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Response of Northwest Douglas-fir Stands to Urea: Correlations with Forest Soil Properties¹

ABSTRACT

Replicated forest floor and surface soil (0–15 cm) samples were obtained from control plots at 160 field installations in western Washington and Oregon. Six-year growth responses of thinned and unthinned Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] installations treated with 0, 224, and 448 kg of urea-N ha^–1 were correlated with 18 forest floor and surface soil properties of the control plots. Correlations for response with respective forest floor and soil data were produced for levels of fertilizer application and both basal area and volume response. Observed and estimated expressions of absolute and relative response were used in analyses. Forest floor nitrogen properties were the most highly correlated with various estimates of response in both thinned and unthinned stands; these correlations were generally independent of methods used to estimate response. For unthinned stands, C/N ratios of both forest floor and surface soil were well correlated with growth response to fertilizer, whereas for thinned stands, N content (kilograms per hectare) of the forest floor was consistently correlated with response.

¹ Sponsored by the Regional Forest Nutrition Res. Project, College of Forest Resources, Univ. of Washington, Seattle, WA 98195.

² Mensuration Director of Regional Forest Nutrition Res. Project, former Graduate Student, and Professor of Forest Soils, respectively.

Received for publication April 4, 1983. Accepted for publication August 22, 1983.