The distinctive leaf and twig heteromorphism in Euphrates poplar (Populus euphratica Oliv.) reflects its adaptive strategies to cope with arid environments across ontogenetic stages. In the key distribution area of P. euphratica forests in China, we sampled P. euphratica twigs (which grow in the current year) at different age classes (1-, 3-, 5-, 8-, and 11-year-old trees), then analyzed their morphological traits, biomass allocation, as well as allometric relationships. Results revealed significant ontogenetic shifts: seedlings prioritized vertical growth by lengthening stems (32.06 +/- 10.28 cm in 1-year-olds) and increasing stem biomass allocation (0.36 +/- 0.14 g), while subadult trees developed shorter stems (6.80 +/- 2.42 cm in 11-year-olds) with increasesd petiole length (2.997 +/- 0.63 cm) and lamina biomass (1.035 +/- 0.406 g). Variance partitioning showed that 93%-99% of the trait variation originated from age and individual differences. Standardized major axis analysis demonstrated a consistent "diminishing returns" allometry in biomass allocation (lamina-stem slope = 0.737, lamina-petiole slope = 0.827), with age-modulated intercepts reflecting developmental adjustments. These patterns revealed an evolutionary trade-off strategy where subadult trees optimized photosynthetic efficiency through compact architecture and enhanced hydraulic safety, while seedlings prioritized vertical space occupation. Our findings revealed that heteromorphic twigs play a pivotal role in modular trait coordination, providing mechanistic insights into P. euphratica's adaptation to extreme aridity throughout its lifespan.