Productivity of mixed stands of Norway spruce and birch affected by population dynamics: a model analysis

Bolewood volume accumulation patterns and levels of mixed Norway spruce and birch forests in Estonia based on modelled population dynamics are discussed in this paper. Trees were divided into separate populations according to species and canopy class. Diameter growth was estimated from increment cores in ten 0.1 ha-sample plots to provide the growth component of the population model. Multiple regression was used to estimate the effect of tree attributes and stand composition on the individual tree diameter growth. The stand growth in the present model is described by modelling the growth of a mean tree of the population. Since birch has a greater diameter growth than spruce, stand productivity should increase when the birch proportion is greater. However, this conclusion is true only in middle-aged stands because more birch in a stand inhibits the diameter growth of both species. These two components (greater birch increment and growth inhibition by birch) have been quantified and the dependence of growth patterns on these factors is investigated. Trials with a simulation model demonstrate the increase in stand productivity accompanying birch population decline. Decreasing birch percentage occurs naturally in the course of secondary forest succession in Estonia.