Energy requirement for foliage formation is not constant along canopy light gradients in temperate deciduous trees

Foliage construction cost (glucose requirement for formation of a unit foliar biomass, G , kg glu kg^?1), chemical composition and morphology were examined along a light gradient across the canopies in five deciduous species, which ranked according to increasing shade-tolerance as Populus tremula < Fraxinus excelsior < Tilia cordata = Corylus avellana < Fagus sylvatica . Light conditions in the canopy were estimated by a hemispheric photographic technique, allowing ranking of sample locations according to long-term light input incident to the sampled leaves (relative irradiance). G and foliage carbon concentration increased with increasing relative irradiance in F. excelsior , T. cordata and C. avellana , but wereindependent of irradiance in F. sylvatica and P. tremula . However, if G of non-structural-carbohydrate-free dry mass was considered, it also increased with increasing relative irradiance in P. tremula . A positive correlation between the concentration of carbon-rich lignin and irradiance, probably a result of the acclimation to greater water stress at higher light, was the major reason for the light-dependence of G . Lignin concentrations were highest in more shade-tolerant species, resulting in greatest carbon concentrations in these species. Since carbon concentration and G are directly linked, the leaves of shade-tolerant species were also more expensive to construct. As the result of these effects, G increased faster with increasing leaf dry mass per area which was mainly determined by relative irradiance, in shade-tolerators. Given that shade-tolerant species had lower leaf dry mass per area at common irradiance and that this saturated at lower relative irradiance than leaf dry mass per area in the intolerant species, it was concluded that enhanced energy requirements for foliage construction might constrain species morphological plasticity and the upper limit of leaf dry mass per area attainable at high light.