Influence of vegetation restoration strategies on seasonal soil water deficit in a subtropical hilly catchment of southwest China

Abstract

Large-scale vegetation monitoring enabled by satellite technology has indicated a global increase in vegetation greenness. The increased soil moisture consumption associated with vegetation restoration may trigger seasonal soil water deficits, limiting sustainable ecosystem recovery. However, the inherent mechanisms of this phenomenon remain unclear, particularly in subtropical hilly regions with frequent seasonal drought occurrences. In this study, we conducted a three-year field observation of volumetric soil water content for three restoration strategies in a re-vegetated hilly catchment of southwest China. These restoration strategies included planted forest (PF) and natural restoration (naturally regenerated forest, NF; deforested pasture, DP). By comparing the soil water storage (SWS) and soil water deficit index (SWDI) among different restored vegetation in the growing and non-growing seasons, the effects of these land-use managements on soil water deficits and their controlling factors were investigated. The results showed that, average SWS of PF was significantly (p < 0.05) lower than that of NF and DP during both growing and non-growing seasons, regardless of slope position. Besides, the soil water deficits primarily occurred during the vegetation growing seasons from May to October, and the frequency of drought occurrences was higher in PF (46.2 %) in comparison to NF (45.3 %) and DP (44.7 %). Particularly, the frequency of severe drought occurrences in PF was 12.0 %, significantly (p < 0.05) higher than that in NF (2.9 %) and DP (1.6 %). The dominant meteorological factor controlling soil water deficits was precipitation (contribution rate = 38.8 %) during the growing seasons. Contrastingly, soil water deficits during the non-growing seasons were mainly controlled by potential evapotranspiration (contribution rate = 32.7 %). Overall, the indexes of SWS and SWDI offer a quantitative framework for assessing the extent of seasonal soil drought. This study highlights the higher vulnerability to soil drought associated with afforestation as compared with natural regeneration, which should be considered when evaluating the ecosystem services provided by vegetation restoration.