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Fertilizer trials in coast Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) in the Oregon Coast Range usually indicate small and statistically nonsignifi-cant response to nitrogen (N) fertilizers. Inherently weak experimental designs of past trials could make them too insensitive to detect growth differences that actually exist. Ability to detect real differences among treatments should be improved by having more than two replications per treatment and by using covariance analysis to adjust observed treatment means for unequal starting conditions among experimental treatments. To demonstrate these assumptions, we used size at fertilization and a pre-fertilization (calibration) period of growth as covariates when analyzing data from five coastal plantations. The trials had three to six replications per treatment and calibration periods of 6 or 7 years. Nitrogen fertilizer was assigned randomly to half the plots at each location when trees were 16 or 17 years old from seed. Our objectives were to quantify 4-or 7-year response to N fertilizer and to demonstrate practical means for detecting response. Effects of fertilization on tree diameter and height, and on basal area and volume growth per acre were estimated. Among the five nonthinned plantations , observed gross basal area growth was changed by-2 to 13 percent in the 4 or 7 years after fertilization. Observed responses were increased substantially by covariance analyses at some plantations but decreased at others. Random assignment of three to six plots per treatment did not ensure balanced or comparable plots for fertilized and nonfertilized treatments. Most fertilizer trials in coast Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) stands in the Oregon Coast Range indicate small and statistically nonsignifi-cant response to urea or ammonium nitrate. Small response to nitrogen (N) could be explained by nitrogen sufficiency in this area as suggested by above-average site quality. Alternatively, weakly replicated experimental designs of past trials could make them too insensitive to detect differences that actually existed among treatments. We speculated that ability of field trials to detect real differences among treatments could be improved by more replications per treatment and by using covariance analysis to adjust observed treatment means for unequal starting conditions among experimental treatments. Accordingly, we used stand basal area and volume at fertilization and pre-fertilization (calibration) growth as covariates when analyzing data from a total of five plantations at three locations in the Oregon Coast Range. These plantations of 30 half-sib families (from 30 open-pollinated mother trees) had three to six …

Detecting response of Douglas-fir plantations to urea fertilizer at three locations in the Oregon Coast Range.