Quantifying functional trait assembly along a temperate successional gradient with consideration of intraspecific variations and functional groups

Advances in functional trait ecology allow us to evaluate community structure and community assembly in a more quantitative and generalised manner. Functional traits are known to differ greatly across local canopy openness, but previous studies often ignored such intraspecific variations when plant community structures are evaluated. We examined how considerations of canopy openness affect evaluations of community structure along a secondary successional gradient in a temperate climate zone in Japan. For 59 woody species found in nine forest plots aged from ca. 10 to 100 years old, we measured 12 functional traits with considering local canopy openness to capture their intraspecific variations across light gradients. Functional traits varied extensively both within and across species, and intraspecific variations were often tightly correlated with local canopy openness. Niche differentiation between deciduous and evergreen species in resource use strategy, as indicated by leaf mass per area (LMA), was more evident when taking canopy openness into consideration. Community-weighted means of the canopy-layer communities differed from those of the subordinate-layer communities, suggesting another niche differentiation along the vertical light gradient. Functional richness was often greater in the subordinate layer than in the canopy layer due to the co-occurrence of small individuals of canopy species and shade-tolerant specialists. Our study emphasises the importance of canopy openness when evaluating and understanding niche differentiation and forest community structures.