Variability in Leaf Morphology and Chemical Composition as a Function of Canopy Light Environment in Coexisting Deciduous Trees

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Abstract
Morphology, chemical composition, and photosynthetic capacity of leaf laminas were investigated in Populus tremula L. and Tilia cordata Mill. along a canopy light gradient. Variables determining the thickness of boundary layer for heat and water exchange at a given wind speed-effective leaf width (Ww) and length (Wd)-scaled positively with daily integrated quantum flux density averaged over the season (Qint, mol m-2 d-1) in T. cordata, but Wd decreased and Ww was constant with increasing Qint in P. tremula, bringing about a moderately improved capacity for convective cooling at greater irradiances in the latter species. Foliar stable carbon isotope discrimination (Delta) decreased with increasing Qint, demonstrating that, possibly because of more severe foliar water stress, leaves operated at a lower intercellular CO2 concentration in the upper canopy. Further analysis of foliar characteristics provided additional evidence of the interaction between water stress and Qint. Leaf dry matter content and both components of lamina dry mass per area (MA)-lamina thickness and density (dry mass per unit volume, rhoB)-increased with increasing Qint in both species. The rhoB and lamina dry matter content were also positively related to lamina carbon concentration, variability in which along the canopy was related to changes in carbon-rich lignin concentration. Since both increases in lamina density and lignin concentration improve leaf tolerance of low-water potentials, these foliar modifications were interpreted as indicative of acclimation to enhanced water limitations in high light. For the whole material, foliar nitrogen concentrations decreased with increasing rhoB, suggesting that an improvement of foliar mechanical strength may result in declining foliar assimilative potential. However, foliar photosynthetic electron transport capacity per unit area increased with increasing rhoB, possibly because increases in rhoB with light are not only attributable to greater cell wall lignification but also to denser packing of leaf cells, in particular, in fractional increases in palisade tissues with Qint. Because of a positive scaling of leaf thickness and density with total tree height, MA was greater in taller trees of T. cordata, foliage of which also had lower Delta and was likely to function with less open stomata. In summary, we conclude that leaf water stress, which scales with both Qint and total tree height, is a major factor altering foliage structure and assimilative capacity.