This study investigates the growth of forest stands (m³ha^-1a^-1) and its temporal variation during the five decades of the period 1961‒2010 as measured in the national forest inventories by the Finnish Forest Research Institute, in three regions, comprising the southern half of Finland. This growth was explained by sum of the effective temperatures and winter maximum soil frost depth; the correlation coefficient between the measured and modelled growths for the regions and decades studied being 0.97. It was also shown that the regional differences in soil frost depth are related to the differences between the temperature sum in the air and that in the upper soil layer during the growing season. Further, the residuals of the main test were analyzed for secondary factors. The temporal and regional variation could be accounted for by the relatively low growth in the peatland-forests created by the extensive drainage measures of 1961‒1980, the response of the growth-rate to May+June precipitation, and the extensive application of fertilizers in 1968‒1980. The regional differences and temporal changes in climate are also reflected in the understorey vegetation.

Keywords: Boreal forests; Forest growth and climate; Snow and forest growth; Soil frost and forest growth