Stem respiration of two forms of Betula pendula Roth [Silver birch (B. pendula var. pendula) and Karelian birch (B. pendula var. carelica)] was investigated. 5-6-years old trees and 2-3-years old seedlings, grown in pots, were studied. Among objects of Karelian birch, there were specimens both without visible deviations from normal growth and with structural anomalies of stems. Along with the relationship with environmental factors (mainly temperature), stem respiration rate was strongly related with cambial activity and stem width growth. Before cambial growth (in June) mean respiration rates of twigs and stems in seedlings with diameters 10-14 mm were 5,3 and 4,6 μmol•m-2•s-1 in Silver birch and Karelian birch respectively, during cambial growth (late June – first half of August) – 12,3 and 17,6 μmol•m-2•s-1, and during period of growth attenuation (second half of August – early September) – 2,6 and 3,1 μmol•m-2•s-1. Maximal respiration rates (20 μmol•m-2•s-1 and 28 μmol•m-2•s-1 for Silver birch and Karelian birch respectively) registered in late July, were higher (in absolute value) than net CO2 gas exchange of leaves.
In about 30% of measurements, respiration rate in Karelian birch was more than two times higher than in Silver birch. Higher respiration rate in Karelian birch than in Silver birch is mostly related with higher metabolic activity in cambial zone and more amount of live respiring tissues per unit of a stem length in stems of the same width in this two forms of birch. Measurements have shown that thickness and volume of the bark in Karelian birch were higher by 16-20% than in Silver birch. There were no significant differences in stem respiration of 6-years old Karelian birch trees, which already had visible thickenings of stems, and seedlings without visible signs of structural anomalies of stems. In rare cases there were higher stem respiration rate in Silver birch than in Karelian birch. The causes of this phenomenon are analyzed.