As a result of logging, fires, and floods, the conifers that once dominated riparian zones in the Oregon Coast Range have been largely replaced by stands of red alder. Typically, few conifers become reestablished beneath the alder canopy, and thus coniferous large woody debris, a desirable component of fish habitat in streams, is essentially unavailable in many riparian zones.
This study was established to determine the most successful methods for reestablishing conifers in these riparian zones. The study is now in its fourth year. Survival data and third-year height and diameter growth responses were collected, analyzed, and reported at the Northwest Science Association meeting, held in March of 1994 at Ellensburg, Wash., and at the COPE symposium, Ecology and Management of Oregon Coast Range Forests, held March 29-31, 1994, at Gleneden Beach, Ore.
Six sites were selected along the west side of the Coast Range. Four conifer species, Douglas-fir, western redcedar, western hemlock, and grand fir, were underplanted 2 m apart in each subplot. Half the trees in each planting were encased in VexarTM tubing to protect them from browsing by animals. The overstory was treated by killing either 50 or 100 percent of the alder trees by girdling or injecting with herbicide. Understory vegetation was also controlled in half of each overstory treatment area. Seedlings were measured for height, diameter at ground level, and diameter at 15 cm above the ground after planting and after the first, second, and third growing seasons.
By the end of the third growing season, estimates of overhead cover were 95, 55, and 8 percent for the undisturbed control areas, partial treatment areas, and total treatment areas, respectively. Survival of conifers on good sites was 68-73 percent, on poor sites 52-60 percent. Survival in the 50-percent removal areas was similar to that in the 100-percent removal areas. Redcedar survival was better than survival in the other species. Controlling of vegetation in both the overstory and understory increased survival. The plastic tubing had no effect on either survival or growth. Growth was similar among species. Hemlock had the greatest diameter and height growth, while Douglas-fir had the least diameter growth.
After 3 years, trees in both overstory treatments had better height growth than trees in untreated control areas. Trees in areas where the entire overstory was girdled had better diameter growth. The beneficial effects of understory treatment were still seen in conifer survival and growth after 3 years, even though the understory vegetation had rebounded to near original conditions by that time.
Results of this and other survival and growth trials has confirmed that underplanting without treating the overstory has little prospect of producing large conifer trees. On the other hand, reducing overstory competition by girdling either half or all of the overtopping alder, coupled with control of understory vegetation, produced promising survival and growth.
Thinning alder-dominated stands appears to be a good option for restoring conifers in the short run, but the duration of the effect of this treatment--which would depend on age and density of the alder and condition of the site--should be assessed first. In this study, the more shade-tolerant conifers did better, but Douglas-fir survived and grew with modest initial success.
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