Photosynthetically active radiation (PhAR) transmitted through the bark and phellema was studied in 13 years old silver birch (Betula pendula Roth.) and Karelian birch (B. pendula Roth var. carelica (Mercklin) Hämet-Ahti). Quantum sensor of the LI-6200 Portable Photosynthesis System was used. Relationship between PhAR transmission (Tr) through phellema and the stem diameter was found. In two years old shoots, Tr in Betula pendula var. carelica was 20-30% less than in Betula pendula. With increasing stem diameter, PhAR transmission through phellema decreased considerably. Probably it was due to betuline created. When stem diameters were 20-40 mm, PhAR on chlorenchima was 200 and 40 μmol m-2 s-1 in B.pendula and B. pendula var. carelica respectively. When stem diameter was 50 mm, these values were 60 and 10 μmol m-2 s-1. Even more differences in Tr were observed for PhAR transmission through bark. In Betula pendula var. carelica, in connection with significant decrease in PhAR on chlorenchyme and in the inner parts of a bark, absorption of CO2 is weak, and in the external xylem layers in the bottom part of a trunk can be absent at all already at early age. The assumption is stated that it can lead to the delay of recovery processes in water columns, broken during a drought because of embolism. This circumstance leads to limitation in the growth processes in the apical stems.