Distribution of leaf photosynthetic properties in tree canopies: comparison of species with different shade tolerance

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Abstract
1. Parameters of the photosynthesis vs CO2 response curve, nitrogen and chlorophyll contents were studied in Populus tremula, Tilia cordata and Corylus avellana leaves along a natural light gradient in the canopy. 2. In all species, leaf nitrogen content per unit area (Ns) declined with decreasing fractional transmission of photon flux density (Ksum), measured by a hemispheric photographic technique. However, the shape of the relationship between Ns and Ksum depended on species. Ns was almost linearly related to Ksum in the most light‐demanding species P. tremula but this relationship was curved in two other species. Populus tremula possessed the largest values of Ns at given Ksum, while C. avellana, the most shade‐tolerant species, had the lowest values of Ns. 3. Both the initial slope, characterizing the maximum carboxylation efficiency, and the plateau value, providing an estimate of the photosynthetic electron transport capacity, of the photosynthesis vs CO2 response curves, were linearly related to Ns and only minor differences were observed among the studied species. Leaf chlorophyll content per area was also related to Ns but the ratio of Chl/N increased substantially at low Ksum in the lower canopy. It was concluded that the partitioning of nitrogen between light harvesting, electron transport and carboxylation components of the photosynthetic apparatus may change with depth in the canopy as a result of a relative increase in the light‐harvesting complexes in relation to the rest of the photosynthetic apparatus. 4. The interspecific differences in species’ photosynthetic potentials are mainly associated with different nitrogen distribution patterns along the canopy and in minor part with differences in nitrogen partitioning among photosynthetic machinery.