1. Distribution of leaf chlorophyll (Chl), nitrogen and non-structural carbohydrates (NSC, ethanol-soluble sugars and starch) in leaves along a natural light gradient was studied in saplings of four deciduous woody species: Acer platanoides, Padus avium, Populus tremula and Quercus robur (from most shade tolerant to intolerant).
2. Leaf dry mass per area (LMA) increased linearly with increasing relative irradiance (RI) in all species and therefore it may be considered as a species-specific estimate of long-term light conditions.
3. Decreasing irradiance enhanced chlorophyll synthesis, and consistently resulted in highest Chl per leaf dry mass in low-light environments. Chl content per leaf area was correlated negatively with RI in A. platanoides and was independent of RI in three other species.
4. Leaf nitrogen content per leaf area (Na) increased linearly with LMA in all species, and with RI in all except A. platanoides.
5. The slopes and intercepts of Na vs LMA relationships were negatively correlated. Moreover, it appeared that the slope decreased and the intercept increased with increasing species shade tolerance. Thus, average nitrogen content per mass (Nm) increases and maximum concentrations of leaf nitrogen shift towards more open habitats with decreasing shade tolerance. Distribution of foliar nitrogen in this way is likely to contribute to greater photosynthetic potentials at higher irradiances in shade-intolerant species. Inversely, more tolerant species have greater concentrations of foliar nitrogen at low irradiance — an investment pattern, which provides resources to enhance the capacity of light harvesting in light-limited conditions.
6. NSC per leaf dry mass was positively related to relative irradiance and the slope of this relationship was highest for shade-intolerant species, which have intrinsically higher photosynthetic capacities. Nevertheless, the conclusions were robust and were not significantly biased by using the parameters expressed either on a total or on a NSC-free dry mass.
7. The analysis was further extended by including a number of other species from several independent studies. Examination of both sets of data resulted in identical conclusions. Based on this evidence it may be concluded that the variability in leaf nitrogen concentration in relation to irradiance plays a central role in species shade tolerance.