Carbon assimilated during photosynthesis is allocated to basic needs, such as growth, defense, and storage of nutrients. The aim of this study was to explore potential relationships between carbon allocation and light conditions during growth, including shade tolerance of species. We studied species that represent light-demanding (Sorbus aucuparia, Betula pendula), intermediate (Carpinus betulus, Quercus robur), and shade-tolerant (Acer platanoides, Fagus sylvatica) trees. We exposed seedlings to two light treatments (full sunlight and shade), and explored how these conditions affect plant growth and biomass allocation, as well as the levels of phenolic compounds, nonstructural carbohydrates, carbon, and nitrogen. We hypothesized that light-demanding species invest less carbon in chemical defenses against pathogens and/or herbivores compared to shade-tolerant species. On the other hand, light-demanding species showed the greater part of assimilated carbon allocate to growth processes. As a result, the stem diameter above the root collar, the mass of leaves, stems, coarse, and fine roots were larger under full-sunlight conditions in all species, except for greater height of A. platanoides and Q. robur under shade conditions. Leaves from full light were characterized by lower nitrogen content, higher carbon and phenolic contents, and a higher carbon/nitrogen ratio compared with leaves from seedlings grown in shade. In the case of shade-tolerant species, a trade-off mechanism can be proposed that such species restrict their usual allocation of carbon to defense and radial growth, while instead of investing it in increasing their heights and storage capacities. According to the light-demanding species, it was not possible to identify a trade-off mechanism and how carbon allocationis restricted upon exposure to shade conditions, except for the reduced allocation to the root mass.
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