Graduate Thesis Or Dissertation
 

Post-eruption species selection and planting trials for reforestation of sites near Mount St. Helens

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  • The 1980 eruptions of Mount St. Helens damaged or destroyed vegetation on more than 66,100 hectares (270,000 acres) within the Gifford Pinchot National Forest. The effects of the eruption on planting sites and seedling performance were unknown. Regeneration of "cut over" lands near Mount St. Helens has historically been limited by high soil-surface temperatures in the summer; a short growing season; and young, easily erodible, nutrient deficient, volcanic soils. Reforestation following the 1980 eruptions was expected to be far more difficult and exacting than pre-eruption reforestation. This study monitored survival and growth of seedlings planted on sites to the east and northeast of Mount St. Helens in the devastated, blowdown, scorch, and ashfall zones. Seven conifer species were planted with shading and fertilization treatments on six "low elevation" sites (685 to 1097 m 2250 to 3600 ft. elevation) and five "high elevation" sites (1067 to 1280 m 3500 to 4200 ft. elevation). First- and second-year survival and growth results were as good as or better than those for undisturbed sites. Seedling second-year survival rates were high: averaging 85 percent on the low elevation sites and 75 percent on the high elevation sites. Raval damage was the most common cause of seedling mortality, especially when slopes exceeded 60 percent. Known autecological differences among the species tested were maintained on the post-eruption sites. Lodgepole pine (Pinus contorta var. murrayana Dougi.) survived best and Douglasfir (Pseudotsuga menziesii (Mirbel) Franco.) exhibited the fastest height growth at elevations below 1100 m (3600 ft). Western white pine (Pinus monticola Dougl.), lodgepole pine, and Engelmann spruce (Picea Englemanii Parry) survived equally well on the high elevation sites, but lodgepole pine grew faster. Lodgepole pine and Engelmann spruce had the highest survival rates on the harshest, exposed sites near the volcano. Ravel caused mortality was severe on the steepest site, especially when slopes exceeded 60 percent. Fertilization usually increased seedling growth rates but reduced first-year survival by as much as 15 percent. Lowered survival rates with the fertilization treatment were most extreme for true fir (Abies spp.) seedlings. Although the growth of shaded seedlings was slower than that of fertilized seedlings, survival rates of the former were significantly higher. Shading was most effective in reducing drought or heat stress on south and southwest facing sites in the low elevation study. Twoyear survival for shaded trees in this experiment was 21 percent higher than that of fertilized trees. The high survival and growth rates for planted seedlings on all planting sites were much better than expected. The good one- and twoyear survival and growth results on the extensively damaged sites in the devastated areas suggested that site conditions were not as harsh as they initially appeared. This study indicated that reforestation procedures used for pre-eruption regeneration should be adequate for successful reforestation of most of the damaged and ash impacted forestland near Mount St. Helens. Shading may be beneficial on hot exposed sites or southerly aspects but fertilization is not recommended.
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