Variability in leaf morphology and chemical composition as a function of canopy light environment in coexisting deciduous trees

Morphology, chemical composition, and photosynthetic capacity of leaf lamin as were investigated in Populus tremula L. and Tilia cordata Mill. along a canopy light gradient. Variables determining the thickness of boundary laye r for heat and water exchange at a given wind speed-effective leaf width (W -w) and length (W-d)-scaled positively with daily integrated quantum flux d ensity averaged over the season (Q(int), mol m(-2) d(-1)) in T. cordata, bu t W-d decreased and W-w was constant with increasing Q(int) in P. tremula, bringing about a moderately improved capacity for convective cooling at gre ater irradiances in the latter species. Foliar stable carbon isotope discri mination (Delta) decreased with increasing Q(int), demonstrating that, poss ibly because of more severe foliar water stress, leaves operated at a lower intercellular CO2 concentration in the upper canopy. Further analysis of f oliar characteristics provided additional evidence of the interaction betwe en water stress and Q(int). Leaf dry matter content and both components of lamina dry mass per area (M-A)-lamina thickness and density (dry mass per u nit volume, rho(B))-increased with increasing Q(int) in both species. The r ho(B) 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 poten tials, these foliar modifications were interpreted as indicative of acclima tion to enhanced water limitations in high light. For the whole material, f oliar nitrogen concentrations decreased with increasing rho(B), suggesting that an improvement of foliar mechanical strength may result in declining f oliar assimilative potential. However, foliar photosynthetic electron trans port capacity per unit area increased with increasing rho(B), possibly beca use increases in rho(B) with light are not only attributable to greater cel l wall lignification but also to denser packing of leaf cells, in particula r, in fractional increases in palisade tissues with Q(int). Because of a po sitive scaling of leaf thickness and density with total tree height, M-A wa s greater in taller trees of T. cordata, foliage of which also had lower De lta and was likely to function with less open stomata. In summary, we concl ude that leaf water stress, which scales with both Q(int) and total tree he ight, is a major factor altering foliage structure and assimilative capacit y.