Responses of leaf-level physiological traits and water use characteristics to drought of a xerophytic shrub in northern China

Abstract

Frequent drought distinctly affects the water use characteristics and physiological responses of xerophytic shrubs, which has significantly altered the regional water cycle in drylands. It is essential to explore the coordination between leaf-level physiological traits and root water uptake patterns of xerophytic shrubs to adapt drought. In this study, the soil water dynamics in 0–200 cm, sap flow, leaf-level physiological traits, and stable isotopes (δ²H, δ¹⁸O, and δ¹³C) of Salix psammophila were observed over three different hydrological years (2019–2021) in the semi-arid Loess Plateau of China, and the responses of water use patterns and physiological traits to drought were investigated. Results showed that in wet (2019) and normal (2020) years, S. psammophila mainly utilized soil water in shallow layer (0–40 cm) with proportion of 46.0 ± 12.3 % and had a higher transpiration rate and intrinsic water use efficiency (iWUE). In dry year (2021), the main soil water source shifted to the middle layer (40–120 cm) with proportion of 51 ± 1.5 % and utilization of soil water in deep layer (120–200 cm) also increased, while iWUE decreased by 10 %. During the three years, an average of 39.4 % and 40.2 % of transpiration were provided by soil water in shallow and middle layers, respectively. S. psammophila could not completely alleviate drought stress by utilizing deep soil water due to persistent deep soil desiccation. In contrast, S. psammophila regulated leaf-level physiological traits (such as reducing leaf water potential and leaf area index) to reduce transpiration for adapting drought in dry year. The findings highlight that deep soil water is not a sustainable water source for xerophytic shrubs during drought, and deep soil water desiccation is the main threat for xerophytic shrub mortality. This study deepens understanding of drought resistance mechanisms of xerophytic shrubs, thereby providing essential insights for shrub ecosystem management to adapt global warming in drylands.