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Against the backdrop of intensifying drought risks and hydrological fluctuations driven by global climate change, the productivity enhancement of plantation forests—as vital carbon sinks and timber resources—increasingly relies on water management. However, how long-term water supplementation influences nutrient allocation and chemometric balance at the tree organ level remains a critical knowledge gap. We conducted a five-year experiment in a Populus tomentosa plantation to examine how long-term water supplementation, interacting with stand development, shapes nutrient allocation and stoichiometric ratios among fine roots, coarse roots, stumps, stems, branches, and leaves.