Trait-specific environmental regulation of multi-element leaf stoichiometry in threatened wild apple across arid Central Asian mountains

Wild apple ( Malus sieversii ) is a threatened relict tree and the primary wild progenitor of cultivated apple. Understanding its leaf nutrient status and environmental relationships is important for assessing nutrient constraints and ecological variation across its transboundary range. Although nitrogen and phosphorus are commonly emphasized in plant stoichiometric studies, incorporating potassium may provide a more complete view of multi-element nutrient coordination under arid mountain conditions. Here, we examined large-scale variation in leaf carbon, nitrogen, phosphorus, and potassium stoichiometry of M. sieversii across the Tianshan Mountains of Central Asia. We sampled 155 individuals from 13 sites across Xinjiang, Kazakhstan, and Kyrgyzstan, spanning approximately 1,300 km. Leaf stoichiometric traits showed pronounced spatial variation. Across the sampled range, leaf N concentrations were comparatively low, P concentrations were moderate, and K concentrations fell within the range reported for regional drought-adapted plants. The combination of relatively low leaf N status and low N: P ratios suggested that some populations may experience potential N constraint. Allometric analysis showed that only the N–P and K–P relationships were significant, and both were hyperallometric, indicating that N and K increased disproportionately relative to P. Patterns of variation also differed markedly among traits: C showed the lowest within-site variability, N:K the highest, and P the strongest site-level structuring. After accounting for site-level clustering and multiple testing, the most robust univariate environmental associations were concentrated in leaf C. Mixed-effects SEMs further showed significant direct soil pathways to most leaf traits, whereas direct climatic pathways were more trait-specific and concentrated mainly on K and K-related stoichiometric ratios, with additional indirect associations through soil. Geographic gradients primarily represented an upstream spatial background associated with regional climatic and edaphic variation. Leaf stoichiometry in M. sieversii reflects potential N constraint, element-specific nutrient coordination, and hierarchical environmental relationships. The hyperallometric N–P and K–P relationships reveal distinct modes of elemental coordination, while the environmental analyses highlight broad soil associations and more selective climatic sensitivity of K-related traits. Integrating multi-element leaf stoichiometry with soil nutrient conditions may provide complementary ecological information for assessing nutrient status and environmental sensitivity among wild apple populations and for informing conservation and restoration efforts.