Graduate Thesis Or Dissertation

Douglas-fir, grand fir and plant community regeneration in three silvicultural systems in western Oregon

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  • Douglas-fir and grand fir seedling establishment and plant community regeneration were examined in a western Oregon forest following harvest in three different silvicultural systems: clearcut, two-story and patchcut. The two-story system consisted of removing all but 10 to 12 trees per acre. The patchcut system consisted of harvesting 1/2 acre patches in a grid like manner resulting in the removal of approximately 1/3 of the trees within a stand. The response of Douglas-fir and grand fir seedling growth to four vegetation control treatments nested within each silvicultural system was evaluated. The four treatments were: no-treatment, herbicide treatment, intensive treatment (herbicide treatment in addition to scalping of competing vegetation and browse protection (vexar tubing) and manual treatment (surrounding each seedling with paper mulch and slashing near-by shrub growth). Plant community regeneration in the three silvicultural systems and the effect of the no-treatment and herbicide treatment on plant growth habits and life forms was evaluated and compared to control stands that were not harvested. Successful artificial regeneration occurred in all three silvicultural systems. However, Douglas-fir had significantly greater height to diameter ratios in patchcuts than either two-stories or clearcuts. Similar trends were observed for grand fir. This suggests that future growth rates in patchcuts of both Douglas-fir and grand fir may be reduced relative to two-stories and clearcuts. Stocking of natural seedlings was satisfactory in two-story stands but was poor in both clearcuts and patchcuts. A lack of seed source is probably responsible for the low stocking in clearcuts. Restocking of patchcuts was low even though the potential for seedfall was high. Low levels of soil disturbance in patchcuts likely limited the amount of successful natural regeneration in this system. The intensive control of competition was the only treatment which resulted in significantly greater Douglas-fir growth than no control of competition. All active vegetation control treatments resulted in greater grand fir growth than the no treatment. However, no differences in the effectiveness of these treatments in increasing grand fir growth were observed. These findings suggest that grand fir has a lower minimum competition response threshold than Douglas-fir. That is, greater levels of vegetation management are needed to get a positive growth response with Douglas-fir than grand fir. Over all, the best seedling growth of both species can be expected when planting stock is large and overtopping is kept to a minimum. Deer showed a marked browse preference for Douglas-fir versus grand fir. However, browsing was not a significant factor in reducing overall growth of Douglas-fir seedlings in this study. Vegetation response to silvicultural system did not differ significantly. The herbicide treatment significantly reduced the vegetation volume. Operational restraints imposed by the different systems resulted in less effective vegetation control in patchcuts than in either the two-story or clearcut systems. Total herb cover increased following harvest and annual herb cover increased more than perennial herb cover. In the absence of vegetation control, total herb cover, annual herb cover, and perennial herb cover are not affected by silvicultural system. However, with the herbicide treatment, a general increase in these three cover parameters from clearcut to two-story to patchcut was observed. Shrub cover is not influenced by silvicultural system in no-treatment plots although in herbicide treated plots a loose relationship resulting in less shrub cover in harvested sites is evident. Over all, vegetation treatment results in no significant reductions in shrub cover.
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