Graduate Thesis Or Dissertation
 

Stocking and growth of noble fir in the Cascade Mountains of Oregon and Washington

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  • Stocking levels and height growth rates of noble fir and associated species were measured on 51 clearcut study areas within the Pacific silver fir zone of the Oregon and Washington Cascades. Clearcut study areas were approximately 20 years old and had either planted noble fir or natural regeneration of noble fir from a seed source adjacent to the clearcut. Study areas occupied a variety of slope aspects, Inclinations and elevations. Clearcut study areas were grouped by ecological types (groups of environmentally similar plant associations) and the regeneration method used to reforest the clearcut. Stocking levels were generally high on 1/400 hectare plots (1/162 acre). The average percent stocking (the ratio of unstocked versus stocked plots) for naturally regenerated clearcuts ranged from 71.6 percent on cool-dry sites to 93.3 percent for the cool-moist ecological types. Planted clearcuts had less variation, with the average percent stocking ranging from 78.3 to 89.0 pecent on cool-dry and cool-moist ecological types, respectively. The stocking density on the Pacific silver fir zone clearcuts sampled was very high, ranging from 528 to 13,696 trees per hectare. The mean density for all the clearcuts surveyed was 3962 trees per hectare (1604 trees per acre). The relative stocking by individual tree species was clearly dominated by noble fir. The relative composition (the proportion of each species sampled on a clearcut) for noble fir was 44.6 percent on naturally regenerated clearcuts compared with 39.2 percent for planted clearcuts. Other species ranged from less than 1 to 21 percent relative composition. In comparisons with Douglas-fir, noble fir stocking was significantly greater on naturally regenerated clearcuts at the south and north extremes of its natural range, the Willamette and Mount Baker-Snoqualmie National Forests, respectively. The difference in composition for noble fir and Douglas-fir was significant on planted clearcuts only on the Gifford Pinchot National Forest. Mean height growth for the trees sampled varied little among tree species and ecological types. Only western hemlock was significantly less than the other species, and then, only at locations north of the Columbia River. Noble fir, Douglas-fir and Pacific silver fir were nearly equal in height growth on all ecological types for all National Forests. There was however a trend for height growth to be more on low elevation, warm ecological types and less on the high elevation, cool-dry ecological types. The use of broadcast burning for site preparation caused Pacific silver fir to have lower densities and relative composition than on clearcuts that had not been broadcast burned. The use of broadcast burning on severe upper-slope clearcuts, where advance regeneration Pacific silver fir is present, should probably be restricted. On clearcuts where adjacent forested stands contain substantial densities of mature noble fir or where noble fir has been planted, noble fir is able to survive and grow well (greater than 23 centimeters per year) on Pacific silver fir zone clearcuts of the Oregon and Washington Cascades. Implications based on the results of this study are that plant associations may not be good predictors of regeneration stocking and growth of noble fir and other upper-slope species over large areas. Predictions may be possible on a more local basis.
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