Individual tree basal area growth models for young stands of Norway spruce in Sweden

Abstract Models for predicting the basal area growth of individual trees of Norway spruce ( Picea abies (L.) Karst.) in young stands in Sweden were developed. The models were based on data obtained from 146 operationally planted stands with Norway spruce, mainly established during the 1950s. The main feature of the study was to compare different approaches to include site effects in the models (site index or primary site factors). Models were developed both with and without site index, competition index and tree age included in the functions. There were no apparent differences in fit statistics between models using site index according to site properties, and models using an independent variable based on average yearly temperature during the growing season. Models based on a distance-dependent competition index and tree age explained about 79% of the variance in basal area increment, while models without distance-dependent competition index and tree age explained 74%. Inclusion of tree age in the models accounted for about 3% of the explained variation and the competition index for about 2% of the explained variation in basal area growth.