Pivotal role of root water uptake pattern in shaping leaf economic and water-use strategies of subtropical native trees

Water uptake depth is often coordinated with leaf morphology, nutrient and water-use traits across dryland plant species, but such trait coordination remains largely unexplored in plants from more humid but nutrient-poor habitats. We assessed how the year-round water uptake pattern influences the leaf economics spectrum (LES) and isotopic water-use traits across five representative native tree species inhabiting limestone soils and sandstone-derived yellow soils in humid subtropical SW China. We used xylem water isotopes (δ18O, δ2H) to infer water uptake depth; leaf δ13C and Δ18O as proxies for time-integrated water-use efficiency and stomatal conductance, respectively; and key LES traits (specific leaf area, Nmass and Narea) as indices of carbon-nutrient economy. Soil water uptake depth strongly influenced the inter-specific variations in leaf economic and water-use traits, especially during the dry winter-spring period. Shallow-rooted species using water stored in fertile topsoil layers exhibited lower carbon investment per leaf area, higher leaf N and water contents, and higher δ13C values. Conversely, deep-rooted species using deeper soil/bedrock water exhibited thicker and more sclerophyllous leaves combined with lower leaf N, water contents and δ13C values. Across species, leaf δ13C increased with N content, revealing that N-induced differences in photosynthetic capacity are the dominant control over interspecific variation in intrinsic water-use efficiency. Shallow-rooted species exhibited lower foliar Δ18O values (indicative of looser stomatal regulation and water-spender strategy), potentially facilitating nutrient uptake from fertile topsoil. Specifically, Zanthoxylum bungeanum played a central role in shaping the broad water-spender-to-water-saver continuum observed across the target species. Our findings highlight how shallow-rooted tree species can adopt a resource-acquisitive strategy through coupled enhancement of soil water and nutrient capture, stomatal conductance, photosynthetic capacity and water-use efficiency. We provide novel insights into key ecophysiological mechanisms that may help maintain tree species diversity and coexistence in humid but nutrient-poor subtropical habitats.