Tree architecture in relation to leaf dimensions and tree stature in temperate and tropical rain forests

1 The architecture and leaf dimensions (petiole length, blade length and blade width) of 403 tree species were studied at six sites ranging from lowland equatorial to temperate Australian rain forest. The branching patterns of understorey saplings and exposed canopy trees were classified as orthotropic (three-dimensional arrangements of leaves about ascending twigs) or plagiotropic (leaves arranged in planes, usually along the sides of horizontal twigs). 2 Among understorey saplings, orthotropic species had larger leaves and longer petioles than plagiotropic species. Discriminant analysis showed that the two groups could be separated by petiole length and the ratio of blade width to length, with those orthotropic species having short petioles also tending to have narrow blades. 3 The petiole length distinguishing orthotropy from plagiotropy showed a linear decrease with increasing latitude, as small-leaved short-petioled orthotropic species were relatively abundant at the temperate Australian sites. 4 The likelihood of a species being plagiotropic was weakly related to its shade tolerance and not related to adult stature or abundance, i.e. understorey leaf display was more closely related to leaf dimensions than to life-history attributes. This importance of leaf dimensions may reflect their influence on light interception and mechanical support requirements. 5 A smaller fraction of species was plagiotropic in the canopy than in the understorey, particularly in Australia, where many short-petioled species shifted from plagiotropic to orthotropic on attaining the canopy. 6 This switch from plagiotropy to orthotropy appeared to be associated with excess light levels relative to photosynthetic capacity, slow branch extension rates and low illumination angles. Thus, leaf display is related to leaf dimensions in shaded saplings, but light and physiological factors may have a greater influence on canopy architecture.