Growth of Managed Older Douglas-fir Stands: Implications of the Black Rock Thinning Trial in the Coast Range of Western Oregon

The most recent remeasurement of growth (at approximate total stand age 100 years) from the Black Rock Thinning Trial in western Oregon provides useful information for forest owners interested in accelerating restoration of older forest characteristics in Douglas-fir stands of the Pacific Northwest. Thinnings at several intensities at total stand age of roughly 50 years effectively reset stand growth patterns. With quadratic mean diameters in thinned plots up to 40% higher than those of unthinned controls, thinned plot mean annual increments (MAIs) and periodic annual increments continue to rise 55 years after thinning, with the peak in board foot and cubic foot MAI apparently still decades in the future. Assuming repeatable future thinning responses similar to the Black Rock Trial, financial analysis of the opportunity costs of extending rotations to 100 years indicates that some thinning treatments can reduce opportunity costs by up to half at a 6% discount rate. Study Implications Active management through thinning may be a useful tool for public and private landowners interested in rapid development of stands with older forest characteristics to enhance output of some ecosystem services. Heavy thinning regimes, of the type described here, are compatible with growing trees with large diameter stems, large branches, and large crowns. They also reduce fuel accumulation by lowering mortality rates of stems less than 60 years old and slowing the rate of crown recession, branch mortality, and branch litterfall. Midrotation thinning revenues reduce opportunity costs of holding more rapidly growing stems to older ages, which may be a consideration for some owners.