Sustained water supplementation alters nutrient allocation strategies in a fast-growing plantation forest: Implications for dynamic water-nutrient management

Against the backdrop of intensifying drought risks driven by global climate change, the productivity enhancement of plantation forests increasingly relies on water management. However, how sustained water supplementation influences nutrient allocation and stoichiometric balance at the tree organ level remains a critical knowledge gap. Through a five-year experiment in a Populus tomentosa plantation, we examined how water supplementation interacts with stand development to shape nutrient allocation and stoichiometry across tree organs. Elevated soil moisture did not change organ carbon (C) concentrations, though it differentially affected nitrogen (N) and phosphorus (P) concentrations depending on the organ and the age of the trees. Leaf N:P patterns suggested a possible tendency toward N limitation during stand development. Stand-level C, N, and P stocks responded to water supplementation in an age-dependent manner, with significant increases under DIFI mainly emerging by age 5. These increases were primarily associated with biomass accumulation, yet pronounced P retention in woody biomass suggested increasing plant P demand during later stand development. Nutrient allocation shifted in two phases: before canopy closure, moisture-elevated trees prioritized leaves, while water-limited trees invested more in roots; after canopy closure, moisture-elevated stands allocated nutrients to stems for light competition, whereas water-limited stands maintained a root-conservation strategy. Our findings suggest that soil moisture and stand age jointly influence the transition from early resource-acquisition to later structural competition or resource conservation. Sustained water supplementation should therefore be coupled with stage-specific nutrient monitoring and adaptive fertilization to maintain long-term productivity and nutrient balance.