An increase in the CO2 content of the atmosphere and in the acidity of rain has been detected in large regions of the northern hemisphere. Both these factors can affect the productivity of trees. A three-component model has been employed for the detection of possible trends in the growth potential of trees. The components are (i) the growth intensity associated with tree age, (ii) the effect of weather, and (iii) the effect of possible long-term trends on the radial growth. The model was tested with a limited set of data from conservation stands in southern and central Finland. Preliminary results indicate that an increase in tree growth rate since the beginning of the 20th century has occurred.