Contrasting adaptive strategies by Caragana korshinskii and Salix psammophila in a semiarid revegetated ecosystem

Salix psammophila and Caragana korshinskii are pioneer species used for revegetating farmlands on the Loess Plateau. Understanding the water-use strategies and physiological characteristics of these two species are important for evaluating their adaptability in water-limited ecosystems. We used stable isotopes (D and ¹⁸O), physiological parameters (e.g. midday and predawn leaf water potential and stomatal conductance (ψₘd, ψₚd and gₛ, respectively)) and foliar δ¹³C to analyse water sources, isohydric/anisohydric behaviour, and water-use efficiency (WUE), respectively, of these two shrub species. Our results show that S. psammophila relied on water (mean 60%) from the intermediate soil layer (30-80 cm from the surface) during the dry period (May and June), while C. korshinskii relied on water from the deep soil layer (80-100 cm) (mean 52%). Both species shifted to using water from the shallow layer (0-30 cm) during the wet period (July and August) (mean 61% for S. psammophila and 51% for C. korshinskii). These root water uptake patterns are consistent with C. korshinskii’s physiological performance, which in turn is consistent with a description that C. korshinskii is an anisohydric species as evidenced by a steeper slope of the relationship between ψₘd and ψₚd (σ, 1.88), and a larger difference between ψₘd and ψₚd (Δψ, -1.66 ± 0.10 MPa). In contrast, S. psammophila could be described as an isohydric species (0.82 for σ and -0.77 ± 0.02 MPa for Δψ). Surprisingly, despite being anisohydric, C. korshinski showed greater long-term WUE than isohydric S. psammophila, particularly during the dry season. Insofar as water availability is concerned, these results show that the environmental adaptability of C. korshinskii may generally be more ecologically plastic than that of S. psammophila. Our study serves as a reference for evaluating the persistence of shrub species in relation to the management and restoration of ecosystems.