Long-term Growth Response of Douglas-fir to Ammonium Nitrate Fertilizer

The effect of a single application of ammonium nitrate fertilizer on diameter, height, and volume growth of a Site IV plantation of Pseudotsuga menziesii was measured repeatedly during a 15-year period. Fertilizer dosages of 157, 314, and 471 kg N/ha increased gross volume growth during the 15-year period by an average of 51, 88, and 111 percent, respectively (67, 116, and 146 m³/ha). Quadratic equations best described the relationship between fertilizer dosage and gross and net volume growth during the entire period and during three intermediate periods. Volume growth on fertilized plots exceeded that on unfertilized plots in all periods. Response to all N dosages did not change significantly during the 15-year period of observation. The positive, long-term effect of fertilizer on stand growth is attributed both to improved tree nutrition and to secondary effects of increased tree growth on stand structure. Forest Sci. 29:127-137.