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

Models for predicting basal area growth of individual trees of Norway spruce (Picea abies (L.) Karst.) in young stands were developed. Data collected in a large survey of young forest stands hi Sweden (the HUGIN young stand survey) were used in the construction of the models. In the present study data was obtained from 146 operationally planted stands with Norway spruce. The plantations were mainly established during the 1950's. The existing system of growth models for young stands in the HUGIN system is based on height development. The purpose of the present study was to enlarge this system with models for basal area increment of individual trees. 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 temperature sum as independent variable. Models based on 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 %. There were bigger differences between models with and without age compared with models with and without distance dependent competition index. 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.