Ecosystem effects of Sitka alder in a Douglas-fir plantation [Alnus sinuata, Pseudotsuga menziesii, nitrogen fixation, growth]

The natural seeding of Sitka alder (Alnus sinuata ) into a portion of a Douglas-fir (Pseudotsuga menziesii ) plantation afforded an opportunity to assess the effects of shrubby Sita alder on Douglas-fir growth and nutrient content, on ecosystem biomass, aboveground net primary production, and litterfall, and on soil. Estimated accretion of nitrogen (N) was 30 kg ha super(-1) yr super(-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.