Adaptive Strategies of Leaf Nutrient Stoichiometry in Response to Vertical Canopy Microclimate in a Temperate Forest

Closed forests are usually characterized by a complex canopy structure that results in dramatic vertical variation in micro-environmental conditions within the tree crown. There is still limited understanding of how plants adjust to vertical gradients in biophysical variables within tree crowns to regulate leaf non-structural carbohydrate (NSC) dynamics and leaf elemental stoichiometry. To enhance the understanding of leaf adaptive strategies across different crown positions, we measured leaf NSC concentrations, elemental composition (C, N, P, K, and Ca), and key morphological features like leaf thickness, leaf area, specific leaf area, leaf water content, and equivalent water thickness at the upper and lower crowns of 13 mature temperate tree species. We found that vertical variation in microclimate, particularly the differences in light availability and air temperature, significantly influenced leaf NSC concentrations, with leaves in the lower crown exhibiting consistently higher NSC levels. Leaf nutrient traits also varied with crown position and were closely associated with changes in leaf morphological characteristics, indicating coordinated adjustments in resource acquisition strategies along the vertical canopy gradient. In contrast, while NSC-to-nutrient ratios declined with increasing crown position, the C:N:P stoichiometric ratios remained largely stable across crown positions. Together, these results suggest that trees maintain relatively stable elemental stoichiometry while allowing flexible NSC allocation and trait coordination to cope with strong vertical microclimatic heterogeneity in closed-canopy forests.