Ontogenetic Stage Strongly and Differentially Influences Leaf Economic and Stomatal Traits Along Phyllotactic and Environmental Gradients

Phyllotaxy is a key determinant of intraspecific variation in leaf functional traits, with different leaflet positions often representing distinct strategies of resource acquisition and utilization. Yet, the extent to which such phyllotactic differentiation is modulated by ontogenetic stage remains poorly understood. Here, we examined saplings and adult trees of Fraxinus mandshurica, a dominant compound-leaved species in temperate broadleaf forests, by quantifying four leaf economic traits and four stomatal traits across six phyllotactic positions. We further assessed the relative influences of phyllotaxy and environmental factors, including soil total nitrogen, soil water content, and canopy openness, on trait variation at different ontogenetic stages. Our results showed that economic traits varied significantly along phyllotaxy, whereas stomatal traits were relatively conservative. The effects of ontogenetic stage on traits at a given phyllotactic position were trait-specific. Within-group correlations of economic traits and of stomatal traits remained stable across ontogenetic stages and were consistently stronger than between-group correlations. Sapling traits were more strongly affected by soil total nitrogen and soil water content, whereas those in adult trees were primarily shaped by soil water content and canopy openness. Moreover, both trait–trait and trait–environment associations were weaker at the leaflet level than at the compound-leaf level. Our study highlights the critical role of ontogenetic stage in shaping leaf trait responses to phyllotaxy and environmental change, providing new insights into the mechanisms underlying intraspecific trait variation in compound-leaved tree species.