In the conditions of long atmospheric drought the СО2-exchange of seedlings of silver birch (Betula pendula var. pendula) and Karelian birch (Betula pendula var. сarelica) was investigated. Values of photosynthesis of the Karelian birch and silver birch were similar. A substantial difference was found in the parameters of the light response curves of photosynthesis during a soil drought. The Karelian birch more intensively took up CO2 in condition of soil water deficit during an atmospheric drought, and the low light intensity, and this was reflected in the higher slopes of the light response curves of photosynthesis. Effective absorption of CO2 early in the morning and in cloudy and dark weather in the conditions of atmospheric drought allows to support carbon balance at the acceptable level and increase competitive ability of Karelian birch.