Authors Miller, Richard E.; Smith, Jim; Anderson, Harry. 2001. Detecting response of Douglas-fir plantations to urea fertilizer at three locations in the Oregon Coast Range. Res. Pap. PNW-RP-533. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 20 p. Fertilizer trials in coast Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) in the Oregon Coast Range usually indicate small and statistically nonsignificant response to nitrogen (N)

more »

... 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 prefertilization (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. Abstract Summary Uniformly planted and intensively managed, these plantations provided an opportunity to estimate response to N fertilizer in uniform stand conditions. Each plantation contained 6 to 12 plots with the same 30 half-sib families. Families differed among the five plantations, however. Each plot contained four to five randomly located trees for each family as noncontiguous (single tree) family subplots. After planting at a 9-by 9-or 9by 10-foot spacing within 5-to 10-acre areas surrounded by an 8-foot tall fence, the 1-year-old, container-grown seedlings at each location were subsequently safeguarded from weeds, disease, and animal damage. These actions and some replacement planting resulted in uniformly spaced, well-stocked stands at age 16 or 17 years from seed (figs. 1, 2, 3, and 4).