Self-organization shapes divergent water use strategies among co-occurring shrubs for drought resilience in drylands

Differentiation in water use strategies is essential for the function and resilience of dryland ecosystems. Under prolonged drought, dominant shrubs self-organize into two spatial configurations: scattered and clumped. However, the mechanisms by which root–leaf trait coordination drives these divergent water use strategies remain poorly understood. In this study, the soil moisture, morphological and root traits, leaf-level physiological traits, and stable isotope (δ²H, δ¹⁸O, and δ¹³C) of scattered and clumped Vitex negundo were observed during the 2022–2024 growing seasons in the semi-arid Loess Plateau, to elucidate the water use strategies and physiological responses of self‑organized shrubs. Our findings indicate that scattered shrubs primarily utilized middle and deep soil water (69.4±7.8%), facilitated by isolated canopies that promote precipitation infiltration and recharge deeper soil layers. In contrast, clumped shrubs predominantly relied on shallow and middle soil water (82.0±6.5%), supported by their aggregated canopies and root systems. Scattered shrubs adopted a conservative strategy, exhibiting higher intrinsic water use efficiency (iWUE) and stable midday water potential during dry seasons, due to lower specific leaf area and moderate stomatal conductance. Conversely, clumped shrubs exhibited an opportunistic strategy, characterized by larger specific leaf area and higher stomatal conductance, enabling rapid photosynthetic accumulation and peak iWUE during rainy seasons. However, under drought, clumped shrubs accelerated the depletion of shallow soil water, leading to depressed midday water potential and constrained photosynthesis. These shrub types illustrate complementary mechanisms for drought adaptation: scattered shrubs enhance  resilience, while clumped shrubs improve precipitation capture efficiency, collectively promoting the stability of dryland ecosystems.