Seasonal drought amplifies stand density effects on water-use strategies in Populus tomentosa plantations

Stand density is closely linked to tree water uptake strategies by modifying soil water availability and ultimately influences forest drought resilience. However, the water uptake patterns of trees in plantations and their responses to fluctuations in soil water availability under the combined effects of stand density and seasonal hydrological variability remain poorly understood. In this study, we employed dual stable isotopes (δ 2 H and δ 18 O) combined with a Bayesian mixing model (MixSIAR) to quantify seasonal variations in water uptake patterns of Populus tomentosa plantations across three stand densities (600, 900, and 1200 trees ha −1 ) in the Lower Yellow River region. Structural equation modeling (SEM) was further applied to evaluate how stand structure and tree physiological traits influenced water uptake patterns. Our findings indicate that during the dry season, P. tomentosa primarily relies on soil moisture at 100–200 cm depth (accounting for 54% of its water uptake), whereas water uptake shifted to the upper 0–100 cm (58%) following wet-season recharge. Across stand density, trees in medium-density stands showed the highest reliance on deep soil water compared with low- and high-density stands during the dry season, while no differences occurred among densities in the wet season, confirming the strong seasonal dependence of density effects. Deep soil water uptake is co-influenced by stand density, soil moisture, and stomatal regulation. The coordination between water-source partitioning and physiological behavior fundamentally shapes the water-use strategies of P. tomentosa . This study reveals that stand density influences drought resistance by seasonally adjusting water uptake depth and physiological behavior, providing a mechanistic basis for optimizing planted forest resilience under increasing drought. • Seasonal drought amplifies stand-density regulation of tree water use. • Medium-density stands show the greatest deep-water uptake in drought. • Water source partitioning is co-regulated by stand structure and physiological traits. • Stand density mediates drought resilience via deep water use and stomatal regulation.