Annual ring data from northern Finland was analysed in order to reveal possible trends in ring width development due to changes in environmental factors. The data was analysed using a four component multiplicative model. The components are: tree age, climatic conditions, tree position and changes in environmental conditions. Since the effect of tree age and position in the stand could be easily eliminated the main problem was thus to eliminate the effect of climatic conditions on ring width. This was based on the dependence of the daily radial increment and daily maximum temperature. The component associated with changing environmental factors, especially to CO2enrichment, was determined using the model. The basal area development of the trees was calculated from measured and estimated ring widths. Depending on the value of the autocorrelation parameter, the effect of changes in environmental factors on the basal area increment of the trees is between 15.5–43.3%during the period from 1950 to 1983.