In Fagus crenata, mean leaf size and difference in sun- vs. shade-leaf morphology are both larger for northern populations. Here, we compared within-crown variation in leaf morphology among five populations to elucidate regional differences in phenotypic plasticity and infer genetic and environmental factors that influence it. Morphological plasticity of leaves was more strongly correlated with relative height in the crown (leaf height / tree height) than with canopy openness. We inferred this was because relative height reflected both light environment and water status of leaves. Regional differences in plasticity of leaf area in relation to canopy openness corresponded to genetic lineages of the populations sampled. Whereas, that of leaf thickness and specific leaf area (leaf area / leaf dry mass) in relation to relative height did not correspond to genetic lineages. In all regions, leaf thickness converged to approximately 0.25 mm at relative height≒0.5 and plasticity resulted from leaves becoming thicker/thinner with increasing/decreasing height, respectively. At the tree top, specific leaf area converged to a minimum value of 0.1 m² g^‒1 and regions with greater specific leaf area in the lower crown had greater plasticity. Leaf morphological plasticity was greater in regions that had more extreme climatic conditions, suggesting that it represents adaptive plasticity in response to climatic variation.

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