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    Description

    Amendment of soil nutrients through fertilization is used to increase wood production of forest stands. Subsequent allocation of growth to individual trees and the resulting increase in stand volume and value, however, depend on stand density at the time of treatment. Our primary research question was: To what extent can volume growth per acre in precommercially thinned or nonthinned portions of a poor-site, 30-year-old coast Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii) plantation be increased by a single application of 200 or 400 lb nitrogen (N)/acre as urea? Each fertilizer treatment was replicated on three 0.2-acre plots in thinned and nonthinned stands located in Rocky Brook drainage on the Olympic Peninsula in Washington. All trees in the interior 0.1-acre plots with a diameter at breast height (d.b.h.) of 1.6 inches and larger were measured for stem diameter, and a subset of trees was measured for total height and height to live crown. We anticipated that poor site quality, prolonged overstocking, and a subsequent severe thinning would influence the responses we observed. In the 20 years after fertilization, net growth in total stem volume on fertilized plots averaged 50 percent greater than on nonfertilized plots in the previously thinned stand and 31 percent greater in the nonthinned stand. In both stands, mortality losses were relatively small, averaging 1 to 15 percent of gross annual growth. Diameter growth of the 100 largest trees per acre was improved by both thinning and fertilization. Present net value (PNV) (in 2014 dollars) at the hypothetical final harvest of the thinned stand was increased by delaying the harvest from 10 years (age 40) until 20 years after fertilization (age 50), and by fertilization, although differences between the 200 and 400 lb N/acre treatments were not significant. Present net value of the nonthinned stand also was increased by delaying final harvest until 20 years after fertilization; however, PNV did not differ significantly among the 0, 200, and 400 lb N/acre treatments, indicating no financial benefit from fertilizing this nonthinned stand. The 90-percent confidence interval for PNV of nonfertilized plots in the thinned stand ($919 ± $278 per acre) did not overlap with that of nonfertilized plots in the nonthinned stand ($2,639 ± $498 per acre), suggesting that the thinning prescription reduced the economic value of this overstocked stand at Rocky Brook. Clearly, the severe thinning at age 30 reduced stocking to the point at which the stand was slow to reoccupy this poor quality site.

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    Citation

    Miller, Richard E.; Harrington, Timothy B.; Anderson, Harry W. 2016. Stand dynamics of Douglas-fir 20 years after precommercial thinning and nitrogen fertilization on a poor-quality site. Res. Pap. PNW-RP-606. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 66 p.

    Keywords

    Douglas-fir plantation, precommercial thinning, forest fertilization, nitrogen, stand growth, economic analysis.

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https://www.fs.usda.gov/treesearch/pubs/50503