. Seven-year response of 35-year-old Douglas-fir to nitrogen fertilizer. Trees Growth; Forest soils Fertilization; Nitrogen fertilizers. x LU // /\ \ / ,4° N _ X \ / ON w • \ X LU YEARS AFTER TREATMENT Figure 5.—Average yearly height growth by treatment and year since treatment re- lated to estimated rainfall during growing season. The method used to determine tree and plot volumes is detailed in the appendix. Although there were initial differences in both total (CVTS) and merchantable (CV4) volume among plots and treatment levels, subsequent volume growth in CVTS, but not CV
. Seven-year response of 35-year-old Douglas-fir to nitrogen fertilizer. Trees Growth; Forest soils Fertilization; Nitrogen fertilizers. x LU // /\ \ / ,4° N _ X \ / ON w • \ X LU YEARS AFTER TREATMENT Figure 5.—Average yearly height growth by treatment and year since treatment re- lated to estimated rainfall during growing season. The method used to determine tree and plot volumes is detailed in the appendix. Although there were initial differences in both total (CVTS) and merchantable (CV4) volume among plots and treatment levels, subsequent volume growth in CVTS, but not CV4, was significantly correlated with volume of the initial growing stock (p < 0. 002). Therefore, CVTS growth was adjusted by covariance to common initial volumes and adjusted growth used to determine fertilizer gain and trends of annual volume growth. Total volume growth.—During the 7 years following treatment, total cubic- volume growth of all trees, 1. 5-inch d. b. h. and larger, was positively related to amount of nitrogen applied (table 4); the response surface was significantly curvi- linear (p <0. 06) (fig. 6). Depending on fertilizer rate, gross growth on fertilized plots averaged 55 to 109 percent greater than that on control plots. Corresponding gains in volume growth averaged 515 to 1,012 cubic feet and increased with nitrogen dosage. The average increase following the 140-N dosage was equivalent to a temporary improvement of site quality by one site class (30 feet on a 100-year basis). The response surface was better described by a curved than a straight line. For example, the adjusted treatment means before smoothing clearly showed that volumes gained from the three dosages gradually declined from through to 2. 3 cubic feet per pound of nitrogen applied. Thus far, the optimum dosage of ammonium nitrate for gross volume production on this area is near 140 pounds of nitrogen per acre. To define the present or future response surface precisely wil
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