SEEING THE FOREST WITH THE LEAVES—CLUES OF CANOPY PLACEMENT FROM LEAF VENATION CHARACTERISTICS

Differences in leaf shape and vein density across a tree's canopy are typically thought of as ‘sun' and ‘shade' forms, but analysis of ontogenetic changes in living broad leaved trees suggest that these characteristics are not light induced, but controlled by hydraulics. Leaves throughout the crown are identical in size and shape at the time of bud break. Morphological adaptation to the local microenvironment takes place during the expansion phase and starts after the determination of the vascular architecture has been completed, so that differences in vein density do not reflect differential vein production but rather the distortion of similar vein networks over different final surface areas. Complementary patterns are seen in both an angiosperm with diffuse leaf growth (Quercus rubra) and a non-angiosperm seed plant with marginal growth (Ginkgo biloba). Acclimation of leaf form and function based upon differential expansion provides an elegant mechanism whereby leaf size and vein density are determined during development by the same local hydraulic properties which will constrain the size of leaf that can be functionally supported at maturity.

Leaves are the most common of plant fossil but typically provide little information concerning the habit and stature of the source plant. A hydraulic based mechanism for leaf differentiation provides consistent expectations of gradients of vein density across leaves from the top and bottom of the canopy which can be used to assess the likelihood that two distinct fossil morphospecies represent sun and shade leaves of the same plant. The expectation that canopy versus understory leaves would have different patterns of vein density across the lamina may also allow the gathering of information concerning canopy structure and whole plant architecture from disarticulated leaves of unknown phylogenetic placement or plant habit. This approach should have applications such as following the ecological spread of angiosperms during their early evolution and constraining the growth forms of important extinct groups for which leaf and reproductive structures are known, but the supporting axis is lacking.