Size-Related Variation in Tree Leaf Traits and Its Effects on Trait-Growth Relationships in a Subtropical Cloud...

Trait-based approaches offer an essential tool for exploring tree growth and adaptation strategies. However, the generality of trait-growth relationships and the role of tree size in influencing their relationships remain uncertain. This study aims to explore size-dependent trait variation and its effects on individual growth models using a trait-based approach. We measured the leaf anatomical characteristics and nutrient content of 322 trees from 18 coexisting species and monitored their growth rates in a subtropical montane cloud forest. Our results showed that between 26% and 62% of trait variance was attributed to intraspecific variation of different sized trees. Larger trees tend to have smaller specific leaf area (SLA) and thicker palisade tissue (PT), while they also exhibit smaller and denser stomata to optimize water utilization and photosynthetic efficiency. As trees increased in size, their basal area growth advantage was attributed to both vertical competitive advantage and functional trait adaptations for light capture. Canopy species enhanced individual tree growth by adjusting the morphological structures of their leaves, such as thicker PT, higher stomatal density, and lower SLA, while understory species increased leaf phosphorus content, reflecting their specialized adaptation strategies to distinct vertical niches in phosphorus-limited environments. In addition, traits measured at the individual level revealed broader trait-growth relationships compared to species average traits. The study highlights that the pronounced effects of size-dependent trait variation are crucial for elucidating trait-growth relationships and understanding tree adaptive strategies under heterogeneous vertical light conditions.