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Ecosystem Effects of Sitka Alder in a Douglas-fir Plantation

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The natural seeding of Sitka alder [Alnus sinuata (Regel) Rydb.] into a portion of a Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantation afforded an opportunity to assess the effects of shrubby Sitka alder on Douglas-fir growth and nutrient content, on ecosystem biomass, aboveground net primary production, and litterfall, and on soils. Estimated accretion of nitrogen (N) was 30 kg ha-1 yr-1 for 23 years in the Sitka alder/Douglas-fir ecosystem. Though Douglas-fir stocking and basal area were not significantly affected by Sitka alder, current average dbh was 13 percent greater, 5-year average basal area growth 33 percent greater, and stem biomass increment 40 percent greater on the Sitka alder site. Nitrogen concentrations in Douglas-fir foliage increased significantly with Sitka alder present, but concentrations of phosphorus and sulfur were markedly reduced. Nutrient content of litterfall was 3 to 7 times greater on the Sitka alder site, and the soil-N availability index was 3-fold greater. Moderate N-fixation rates and shrubby growth form of Sitka alder combine to make it an attractive species for mixture with conifers on N-deficient sites. Forest Sci. 30:26-35.

Keywords: Nitrogen fixation; biomass; nutrient cycling

Document Type: Journal Article

Affiliations: Associate Professor, Department of Forest Science, Oregon State University

Publication date: March 1, 1984

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