Effects of Leaf Nutrients, Non-Structural Carbohydrates, and Microanatomical Structure on Biomass of Three Tree Species Under Drought Stress

Drought stress profoundly affects plant growth and survival, but comparisons of integrated adaptive strategies across multiple tree species remain unclear. In this study, seedlings of Elaeagnus angustifolia (E. angustifolia), Populus euphratica (P. euphratica) and Xanthoceras sorbifolium (X. sorbifolium) were subjected to well-watered (CK), mild (T1), moderate (T2), and severe (T3) drought treatments. Leaf microanatomical traits, non-structural carbohydrates (NSCs), stoichiometric elements, biomass allocation, and key stress indicators were measured. The results showed that P. euphratica seedlings thickened leaves and vascular tissues and accumulated soluble sugars (SSs) and starch (ST) under T1–T2, but under T3, they prioritized root investment (root biomass +26.0%); their antioxidant enzymes were activated only under mild-to-moderate stress and declined under severe stress. E. angustifolia seedlings exhibited moderate leaf structural thickening, sharply increased root biomass (+97.2% under T3) while maintaining stem biomass, continuously elevated activities of superoxide dismutase (SOD) and peroxidase (POD) as well as osmoregulatory substances (soluble protein SP, proline Pro), and showed the lowest malondialdehyde (MDA) content; their leaf carbon (C), nitrogen (N), and phosphorus (P) contents decreased the least, and their stoichiometric ratios remained stable. In contrast, X. sorbifolium seedlings progressively reduced leaf thickness and vascular area, depleted NSC reserves, exhibited unstable antioxidant responses, showed a significant decrease in Pro under severe drought, accumulated the highest MDA, and had the lowest N/P ratio, indicating the strongest nitrogen limitation. These results demonstrate that E. angustifolia combines structural plasticity, efficient nutrient use, robust osmotic adjustment, and sustained antioxidant capacity, conferring the strongest drought tolerance; P. euphratica* shows moderate tolerance through transient structural and carbon investment but suffers under extreme drought; X. sorbifolium has the weakest drought tolerance.