10-YEAR GROWTH-RESPONSE OF COASTAL DOUGLAS-FIR ON VANCOUVER-ISLAND TON AND S FERTILIZATION IN AN OPTIMUM NUTRITION TRIAL

A 27-year-old stand of coastal Douglas-fir (Pseudotsuga menziesii (Mir b.) France) on Vancouver Island was fertilized four times with N as ur ea at six rates from 0 to 250 kg.N ha(-1), with and without S (0-50 kg .ha(-1)). Foliar N levels declined in the control plots over the 10-ye ar period, but remained elevated in the fertilized plots. The increase in foliar N concentrations was commensurate with the amount of N adde d. Stand basal area response increased with increasing rates of N addi tion. The greatest response in relative basal area net increment was 4 .52 m(2).ha(-1) (40%) in plots that received a total of 1000 kg N.ha(- 1) during the 10 years. Over a 60-year rotation, about 62 m(3) of extr a wood would be produced under this regime. There was no additional re sponse to S added in conjunction with N, so the N + S plots received o nly a single application of fertilizer. Mortality was confined to smal ler suppressed stems, mostly of species other than Douglas-fir. Foliar N concentrations and basal area increment declined steadily following cessation of fertilization in these plots, but remained greater than those in control plots for the 10-year measurement period. The results suggest that sustained increases in growth response of Douglas-fir ca n be achieved through repeated additions of N that maintain elevated c oncentrations of N in foliage.