Percentile based basal area diameter distribution models for Scots pine, Norway spruce and birch species

Information about diameter distribution is used for predicting stand total volume, timber volume and stand growth for forest management planning. Ofte n, the diameter distribution is obtained by predicting the parameters of so me probability density function, using means and sums of tree characters as predictors. However, the results have not always been satisfactory: the pr edicted distributions practically always have a similar shape. Also, multim odal distributions cannot be obtained. However, diameter distribution can a lso be predicted using distribution-free methods. In the percentile method, the diameters at certain percentiles of the distribution are predicted wit h models. The empirical diameter distribution function is then obtained by interpolating between the predicted diameters. In this paper, models for di ameters at 12 percentiles of stand basal area are presented for Scots pine, Norway spruce and birch species. Two sets of models are estimated: a set w ith and one without number of stems as a predictor. Including the number of stems as a predictor improved the volume and saw timber volume estimates f or all species, but the improvements were especially high for number of ste ms estimates obtained from the predicted distribution. The use of number of stems as predictor in models is based on the possibility of including this characteristic to measured stand variables.