DEVELOPMENT OF A TREE HEIGHT GROWTH-MODEL FOR DOUGLAS-FIR

Using a sample of 866 Douglas-firs, a predictive model is presented which expresses 5-year height growth as a nonlinear function of potential height growth, crown ratio and height of the subject tree divided by dominant stand height. The adjusted coefficient of multiple determination .hivin.R2 of this model exceeded 0.70, which was superior to four alternative models derived from previous studies. The final model also fit well to the data from each crown class. A number of different techniques for expressing competitive effects were also studied. The most influential variables in the height growth analysis were related to tree position, crown competition factor in larger trees and height divided by dominant stand height. Crown ratio, considered an index of tree vigor, was statistically significant but only accounted for about 8% of the residual variation in height growth.