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.

1987年至1994年间，研究人员于华盛顿州与俄勒冈州的奥林匹克山脉（Olympic Mountains）及喀斯喀特山脉（Cascade Range）西侧的真冷杉-铁杉林分（true-fir-hemlock stands）中，设立了18项商品材前疏伐（precommercial thinning）试验。本次试验选取的林分，可代表未来拟经营林分的林木状况（树种组成、密度、林分起源）与立地条件。18个试验点均设置1组或2组共6个处理样地：每组包含1个未疏伐的对照样地（control plot），以及5个分别按7.9、10.1、12.8、16.4、20.9英尺的近似方形株行距（square spacings）开展疏伐的样地。处理样地的面积介于0.59至0.92英亩之间，株行距越宽，所用样地面积越大。处理样地内部设置面积为0.25英亩的测量样地作为缓冲区。在经疏伐的样地中，疏伐后留存的所有林木均被挂牌标记；对于胸径（DBH）≥0.6英寸的林木，需记录其胸径与健康状况。在未疏伐的对照样地中，则对所有胸径≥0.6英寸的林木开展测量。在试验后5年、10年及19-20年时，针对跨物种与径级选取的部分林木，测量其树高与活树冠高（height to live crown）。仅在最后一次测量时，需在每个样地内按树种选取4株不同高度的林木测量冠幅。在试验第5年与第10年时，对所有样地内胸径≥1.6英寸的进界木（ingrowth trees）进行挂牌并测量。在最后一次测量（19-20年）时，除挂牌测量胸径>4.6英寸的进界木外，还需在每个样地内设置4个1/50英亩的副样地（subplots），以按树种与1英寸直径阶统计胸径介于1.6英寸至4.6英寸之间的进界木——部分宽株行距样地的进界木数量较多，故增设副样地以提升统计精度。在最后一次测量期间，每个样地内每个树种的树高与活树冠高测量样本量将缩减至约30株，且样本需覆盖该树种的全部径级范围。

此前针对华盛顿州西部与俄勒冈州中高海拔林分的林木生长速率的公开资料极为有限。本次试验覆盖的林分包含天然更新林与人工林，涉及的树种包括太平洋银冷杉（Pacific silver fir）、壮丽冷杉（noble fir）、亚高山冷杉（subalpine fir）、西部铁杉（western hemlock）与山地铁杉（mountain hemlock）。真冷杉类树种的冠幅通常较同区域低海拔林分的树种更窄，因此相较于花旗松（Douglas-fir），其可在更窄的株行距下良好生长；该类树种的幼龄生长速率较慢，但持续显著高生长的周期极长，可延续至林分发育极晚的阶段。由于木材价值较低，且需关注树皮较薄林木易引发的树干腐朽病，真冷杉类林分的商品材前疏伐未必具备可行性，因此森林经营者亟需评估幼龄林分在宽泛株行距范围内的林木生长响应情况。