Twenty-year tree measurement data from true fir spacing trials in the Pacific Northwest

A series of 18 precommercial thinning trials were established in true-fir-hemlock stands in the Olympic Mountains and along the west side of the Cascade Range in Washington and Oregon, from 1987 through 1994. The stands were selected to represent the tree conditions (species, density, stand origin) and site conditions of stands likely to be managed in the future. Each of the 18 installations contained either 1 or 2 sets of 6 treatment plots with each set including an unthinned (control plot) and 5 plots thinned to different approximate square spacings: 7.9, 10.1, 12.8, 16.4, and 20.9 feet. Treatment plot sizes ranged from 0.59 to 0.92 acres with larger plots used for wider tree spacings. Measurement plots (0.25 acres) were buffered inside the treatment plots. On thinned plots, all trees remaining after thinning were tagged and diameter at breast height (DBH) as well as tree condition were recorded if the tree had a DBH at or above 0.6 inches. On unthinned (control) plots, all trees with a DBH at or above 0.6 inches were measured. Tree height and height to live crown were also measured for a subset of trees (selected across species and size categories) after 5, 10 and 19-20 years. Crown width was measured on 4 height trees per species per plot at the last measurement only. Ingrowth trees with a DBH of 1.6 inches and larger were also tagged and measured on all plots at ages 5 and 10. At the last measurement (years 19-20), ingrowth trees > 4.6 inches DBH were tagged and measured and four 1/50 acre subplots were established in each plot to record ingrowth > 1.6 inches and < 4.6 inches by species and 1 inch diameter classes (due to large numbers of ingrowth trees present on some widely spaced plots). In addition, during the last measurement period, trees measured for height and height to live crown were reduced to approximately 30 per species per plot distributed across the range of diameters.
Very little information was available on growth rates on tree species in mid to high elevation stands in western Washington and Oregon. These stands, naturally regenerated or planted, included Pacific silver fir, noble fir, subalpine fir, western hemlock, and mountain hemlock. True fir species tend to have narrower crowns than the species present at lower elevations so may grow well at narrower spacings than Douglas-fir, and have slower juvenile growth but a long period of appreciable height growth extending to very advanced ages. Commercial thinning may or may not be feasible in true fir stands due to low timber value and concern about stem rots associated with thin bark; thus, managers were interested in evaluating tree growth response at a wide range of spacings in young stands.