Effects of thinning on soil water content and water use characteristics of artificial forest in the Loess Plateau of China

Abstract

Stand thinning, an important means of forest management, can alleviate soil drying and vegetation degradation caused by excessive afforestation in semi-arid regions. However, how thinning affects leaf-scale and root water use characteristics of artificial forests in different hydrological years remains unclear. In this study, the soil moisture, sap flow, and stable isotopes (δ2H, δ18O, and δ13C) were observed to investigate the effects of thinning (heavy to light) on stand transpiration, soil water sources, and leaf-scale water use efficiency (WUEi) of Robinia pseudoacacia within three years (2020–2022) in the Loess Plateau. The results showed a relatively higher contribution of shallow-middle soil water (0–300 cm) to R. pseudoacacia and a higher soil water content in deep layer (300–500 cm) in thinning plots compared to non-thinning plot. In the normal (2020) and wet (2021) years, an “upward shift” water uptake pattern from deep to shallow-middle soil layers of R. pseudoacacia in thinning plots was observed. In the meantime, thinning led to a significant decrease in transpiration (45.1 % and 40.3 %) and an increase in WUE_i (12.0 % and 10.9 %) of R. pseudoacacia in heavy and light thinning plots. However, R. pseudoacacia in turn shifted water uptake depths to deep layer (37.6 %-47.2 %) in the third year with less precipitation (2022). The thinning effects on transpiration and WUEi of R. pseudoacacia also weakened in 2022. There was a linear positive correlation of WUEi with transpiration rate and shallow soil water use, while WUEi was negatively related to the middle and deep soil water contributions to R. pseudoacacia, with a higher slope in thinning plots compared to non-thinning plot, indicating the shifting water absorption strategy of R. pseudoacacia to improve ecological adaptability by thinning. Overall, the temporary effects of thinning on soil moisture and water use characteristics should be fully integrated into forest ecosystem restoration and management efforts.