ESTIMATION OF THE PARAMETERS OF THE WEIBU LL FUNCTION FOR GENERATING DIAMETER DISTRIBUTIONS

Knowing the distribution of tree diameters is of great importance in t he evaluation and management of forest stands. Modern single tree simu lators often require as input a list of individual trees with, their a ssociated sizes (diameter and height, for example). Diameter distribut ions are useful in this regard because they can provide a list of tree s and diameters for simulation based upon simple stand summary statist ics. Height/diameter equations can then be used to supply the missing heights for each of the trees generated from the diameter distribution . Then the individual trees are grown in height and diameter through t ime in the simulator. One of the most accurate and flexible functions for modelling diameter distributions is the Weibull probability densit y function. It has 2 primary parameters. The b-parameter controls the scale and the c-parameter influences the shape (form) of the Weibull f unction. In some formulations there is also an a-parameter which is a location parameter. In this study these parameters are predicted as a function of two easily measured stand parameters: Dg (quadratic mean d iameter) and Dmax (maximal diameter). Finally a diameter distribution is created from the estimated b- and c-parameters. Trees are sampled f rom this distribution until a specified basal area is obtained. The ac tual number of trees was then compared to the number of trees predicte d with this method and good results were obtained. In most cases the p ercentage differences between observed and predicted number of trees w ere under 10 %. For stands greater than 80 years in age the difference s were under 5 % for all species studied.