Modelling crown shape of Picea abies: spacing effects

A model of branch length increment and inclination was developed for Picea abies (L.) Karst. to describe the dynamics of crown shape. Four trees belonging to the provenance Istebna, from Poland, were sampled in an experimental plot where density varied continuously from 200 to 10000 stems/ha. Sampling along the density gradient enabled us to study the effect of spacing on tree morphology and development. The model of branch extension is based on height growth and on the year of elongation of the branch. A second model describes the change in branch inclination. Branch spread increased with between-tree spacing, whereas branch inclination was affected by crown contact, competition was then divided into two components: (i) direct mechanical contact and (ii) global resource depletion. The sample trees were young (26 years), so height growth was mainly linear and the effect of apical control could not be tested. This study provides a simple model of crown shape development that can be used in a more detailed process-based model. Simulations are presented based only on height growth. For wood quality purposes, the relationship between branch diameter and branch length is provided to give access to the knottiness of the stem. The limitations of the sample are discussed in view of a more general use of these models.