Ontogenetic Variation in Multidimensional Leaf Traits and Stage-Specific Soil–Trait Associations in Fraxinus mandshurica

Multidimensional trait frameworks are increasingly used to understand how plant functional strategies change across ontogeny. However, whether leaf economic, stomatal, and non-structural carbohydrate traits show consistent or dimension-specific ontogenetic patterns, and how their associations with soil factors vary across ontogenetic stages, remains unclear. We measured 11 leaf functional traits and five soil factors across sapling, young, and adult tree stages of Fraxinus mandshurica in a temperate mixed broadleaf–Korean pine forest. Principal component analysis was used to integrate the measured traits into leaf economic, stomatal, and non-structural carbohydrate axes, whose ontogenetic patterns and stage-specific associations with soil factors were subsequently evaluated. The results showed that the dominant structure of trait covariation shifted from a leaf economic dimension in saplings and young trees to a stomatal size–density dimension in adults. The three functional axes showed distinct stage-specific patterns: scores on the leaf economic axis decreased, and those on the stomatal axis increased from saplings to young trees, with neither differing significantly between young and adult trees, whereas scores on the non-structural carbohydrate axis did not differ significantly between saplings and young trees but were higher in adults. Stage-specific analyses revealed contrasting patterns of soil–trait associations among ontogenetic stages and functional axes, with a broader set of significant associations in saplings and young trees than in adults. Notably, soil water content was the only soil factor associated with all three functional axes in saplings, whereas soil total phosphorus made the largest independent contribution to all three axes in young trees; in adults, only scores on the leaf economic axis were significantly associated with soil pH. Our findings highlight that leaf functional variation is multidimensional, with leaf economic, stomatal, and non-structural carbohydrate dimensions exhibiting distinct ontogenetic patterns, and that soil–trait associations vary among ontogenetic stages and functional axes. These results improve our understanding of variations in multidimensional leaf functional strategies and soil–trait associations across ontogenetic stages.