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 (SLA, Nmass, 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 schlerophyllous 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 inter-specific 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.