Weibull diameter distribution models for managed stands of Douglas-fir in Washington and Oregon

Abstract

The two-parameter Weibull function was used to predict forest stand diameter distributions and growth. Diameter distribution models were developed for even-aged Douglas-fir stands, 20 to 40 years old, in Oregon and Washington. In order to test if the two-parameter Weibull function can adequately describe the diameter distributions of such stands, maximum likelihood estimated parameters of the two parameter Weibull function were compared with observed diameter distributions. Two sets of models were developed. The first set of models predicted the diameter distributions of unthinned forest stands from stand variables. Models were developed to predict each of the two parameters of the Weibull function from stand variables. The Weibull function with the two estimated parameters became the diameter distribution model for the stand. The second set of models predicted the growth of a stand of trees after thinning. A Weibull diameter distribution model was developed for a forest stand after a growth period. The parameters of the Weibull function were estimated from stand variables at the beginning of the growth period and the length of the growth period. Results showed that the two-parameter Weibull function can describe the diameter distributions of even-aged stands of Douglas-fir, 20 to 40 years old, in Oregon and Washington. The diameter distribution model for unthinned stands predicted the observed diameter distributions in an independent data set quite well. The diameter distribution model for a thinned stand after a growth period gave a satisfactory prediction for 93 percent of the observed diameter distributions in the independent data set. The two-parameter Weibull function in this study gave at least as good results as that which has been obtained with the three-parameter Weibull function in previous studies.