Leaf cluster morphology is altered along a local light gradient in different manners between young and old Fagus crenata crowns

Leaf cluster morphological variation within and among Fagus crenata crowns was examined by analysing hierarchical leaf cluster structure. Leaf clusters were defined as a continuous group of leafy voxels, where each voxel was classified as part of a cluster if its effective leaf area density (eLAD) exceeded a threshold value. This threshold value ranged from sparse (10−1.3 m2 m−3) to dense (101.2 m2 m−3). Compared with young crowns, old crowns had a smaller effective leaf area index, a higher proportion of leafless space, more highly fragmented leaf clusters, and thinner clusters at all hierarchal levels, from smaller clusters with higher eLAD to large clusters with lower eLAD. Leaf clusters exhibited structural adaptability to the local light gradient within the crowns. For instance, we observed a positive correlation between leaf cluster thickness and inclination with relative light intensity. We hypothesized that the leaf cluster responses represented adaptations to changes in the light environment within crowns, particularly the increased light interception in lower crowns and the corresponding responses of leaves and twigs. However, cluster adaptation to local light conditions differed between young and old crowns. In young crowns, cluster thickness markedly varied with local light conditions, whereas cluster inclination did not respond. In old crowns, cluster inclination clearly adapted to local light conditions, whereas cluster thickness had little response.