Comparative effects of urea fertilizer and red alder in a site III, coast Douglas-fir plantation in the Washington Cascade Range.

Abstract

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.