Title: Investigation of PhAR transmission through the bark in silver birch and Karelian birch
Abstract: 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 ( T r ) through phellema and the stem diameter was found. In two years old shoots, T r 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 T r 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 CO 2 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.