Leaf and twig photosynthesis of young beech (Fagus sylvatica) and aspen (Populus tremula) trees grown under different light regime

Sunlight adapted pioneer trees (trembling aspen) and shade-tolerant beeches were exposed to different light conditions (20% and 100% sunlight) throughout an annual cycle. Anatomical and morphological changes of leaves and stem segments were followed besides physiological parameters (photosynthesis, respiration, light transmittance) of these different photosynthesising organs. Buds and leaves of both species responded in differentiation and growth even within the first year of the treatment. While area, stomatal density, and mesophyll thickness clearly responded in leaves, the corresponding parameters in twigs varied only slightly. In the shaded trees plant increment and stem diameter were dramatically reduced. In shade-treated aspen, stem chlorophyll increased by ca. 40%, while only minor changes were recorded in beech. Independent of light conditions during growth, positive net photosynthesis was rarely to be seen in intact twigs and branches. Nevertheless, apparent twig respiration (measured as CO2 release from the twig) was clearly reduced in the light because of the light-driven carbon re-fixation within the chlorenchymal tissues of twigs and stems. Calculations of net photosynthesis in illuminated current-year and one-year-old twigs revealed stem-internal CO2 re-fixation to transiently exceed 90%. At least in young twigs and branches, and thus in the outer parts of tree crowns, the respiratory CO2 losses may efficiently be reduced. Although surely different in young and mature trees, re-fixation of carbon dioxide may be of great importance for carbon budgets in the environmentally controlled leafless states of deciduous trees.