Differences in plant water use between check-dam land and slope land on the Loess Plateau: Significance for vegetation restoration

Abstract

Check dam lands (DLs) and hillslope lands (SLs) are two typical landscape units in the semi-arid Loess Plateau of China (CLP). Knowledge of the water resource distribution and water use characteristics of plant species used for revegetation in SL and DL has profound implications for understanding plant–water relations and guiding revegetation strategies in water-limited ecosystems. To assess the spatiotemporal characteristics of water resource and investigate water use strategies of two widely used species for revegetation (i.e., Medicago sativa and Caragana korshinskii) in the SL and DL, numerous types of water samples (including rain-, soil-, plant-, and groundwater) were collected from a watershed in the semi-arid CLP for analysis. The oxygen and hydrogen stable isotopic ratios (δ18O and δD) of the water samples were estimated for three consecutive hydrological years (2018−2020). The soil water storage in the unsaturated zone of 0–600cm in the DL (mean 1008mm) was estimated to be 64% higher than that in the SL (mean 613mm). Below 600cm, soil was saturated with water (i.e. groundwater) in the DL and remained unsaturated in the SL. The Bayesian mixing model was applied to the stable isotopic ratios of H and O, which revealed significant differences (p < 0.05) in the proportional contributions of water resources between the two revegetated species. Although M. sativa (mean 76% for SL and 61% for DL) and C. korshinskii (mean 72% for SL and 47% for DL) relied on water from the upper 100-cm soil layer during the observation period, C. korshinskii generally utilised more water from the deep soil layer (>100cm) than M. sativa. Furthermore, the mean water uptake fractions of C. korshinskii in both the SL and DL for the deep soil layers and groundwater significantly increased with decreasing annual rainfall, whereas this was not observed for M. sativa, suggesting that the environmental adaptability of C. korshinskii may be more ecologically plastic than that of M. sativa. Additionally, the contribution of groundwater (>600cm) to root water uptake in M. sativa (mean, 21%) and C. korshinskii (mean, 28%) was comparable in the DL, indicating the importance of groundwater in combating possible water stress for revegetated plants. These results indicate that natural and engineering measure-induced water resource distributions result in different plant water-use characteristics and responses to variations in precipitation. Therefore, engineering measures and species are essential factors in ecological restoration management for enhancing the effectiveness of vegetation restoration strategies.