GROWTH AND MORTALITY IN MANAGED DOUGLAS-FIR - RELATION TO A COMPETITION INDEX

Twelve-year increments of Dbh height and volume in thinned and urea-fertilized Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) were related, by regression, to degree of thinning, amount of fertilization, initial Dbh at treatment, and competition index to evaluate the use of the competition index as a predictor of growth. In general, initial Dbh and fertilization treatment outperformed the competition index in this respect. This may be partly due to the spatial regularity, narrow range of competitive stress index, and narrow age distribution of the trees. Tree mortality was also documented and related to the competition index. Most trees that died did so as a result of suppression, and this mortality was related to the competition index; however, there was no clear indication of an upper limit to the competition index that was compatible with continued life of the tree. The variability of tree heights generally became less over the 12 years following treatment. This is more consistent with the predictions of two-sided competition than with one-sided competition. This is somewhat counterintuitive since Douglas fir is a shade-intolerant species, and implies the existence of considerable competition for nutrients. © 1990.