Growth and foliar nutrient response to fertilization and precommercial thinning in a coastal western red cedar stand

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Abstract
Seven silvicultural treatments were applied to a young (15- to 20-year-old), naturally regenerated western red cedar (Thujaplicata Donn ex D. Don) stand growing on a poor-quality site in western Washington. The treatments were as follows: unthinned, unfertilized (untreated); unthinned, fertilized with ammonium nitrate, dicalcium phosphate, and potassium sulfate; thinned, unfertilized; thinned, fertilized with urea; thinned, fertilized with ammonium nitrate; thinned, fertilized with ammonium nitrate and dicalcium phosphate; and thinned, fertilized with ammonium nitrate, dicalcium phosphate, and potassium sulfate. Elemental application rates were 300 kg nitrogen, 100 kg phosphorus, 129 kg calcium, 100 kg potassium, and 41 kg sulfur per hectare. Growth and foliar nutrient concentrations were monitored for 5 years. Five-year height growth and diameter growth of the 20 tallest trees per 0.08-ha measurement plot were both substantially greater in all thinned or fertilized treatments than in the unthinned, unfertilized treatment. The three best treatments for height growth (the treatments containing ammonium nitrate and dicalcium phosphate) had 65% more growth than the unthinned, unfertilized treatment. The two best treatments for diameter growth (thinned, fertilized with ammonium nitrate and dicalcium phosphate) had 106% more growth than the unthinned, unfertilized treatment. The effects of thinning alone were small compared with the effects of fertilization alone or fertilization combined with thinning. Initial foliar nitrogen concentrations showed that the site was nitrogen deficient, and both nitrogen sources, urea and ammonium nitrate, increased growth. The addition of dicalcium phosphate resulted in significantly greater incremental growth above that attributed to nitrogen alone, but the further addition of potassium sulfate did not increase growth significantly. Increases in foliar nitrogen and phosphorus associated with fertilization were still evident 5 years after treatment. All sizes of trees responded to treatment, and growth of the total stand followed the same trends as for the 20 tallest trees per plot. Basal area growth on an absolute basis was best in the unthinned, fertilized treatment (more than twice that of the unthinned, unfertilized treatment) and on a percent basis was best in the thinned treatment fertilized with ammonium nitrate, dicalcium phosphate, and potassium sulfate.
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