Stability and density management in Douglas-fir plantations

Limited tree size variation in coastal Oregon, Washington, and British Colu mbia Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantations makes t hem susceptible to developing high height to diameter ratios (H/D same unit s) in the dominant trees. The H/D of a tree is a relative measure of stabil ity under wind and snow loads. Experimental plot data from three large stud ies was used to evaluate the impact of initial planting densities and thinn ing on plantation H/D values. The H/D predictions from the experimental plo t data match spacing trial results closely but are substantially different than distance-independent growth model predictions. The results suggest tha t plantation H/D values can be lowered and stability promoted through reduc ed planting densities or early thinning; however, later thinnings may not b e effective in promoting stability, since they do not appear to lower H/D v alues. Higher initial planting densities shorten the time period during whi ch thinning can be expected to effectively lower future H/D values. Time-se nsitive thinning requirements in dense plantations make their management in flexible. The flexibility with which a stand can be managed describes the r igidity of intervention requirements and (or) potential range of stand deve lopment pathways.