Afforestation may accelerate the depletion of deep soil moisture on the Loess Plateau: Evidence from a meta-analysis

Afforestation is an effective way to restore degraded landscapes and protect ecosystems in arid and semiarid regions. However, the responses of soil moisture content (SMC) to afforestation in deep soil layers as well as the related controlling factors on a regional scale are still poorly understood. In this study, a meta-analysis compiled a total of 3045 individual measurements in 203 sites, was conducted to evaluate the effects of afforestation on SMC to a depth of 300 cm based on a field transect survey covering non-afforested modes (traditional rain-fed cropland as the control) and afforested vegetation types (forestland, shrubland, and grassland) from southeast to northwest across the Chinese Loess Plateau and to examine the relationships between the SMC and potential affecting factors. The results showed that afforestation led to a decreasing SMC across the entire profile, especially in forestland and shrubland. Soil moisture changes were significantly affected by restoration types (forests, shrubs, and grasses) and restoration ages. Mean annual precipitation, mean annual temperature, and initial soil moisture jointly affected soil moisture in all land use types along the precipitation gradient. The artificial restoration mode significantly decreased the soil moisture, while the natural restoration mode partially restored the soil moisture. These results indicate that natural restoration may be the better choice from the soil moisture perspective in the context of future climate change in arid and semiarid regions.

Automated summary

Afforestation is a useful method for restoring degraded landscapes and protecting ecosystems in arid and semiarid regions. However, the impact of afforestation on soil moisture content (SMC) in deep soil layers and the factors controlling it on a regional scale are not well understood. This meta-analysis study compiled data from 203 sites and found that afforestation led to a decrease in SMC across the whole soil profile, especially in forestland and shrubland. The type of restoration and the age of restoration significantly affected soil moisture changes, while precipitation, temperature, and initial soil moisture collectively influenced SMC in different land use types.