Intermountain Forest Tree Nutrition Cooperative - IFTNC Supplemental Report No. 4: Nitrogen Fertilizer Response of Douglas-fir by Geographic Areas across the Inland Northwest

Response to nitrogen fertilization treatments differed significantly among geographic regions. Gross basal area andvolume 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 lbs./a. N plots than on 200 lbs. N plots. Net basal area and volume growth on treated plots in Montana, central Idahoand northeast Oregon were not significantly greater than the controls for either nitrogen treatment. Analysis of two year periodic basal area increment indicated that, while response did decline through time, treated plots continued to produce more gross growth than control plots six years after treatment. Similar operational nitrogen treatments applied to the Douglas-fir population sampled in this study should produce gross responses exceeding ten percent after six years three out of four times