Dose-dependent root phenotypic plasticity in Casuarina equisetifolia: Mechanistic insights into drought adaptation strategies for coastal sand-fixation afforestation

China's 17,000 km coastal shelterbelt system faces challenges in natural regeneration due to drought stress in sandy soils, particularly affecting the pioneer species Casualina equisetifolia, crucial for stabilizing these vulnerable zones. Limited understanding of drought-induced population declines hampers effective conservation strategies. To address this, we conducted pot experiments on one-year-old C. equisetifolia seedlings using polyethylene glycol (PEG-6000) to simulate drought stress, analyzing root biomass, morphology, and anatomy. Our findings reveal dose-dependent drought responses in C. equisetifolia roots. Moderate water deficit boosts root biomass through accelerated elongation, while severe stress causes a 62 % biomass reduction. Morphologically, seedlings increase root surface area (↑34 %) and tissue density (↑18 %) but reduce total length (↓27 %), with a 2.3-fold expansion of submillimeter roots (0–0.5 mm diameter) to optimize soil exploration. Anatomically, primary roots show drought hypersensitivity with a 41 % reduction in xylem conduit numbers, whereas tertiary roots compensate through increased vessel density (↑22 %) and cortical cell expansion. These adaptive strategies provide a mechanistic framework for enhancing seedling resilience in coastal afforestation programs, offering silvicultural insights for improving the survival and adaptation of C. equisetifolia in drought-prone environments.