|Abstract or Summary
- The differential effect of solar radiation on seedling establishment under a forest stand was studied to develop a basis for controlling species composition in the regenerating stand. The study was carried out on the Oregon Coast where current silvicultural practice is to clearcut mature stands and establish Sitka spruce (Picea sitchensis (Bong.) Carr.), western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings in full sunlight. While generally favorable for conifer seedlings, the clearcut environment also favors growth of competing red alder (Alnus rubra Bong.) which often overtops and suppresses the conifers. An exploratory study in a Sitka spruce, western hemlock, and Douglas-fir stand on gentle topography indicated the mechanics of manipulating canopy density could be handled without difficulty but removal of the overstory after seedling establishment needed to be done with care to avoid damage to seedlings. Logging slash was readily disposed of by piling and burning and the mature stand suffered only light storm damage even though a very severe windstorm occurred during the study period. Many Sitka spruce and western hemlock, some Douglas-fir, and no red alder seedlings became established under the forest canopy, but species differences were influenced by differences in seed supply. Competing vegetation increased following thinning of the forest canopy but generally did not prevent establishment of conifer seedlings. In the main study scarified plots were established under a 118-year-old stand thinned to provide a range in canopy density and, therefore, in solar radiation reaching the forest floor. Establishment and growth of seedlings originating from natural seedfall and artificial seeding were measured and related to solar radiation by regression analysis. Sitka spruce, western hemlock, and Douglas-fir seedlings all became established more readily on mineral soil under the forest canopy than red alder. The ratio of viable seeds sown in the spring to established seedlings in the fall was 5.8 for the conifers compared to 46.7 for alder. The intensity of solar radiation had surprisingly little effect on seedling establishment even though intensities ranged from less than ten to almost 70 percent of radiation in the open. Radiation played a more important role in first-season growth. Spruce and hemlock growth increased with radiation up to an average daily radiation of about 150 Langleys or 50 percent of radiation in the open, then decreased at higher radiation levels. Red alder's response was similar with the optimum at about 39 percent of full sunlight. Growth of Douglas-fir continued upward at higher radiation levels, exhibiting a growth response significantly different from the other species. Red alder, generally recognized as an intolerant tree, appeared to be very tolerant of shade during its first and probably its second growing season. Starting from a smaller seed, it outgrew spruce and hemlock at low radiation levels indicating a basic photosynthetic efficiency surpassing these conifers. The downtrend in growth of spruce, hemlock, and alder at the higher radiation levels apparently was caused by high soil moisture tension and did not occur in a supplemental study where seedlings were watered. Even in the moist coastal climate soil moisture is apparently an important environmental factor limiting first-season seedling growth.