Coupled hydrological niche segregation and the leaf economic spectrum explain the coexistence of woody species in a natural secondary forest

Abstract By contrasting water sources and diverging in leaf economic spectrum (LES) traits, plants regulate and balance their use of water, carbon and nutrient resources, thereby potentially promoting species coexistence. However, how these strategies affect species coexistence is unclear. We quantified the relative contributions of different water sources (topsoil water, 0–20 cm; subsoil water, 20–200 cm), intrinsic water use efficiency (WUEi), key LES traits and the importance value index (a proxy for species dominance) across the 10 coexisting plant species in a natural secondary forest at Ziwuling Mountains of the Loess Plateau (LP), Northwest China. Our results showed the large variability in water source utilisation, WUEi, leaf N, P contents and specific leaf area (SLA) among species, indicating diverse resource use strategies. Differences in water sources resulted in varying degrees of water competition between species, ranging from 47% to 81%. The water source utilised by plants was coordinated with the LES. Species that relied on topsoil water, characterised by nutrient‐rich but variable water supply, adopted acquisitive strategies, exhibiting high leaf N, P contents and SLA. In contrast, species relying on subsoil water, which provides relatively low‐nutrient availability but a stable water supply, adopted conservative strategies, displaying opposite traits. Notably, species with conservative strategies dominated in the community. Synthesis . Our findings highlight a tight coupling between plant water sources and the LES, which jointly influence species coexistence and dominance. Our study evaluated the persistence and coexistence strategies of the 10 species in a natural secondary forest, which serves as a reference for the tree species selection of afforestation in the LP, or similar regions.