<p><b>Abstract</b>—The Siberian fir (<i>Abies sibirica</i> Ledeb.) is one of the main forest-forming species of northeastern European Russia. Compared to other forest-forming species, the Siberian fir has been poorly studied and there are no integral models to describe the dynamics of fir photosynthesis depending on environmental factors and growing conditions. Modeling of the intensity of fir photosynthesis depending on vertical gradients of environmental factors and anatomo-morphological structure of Siberian fir needles that vary from the location within the vegetation canopy by height is the purpose of the present study. The study was conducted in the middle taiga subzone of the Komi Republic in a bilberry–sphagnum spruce forest at two altitude levels. The objects of the study are two Siberian fir trees with different positions in the stand structure, one being a part of the main tier and the other being a part of the second tier. The CO₂ assimilation rate of fir needles, the intensity of incident photosynthetically active radiation, air temperature, relative air humidity, and atmospheric CO₂ concentration were measured in the crowns of the selected trees on uncut shoots; the anatomical and morphological structure of fir needles depending on their height in the stand structure was studied. A&#xa0;regression equation modeling the dependence of the CO₂ assimilation rate by Siberian fir needles on environmental factors was obtained on the basis of experimental data. Validation showed the adequacy and statistical significance of the model. The specificity of the model and anatomo-morphological features of Siberian fir needles depending on their height in the stand was revealed.</p>