COMPARISON OF BETA-FUNCTIONS AND WEIBULL FUNCTIONS FOR MODELING BASALAREA DIAMETER DISTRIBUTION IN STANDS OF PINUS-SYLVESTRIS AND PICEA-ABIES

The purpose of this study was to compare beta and Weibull distribution s in describing basal area diameter distributions in stands dominated by Scots pine and Norway spruce. The material of the study consisted o f 535 stands located in eastern Finland. Parameters for both two- and three-parameter approaches of the Weibull distribution were estimated using the method of maximum likelihood. Models for these parameters we re derived using regression analysis. For the beta distribution, regre ssion models were formed for the minimum, maximum and standard deviati on of diameters within individual stands. These models were used when the exponents of the beta distribution were calculated analytically. A lso, some parameter models for beta and Weibull distributions from pre vious studies were compared with the measured diameter distributions. The distributions obtained were compared using diameter sums and an es timate of the proportion of sawtimber. The results did not reveal any major differences between the suitability of the beta and two-paramete r Weibull distributions. There are appropriate models available for bo th of the distributions and the more similar the original data is to t he data of an application, the better are the results. The two-paramet er approach of the Weibull distribution gave better results than the t hree-parameter approach. The poorest results for all the predicted dis tributions were obtained at the extremes of the distributions.