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Assessing the influence of silvicultural practices on Douglas-fir nutrition and stemwood production

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Nitrogen, phosphorus and sulfur concentrations were measured in the soil, foliage and litter of Douglas-fir growing in pure Douglas-fir and mixed conifer stands. On four sites of contrasting fertility and climatic conditions, plots had been modified by the introduction of alder, thinning, or by a single application of nitrogen fertilizer and thinning. Samples were taken to look for differences between plots in soil, foliage and litter nutrient concentrations arid ratios. Changes in canopy leaf area and wood production efficiency. were calculated from estimates of stemwood diameter and sapwood area. Wood production efficiency, an index of stemwood production per unit of leaf area, remained steady or increased under all treatments, despite shading from higher canopy leaf areas. On the less fertile Washington and Vancouver Island sites, the alder/conifer plots, with higher concentration up to a certain 'threshold level', above which higher nitrogen concentrations did not increase wood production as significantly. Stemwood production also tended to increase with higher N:P ratios and then leveled off above a ratio of twelve. A nitrogen use efficiency was calculated as the amount of stemwood produced per unit of N in the foliage. Although higher foliar nitrogen values correlated with higher wood production/m2 on the poorer quality sites, the nutrient use efficiency tended to be lower when the nitrogen concentration per m2 was more than about three grams per m2. Stemwood production per m2 of foliage tended to increase with higher foliar N:P ratios, but the correlation coefficient was low. Soil N (mineralizable and total) plotted against against foliar N/m2 produced a coefficient of .7. Oregon coast site data alone showed a significant correlation between foliar N and percent N retranslocation (r = .94) and between foliar P and percent P retranslocation (r = .91).

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