A basal area increment model for Norway spruce in mixed stands in Estonia

An individual tree basal area increment model was developed for Norway spruce [Picea abies (L.) Karst] in mixed stands of spruce and birch in Estonia. Different regression equations were fitted for different combinations of variables to obtain biologically tractable interactions between growth and factors affecting it. The best fit index of the regression model was pursued in trials with variable combinations. The basal area increment was chosen to describe the tree growth and both the diameter and age of the tree were included as independent variables. The logical growth relationships were obtained. The basal area increment has a culmination introduced by the simultaneous influence of tree size and age explicitly included in the model. The stand level attributes contributed modestly to the explanatory power of the model because of the narrow range of stand conditions sampled. The present model is applicable to Estonian conditions.