Bark photosynthesis has been shown to be an effective mechanism for stem-internal refixation of respiratory CO . In young birch trees (Betula pendula Roth.) this function is clearly modulated by the prevailing light intensity regime. Although positive net photosynthesis was not found in intact birch twigs, apparent twig respiration was reduced upon illumination by 65 % in high-light grown birches and even more in shade grown trees (81 %). Compared on a unit area basis the bark chlorenchyma contained up to 55% of the chlorophyll of the concomitant leaves when grown under 100 % sunlight and even 66 % when trees were grown under low-light (20 % of full sunlight). Light penetration through the periderm of birch twigs and branches is age-dependent and ranges in control trees from roughly 24 % of the incident sunlight in recent-year twigs to 1-3 % in 5-year-old main stems. Peridermal light transmittance was also changed by the light intensity regime. An additional light-reducing peridermal layer present in control trees was not found in shade-grown birches. It was shown that CO -refixation is not limited to the light­ exposed outer parts of tree crowns. Our results show that also inner branches of trees are well adapted to function as a rather efficient system to prevent respiratory carbon loss.