Nitrogen Fertilizer Response of Rocky Mountain Douglas-Fir by Geographic Area Across the Inland Northwest1

Abstract

Response to nitrogen fertilization treatments in Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca) differed significantly among geographic regions within the inland northwest. Gross basal area and volume growth on fertilized plots were significantly greater than growth on controls for all geographic regions, but only in northern Idaho and central Washington was gross response significantly greater on 400 lb/ac N plots than on 200 lb N plots. Net basal area and volume growth on treated plots in Montana, central Idaho, and northeast Oregon were not significantly greater than the controls for either nitrogen treatment. Analysis of 2-year periodic basal area increment indicated that, while response did decline through time, treated plots continued to produce more gross growth than control plots 6 years after treatment. Similar operational nitrogen treatments applied to the Douglas-fir population sampled in this study should produce gross responses exceeding 10% after 6 years three out of four times. West. J. Appl. For. 6(4):94-98.