Do tall trees scale physiological heights?

D oes genotypic variation in leaf form and function among tree species reflect adult height or only the seedling regeneration niche? For trees in closed canopy forests, typical adult tree height directly indicates the position of the species in the light hierarchy of the canopy. However, it is unknown whether trees that characteristically differ in mature height have similar or different canopy attributes. Given that light levels decrease vertically in canopies 1,2 , and that leaf form and function are linked with light availability as a result of adaptation 3–5 and acclimation 2–6 , it is logical to hypothesize that trees that differ in adult height should vary systematically in leaf structure and function. But do they? If so, how and why? This issue has received little attention in spite of the interest in whether species that regenerate and spend their lives in markedly differing light environments vary in leaf traits: this is the foundation for the classic contrast of sun versus shade plants 3–6. However, a new paper 7 by Thomas and Bazzaz advances our understanding of trait variation in trees differing in typical adult height. They compared leaf traits of late-successional Malaysian rainforest species that regenerate in shaded-forest understories, but which occupy diverse canopy strata as mature individuals. They discovered that leaf traits of saplings growing in comparable light environments are correlated with adult tree height (i.e. the vertical position and light environment of the canopy of mature trees). Their study is the first to provide quantitative evidence of scaling of leaf traits with tree size and indicates that the mature-tree phase is an important evolutionary 'axis'. Does tree height select for certain leaf traits? Phenotypic variation in leaf traits across pronounced light gradients has been a major issue in plant ecology for a long time 5,6 , as has genotypic variation that helps to explain species habitat distributions 3,4. Thomas and Bazzaz note that most sun versus shade trade-offs have been interpreted in relation to succes-sional status, with early-successional species tending to exhibit sun-plant characteristics , and late-successional species exhibiting shade-plant characteristics. The examination of genotypic adaptation among species that regenerate in shaded understories is less advanced 6,8. Although phenotypic variation in leaf traits has been examined vertically within canopies 2,9 , Thomas and Bazzaz provide the first test of whether photosynthetic characteristics have evolved in response to these vertical light gradients. They ask whether species that …