Cone-Production and Stem-Growth Response of Douglas Fir to Rate and Frequency of Nitrogen Fertilization

Twenty-year-old Douglas fir (Pseudotsugamenziesii (Mirb.) Franco.) were treated in May with 0, 50, 100, 200,400, 800 and 1600 lb nitrogen per acre (1 lb/acre = 1.12 kg/ha) as ammonium nitrate. In 5 subsequent years, cone production with annual and biennial retreatment at one rate was compared with no retreatment. Heavy rainfall after the initial fertilization appeared responsible for small, similar cone-production responses the next year from the 100–1600 lb rates. Retreatment at 400 lb increased cone production of the third year but overfertilization occurred in combination with the higher initial rates. Initial overfertilization or annual refertilization reduced long-term responses. Responses were greatest when dry conditions followed treatment. It was concluded that 400 lb nitrate nitrogen per acre at the start of vegetative bud break, applied biennially to alternate halves of an area, would provide optimum long-term cone production.Stem growth benefited from increasing foliar nitrogen levels up to about 2%, obtained with 200–400 lb nitrogen per acre, and was adversely affected by foliar nitrogen above 3%. Response to fertilization consisted of: (1) direct effects of improved nitrogen status lasting about 2 years, (2) accumulative increases in leaf area favoring shoot and stem responses up to the 4th or 5th year, and (3) continued stem-area increases due to growth over the greater circumference resulting from factors 1 and 2. Over the full duration of effect in young stands, the optimum economic treatment for increased wood production may correspond closely to the 400 lb nitrogen per acre which provided near-optimum physiological responses. The 200 and 400 lb rates increased 5-year stem-volume increment by 62 and 90%.