A comparative assessment of gaseous CO2 exchange in leaves of 5–6 years old saplings of two forms of silver birch (Betula pendula and Betula pendula var. сarelica) and downy birch (Betula pubescens Ehrh., 1789) growing under different levels of nitrogen nutrition was done. The study was carried out in five plots (control sector, three sectors fertilized with different doses of NH4NO3, and a sector with NPK fertilization) at the Agrobiological Research Station of the Karelian Research Centre RAS located at the southern outskirts of Petrozavodsk. Both in the control sector (where the content of nitrogen was 0.23 ± 0.04 %) and in most of the fertilized sectors, there was no significant difference between the mean values of photosynthesis in B. pendula and B. pendula var. сarelica either for the entire PhAR range or for PhAR > 1400 μmol m-2 с-1. Nitrogen fertilization had a beneficial effect on the growth and the magnitude of gaseous CO2 exchange of the studied objects. The calculated parameters of light response curves for net photosynthesis have shown an improved performance of the photosynthetic apparatus upon fertilization. Where PhAR values were low, its efficiency in Karelian birch was much higher compared with silver birch (B. pendula), and increased with the elevation of the nitrogen dosage. In the conditions of soil moisture shortage, there was no significant difference between chlorophyll content in leaves of fertilized and non-fertilized plants. The carotenoid content in the leaves of non-fertilized plants was 1.3–1.5‑fold that of fertilized plants. Even a small dose of nitrogen fertilizer (20 g per tree) had a favorable effect on the water content of leaves in a dry period. In the second year of fertilization, the midday depression of photosynthesis increased considerably, especially in the sectors with high doses of nitrogen fertilizer. In the control sector, the decrease in photosynthesis in the absence of air drought and with sufficient soil moisture content was 20–25 %, and in fertilized plots there was a more than three-fold decrease in photosynthesis from 13:00 to 16:00. As a result, on sunny days in the second part of the growing season daily photosynthetic production in fertilized plants was lower compared to non-fertilized ones. In the first year of fertilization, there was no strong midday depression of photosynthesis, and nitrogen fertilization promoted CO2 uptake on both cloudy and sunny days.