Ten-year growth response of coastal Douglas-fir on Vancouver Island to N and S fertilization in an optimum nutrition trial

A 27-year-old stand of coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) France) on Vancouver Island was fertilized four times with N as urea 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-year period, but remained elevated in the fertilized plots. The increase in foliar N concentrations was commensurate with the amount of N added. Stand basal area response increased with increasing rates of N addition. 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 extra wood would be produced under this regime. There was no additional response to S added in conjunction with N, so the N + S plots received only a single application of fertilizer. Mortality was confined to smaller 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 can be achieved through repeated additions of N that maintain elevated concentrations of N in foliage.