Understanding how morphological and physiological trait relationships vary with tree size and environmental conditions is fundamental to predicting tropical tree responses to environmental change. We examined trait relationships in 17 co-occurring tree species spanning juveniles to adults in Pasoh, a lowland rainforest, Peninsular Malaysia. We measured multiple leaf and stem traits related to light capture, water relations, and structural support, along with individual tree size, light environment, and neighborhood competition indices.

We first examined how tree size and environmental factors (light availability and competitive neighborhood) affect individual trait values using mixed-effects models controlling for species identity. Preliminary analyses suggest that ontogenetic shifts (size effects) may be a dominant factor influencing trait variation, with more limited effects of light environment on specific traits. Results indicate that environmental responses appear consistent across ontogenetic stages.

We then examined whether environmental factors modulate trait-trait relationships. Results using mixed-effects models suggest that trait coordination may remain relatively stable across environmental gradients, though patterns vary across trait combinations. We further used standardized major axis regression to characterize variation in trait relationships across life-form groups (Emergent, Canopy, and Understory), with a sequential testing framework classifying them into three categories: relationships with fundamentally different allometric slopes; relationships sharing common slopes but exhibiting parallel shifts in intercepts; and universally conserved relationships showing neither slope nor intercept differences. Results suggest systematic patterns in environmental sensitivity across these categories.

Our findings reveal coordinated variation in trait relationships among tropical trees, with ontogenetic development, environmental conditions, and life-form each contributing to trait variation.