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Growth Response of Young Douglas-Fir to Repeated Application of Nitrogen and Phosphorus¹

ABSTRACT

A heavily thinned (to 200 trees/ha) stand of young Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] in southwestern Washington was treated with repeated applications of nitrogen (N) and phosphorus (P) over the 15-year period from 1955 to 1969. Nitrogen treatments resulted in a 20-year growth response of 24% over the controls for the lower application rates (1,120-kg N/ha) and 46% for the higher application rates (2,016-kg N/ha). All nitrogen treatments combined averaged 40% greater volume growth than controls. Plots that received 1972-kg N/ha over the first 10 years of the study still showed 25% greater volume growth than the controls after 10 additional years without nitrogen applications. Phosphorus applications, either alone or in combination with nitrogen, did not increase tree growth rates.

¹ Contribution from the Weyerhaeuser Co., Centralia, Wash.

² Forest Soils Specialist, Weyerhaeuser Co., Western Forestry Research Center, Centralia, WA 98531.

Received for publication August 4, 1980. Accepted for publication February 26, 1981.

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