In the article, we discuss some issues of the stem phytomass formation and energy potential of stands with different proportions of pine and spruce growing in Northwest Russia. The objects of study were mixed coniferous stands of natural origin not affected by economic impact. On the basis of biometric characteristics and wood sampling, the balance of stem phytomass and bioenergy potential of stands with different proportions of pine and spruce were calculated. The largest growing stock and stem mass are formed in plantations with a share of pine of 70-90%, and spruce, 80-90%. The analysis of variance showed a statistically significant difference between plantations with a predominance of pine and spruce. The energy balance of stands with different proportions of pine and spruce is directly related to the stem biomass of these species. Plantations with a predominance of up to 70% of the main species are characterized by high energy potential. The results of the calculation of the energy component of forest stands with different proportions of pine and spruce allow us to optimize species composition in order to obtain a possible effect both for the needs of thermal power engineering and for compiling the energy balance of forest communities.