Comparative effects of precommercial thinning, urea fertilizer, and red alder in a site II, coast Douglas-Fir plantation - Introduction

We varied the number of red alder retained with 300 Douglas-fir per acre on a high-quality site in coastal Oregon. Alder densities of 0, 20, 40, and 8 0 per acre were tested. Our fifth treatment eliminated nitrogen-fixing alde r, but substituted nitrogen fertilizer. Treatment 6 had neither thinning no r alder control. Treatments were randomly assigned within each of three blo cks in a 9-year-old plantation. Stand density was reduced within 15 of thes e 18 experimental units. Surplus conifers were cut, but surplus red alder w ere controlled by the "hack-and-squirt" method. Because numerous trees of o ther species regenerated naturally, combined density of all species before thinning ranged from 1,400 to 5,700 trees per acre. Subsequent 17-year chan ge in number, average height, basal area, and volume of Douglas-fir were co mpared. Retaining 20, 40, or 80 alder per acre reduced numbers of associate d Douglas-fir by about 10, 17, and 23 percent, respectively. In pure Dougla s-fir plots, gross volume growth was similar for nonfertilized and fertiliz ed plots, indicating no measurable benefits of additional nitrogen. In mixe d stands, red alder reduced yield of associated Douglas-fir, but not yield of combined species. Similar comparisons are needed at other locations, esp ecially those with known nitrogen deficiency.