslands. The average SMC of afforested lands was ranked as steppe zone < forest-steppe zone < forest zone, and the results further indicated afforestation decreased SMC compared with the natural grassland. The relative benefit of the species diversity index in afforested land was more extensive than that of soil moisture in the steppe zone but smaller in the forest zone, and the trade-offs between soil moisture and species diversity were smallest in the forest-steppe zone but approached no trade-offs in natural grasslands. Our study suggests that although natural grassland is the ideal ecosystem across all the vegetation zones examined, the forest and forest-steppe zones are most suitable for afforestation, accounting for the human need for timber harvesting. However, afforestation plantations need to be thinned appropriately to maintain high soil moisture and species diversity and low trade-offs between soil moisture and species diversity.