A STUDY OF CO<sub>2</sub> GAS EXCHANGE IN KARELIAN BIRCH UNDER LOW SOLAR RADIATION CONDITIONS IN PLANTATIONS WITH DIFFERENT LEVELS OF SOIL FERTILITY
Comparative estimation of CO<sub>2</sub> gas exchange in leaves on shaded branches of Karelian birch (Betula pendula var. carelica) was carried out in two 40-50-year-old plantations with different levels of mineral nutrition. In both plantations, CO<sub>2</sub> gas exchange parameters were also compared between sunlit and shaded leaves. Characteristics of CO<sub>2</sub> gas exchange in shaded leaves varied depending on the content of mineral substances (N, P, K) in the soil. With increasing soil fertility, mean values of photosynthesis in shaded leaves increased from 1.22 to 2.74 mmol·m<sup>-2</sup>·c<sup>-1</sup>, dark respiration - from 0.43 to 0.69 mmol·m<sup>-2</sup>·c<sup>-1</sup>. Mean values of photosynthesis in sunlit leaves did not differ significantly (p > 0.05) between these two plantations, but respiration varied from -1.54 to -2.63 mmol·m<sup>-2</sup>·c<sup>-1</sup>. The parameters of photosynthetic light response curves changed with an increase in soil fertility: the constant of saturation decreased from 100-120 to 20-60 mmol·m<sup>-2</sup>·c<sup>-1</sup>, light compensation point - from 12-20 to 6-8 mmol·m<sup>-2</sup>·c<sup>-1</sup>, which is evidence of more effective operation of the photosynthetic apparatus where nutrition was not a limiting factor. Significant differences in the parameters of photosynthetic light response curves were revealed between sunlit and shaded leaves in both plantations. In the plantation with a higher NPK level, the rate of photosynthetically active radiation (PAR) utilization for photosynthesis was 6.1 and 53.1 mmol СО<sub>2</sub>· (mmol photons)<sup>-1</sup> in sunlit and shaded leaves, respectively. Dark respiration was -2.63 and -0.66 mmol·m<sup>-2</sup>·c<sup>-1</sup>, light compensation point - 45.8 and 5.91 mmol·m<sup>-1</sup>·c<sup>-1</sup> for sunlit and shaded leaves, respectively. Based on fresh weight, the amounts of carotenoids (c) and chlorophyll chl(a+b) in the shaded and sunlit leaves had no significant differences. Mean values of the chl (a+b)/с ratio in shaded leaves were 1.5 times that of sunlit leaves. Based on leaf area, because of lower thickness of shaded leaves, the amount of chl (a+b) was 1.4 times lower in shaded leaves in comparison with sunlit leaves. Assessment of the carbon balance of the shaded branches showed that the balance remains positive when the daily mean values of photosynthetically active radiation are 18-25 mmol·m<sup>-2</sup>·s<sup>-1</sup>. The Karelian birch trees growing on comparatively fertile soil utilize the low values of solar radiation more effectively than those growing on nutrient-poor soil, and owing to morphological and physiological adaptations the short-stemmed and shrubby birches with figured wood can survive in a very shaded environment.