Seven case studies of 11 ecosystems were used to examine the effects of nitrogen-fixing alders in Douglas-fir plantations. The first case study quantified nitrogen (N) fixation and aboveground net primary production in a young Sitka alder [Alnus sinuata (Regel) Rydb.] ecosystem. At 5 yr of age, the N fixation (C22H2 reduction) rate of 35 kg ha−1 yr−1 was near the middle of the reported range for this shrubby species. The second case study compared N fixation rates of Sitka alder and red alder (Alnus rubra Bong.) on the same sites. These species exhibited similar nodule activities and had similar nodule:leaf biomass ratios of 7-8%. A mixture of Sitka alder and Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] was estimated to have a current N fixation rate of 20 kg ha−1 yr−1 based on acetylene reduction; N accretion measurements indicated an average N fixation rate of 30 kg ha−1 yr−1 for 23 yr. A mixture of red alder and Douglas-fir on the same site had a current N fixation rate of 130 kg ha−1 yr−1 based on acetylene reduction, with an N accretion rate of 65 kg ha−1 yr−1 for 23 yr. The third case study evaluated the effects of Sitka alder on Douglas-fir growth and nutrition, and on ecosystem production and litterfall. Current Douglas-fir stem growth was 40% greater with Sitka alder, and aboveground ecosystem net primary production was increased 70% with the alder. Litterfall nutrient content was 3 to 7 times greater under the mixed canopy. The fourth and fifth case studies looked at 23 yr-old mixtures of red alder and Douglas-fir in comparison with pure Douglas-fir, on an infertile site and on a fertile site. Red alder had little effect on Douglas-fir size or growth rate on the infertile site, but ecosystem net primary production was tripled. Conversely, net primary production was not increased by red alder on the fertile site, and Douglas-fir size and growth were reduced. The final two case studies evaluated: (1) the conclusions reached in the previous case studies for applicability to red alder/Douglas-fir mixtures in general, and (2) trends in production with stand development up to age 50. These case studies were consistent with the general conclusions of site fertility interactions with red alder/Douglas-fir mixtures. With further stand development, red alder continued to enhance ecosystem production and Douglas-fir growth on infertile sites, with opposite effects on fertile sites. Sitka alder demonstrated a high potential usefulness for interplanting with Douglas-fir, and red alder greatly boosted ecosystem production on infertile sites. Both species merit further development as tools for forest management on N deficient sites.

Five randomly assigned treatments were used to quantify effects of adding varying numbers of red alder (Alnus rubra Bong.) or nitrogen (N) fertilization on growth of a 10-year-old conifer plantation at a medium quality site in the western Washington Cascade Range. Zero, 20, 40, and 80 alder trees per acre were retained along with about 300 conifers per acre. Nearly all conifers were coast Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco). A fifth treatment substituted N fertilizer for N-fixing alder. Changes in average tree height, and in numbers of trees, basal area, and volume per acre between plantation ages of 10 and 27 are compared. In pure conifer plots, gross volume growth averaged 26 percent greater on fertilized than nonfertilized plots, indicating measurable benefits of additional N. On both fertilized and nonfertilized plots, an average of 13 percent of the original conifers died. Retaining 20, 40, or 80 alder per acre (7, 13, and 27 percent of the associated conifer trees per acre, respectively) was associated with reduced numbers of Douglas-fir by about 19, 5, and 17 percent, respectively, in the next 17 years. Mortality and growth of Douglas-fir were not related to alder density, but losses of Douglas-fir were especially large on plots where relatively large red alder (20 per acre) were retained. Neither total stand nor conifer yields were changed by retaining alder. Additional comparisons are needed at other locations, especially those with known N deficiency.