Distribution of foliar carbon and nitrogen across the canopy of Fagus sylvatica: adaptation to a vertical light gradient

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Niinemets, Ülo:
Distribution of foliar carbon and nitrogen across the canopy of Fagus sylvatica: adaptation to a vertical light gradient.
In: Acta Oecologica. Bd. 16 (1995) Heft 5 . - S. 525-541.
ISSN 1873-6238

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Abstract

Distribution of foliar carbon and nitrogen was studied across a vertical light gradient in the natural canopy of Fagus sylvatica. Leaf dry and fresh weight per area (LWA and LWA(w), respectively) increased linearly with diffuse site factor (a(d), relative irradiance). Tree size, independent of a(d), was another determinant of leaf morphology, and both LWA and LWA(w) increased with increasing tree dimensions. A positive linear relationship between a(d) and the content of total non-structural carbohydrates per leaf dry weight (TNC, the um of ethanol-soluble carbohydrates and starch) was found, presumably caused by higher photosynthetic production of leaves at greater irradiance. Therefore, to account for confounding variation in TNC with irradiance, it may be necessary in studies on light-related modifications in leaf structure to express the content of leaf constituents on che basis of TNC-free dry weight instead of total dry weight. Total leaf carbon content per dry weight and a(d) were not correlated, however, leaf carbon content, corrected fur changes in TNC with a(d), increased significantly with relative light availability, suggesting increasing investment of leaf carbon for mechanical support. Leaf water content decreased with a(d), probably due to a less favourable water balance of leaves at higher canopy heights. Leaf nitrogen content per dry weight (N-m) decreased with increasing irradiance. If nitrogen content was expressed on the basis of TNC-free dry weight, the relationship became more scattered, though the qualitative tendency remained. No correlation between leaf nitrogen and TNC, considered as an estimate of daily photosynthetic production, was found, and it was suggested that increasing N-m at decreasing values of a(d) was invested for more efficient light harvesting, representing a strategy of nitrogen partitioning inherent to shade-tolerant plants. Because a considerable fraction of leaf magnesium is contained in chlorophyll, decreasing leaf magnes