AMMONIUM-NITRATE, UREA, AND BIURET FERTILIZERS INCREASE VOLUME GROWTHOF 57-YEAR-OLD DOUGLAS-FIR TREES WITHIN A GRADIENT OF NITROGEN DEFICIENCY

In a nitrogen-deficient plantation in southwest Washington, we (1) com pared effects of 224 kg N/ha as ammonium nitrate, urea, and biuret on volume growth of dominant and codominant Douglas-fir (Pseudotsuga menz iesii var. menziesii (Mirb.) France); (2) determined how 8-year respon se of these trees to fertilization was related to their distance from a strip of the plantation interplanted with nitrogen-fixing red alder (Alnus rubra Song.), and (3) observed effects of biuret on understory vegetation. On both sides of the strip centerline, we grouped subject trees into 30 plots of 4 trees each, based on slope position and dista nce from alder. We randomly assigned three fertilizers and a control w ithin each plot. We analyzed separately data from east and west of the mixed stand centerline. Initial volume differed greatly among the 120 trees on each side, so we used covariance analysis to adjust observed treatment means. Adjusted mean volume growth was increased (p less th an or equal to 0.10) by 22 to 28 percent on the east side and by 11 to 14 percent on the west side, with no significant difference in respon se to the three fertilizers. Only biuret stimulated growth within the mixed stand. Biuret had no visible toxic effect on competing vegetatio n during 8 years after application.