Comparative estimation of CO2 gas exchange in leaves of shaded branches of Karelian birch (Betula pendula var. сarelica) was carried out in two 40-50 years old plantations with different levels of mineral nutrition. On both plantations, comparison of parameters of CO2 gas exchange was also made between sunlit and shaded leaves. Characteristics of CO2 gas exchange in shaded leaves differed depending on the content of mineral substances (N, P, K) in the soil. With increasing of soil fertility, mean values of photosynthesis of shaded leaves increased from 1.22 to 2.74 mmol·m-2·c-1, dark respiration – from 0.43 to 0.69 mmol·m-2·c-1. Mean values of photosynthesis of sunlit leaves did not differ significantly (p>0.05) between these two plantations, but respiration has changed from –1.54 to –2.63 mmol·m-2·c-1. With increasing of soil fertility, parameters of photosynthetic light-curves changed: constant of saturation decreased from 100-120 to 20-60 mmol·m-2·c-1, light compensation point – from 12-20 to 6-8 mmol·m-2·c-1, which is evidence of more effective operating of photosynthetic apparatus when nutrition was not a limiting factor. Significant differences in parameters of photosynthetic light-curves were revealed between sunlit and shaded leaves in both plantations. In plantation with a higher level of NPK, index of usage of photosynthetically active radiation (PhAR) for photosynthesis was equal to 6.1 and 53.1 mmol СО2· (mmol photons)-1 in sunlit and shaded leaves, respectively. Dark respiration was -2.63 and -0.66 mmol · m-2·c-1, a light compensation point – 45.8 and 5.91 mmol·m-1·c-1 for sunlit and shaded leaves, respectively. Based on fresh weight, amounts of carotenoids © and chlorophyll хл (a+b) in the shaded and sunlit leaves had no significant differences. Mean values of the хл (a+b)/с ratio in shaded leaves were 1.5 times more than in the sunlit leaves. Based on leaf area, because of decreasing of thickness of the shaded leaves, amount of хл (a+b) was 1.4 times more in the shaded leaves in comparison of the sunlit leaves. Assessment of the carbon balance of the shaded branches showed that the balance remains positive when the daily mean values of the photosynthetically active radiation are equal to 18-25 mmol·m-2 ·s-1. The Karelian birch, growing on the comparatively fertile soil, utilises the low values of solar radiation more effectively than on soil with a low nutrient content, and owing to morphological and physiological adaptations the short-stemmed and shrubby birches with a figured wood can survive in environment of a heavy shade.