The article reveals the role of environmental factors in foliage formation. For a given value of region-averaged climatic characteristics, the stand productivity depends on the forest
site conditions, which predetermine the forest type and soil fertility. An increase (decrease) in the productivity of the stand is accompanied by a decrease (increase) in the specific mass of foliage. Changes in the specific mass of foliage when the productivity of the stand changes by 1 stand quality class in south Karelian conditions amount to an average of 5–20 % for a middle-aged stand, depending on the tree species. In a predominantly forested area, the growth and development of a tree stand of each forest type are constrained by energy availability. Changes in the specific mass of foliage are related to this factor. As the radiation balance grows in the north-to-south direction, the share of foliage in the stand increases, and the quality class of the forest type is improved accordingly. Greater changes happen in tree stands growing in better site conditions. As a result of these changes it may be that within the same stand quality class there will be more foliage in a stand growing under poorer site conditions but at higher radiation balance values. The quality class reaches a maximum where the radiation balance is 1400–1600 MJ/(m² year). As a rule, the methods of calculating foliage mass use the tree stand valuation characteristics as the main parameters. To develop a foliage mass estimation method unified for different climatic and site characteristics in the given forest zone one should take into account the environmental factors and the forest type.