Effects of density control and fertilization on growth and yield of young Douglas-fir plantations in the Pacific Northwest

Abstract

To examine the effects of density control and fertilization on stand growth and yield of young Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) plantations, seven treatment regimes were applied in sixty-three 9-year-old plots from nine installations across western Washington and Oregon. Fertilizer was applied at the rate of 220 kg N·ha–1 (as urea) at stand establishment and every 4 years thereafter. Results after 12 years showed that widely spaced stands exhibited significantly larger quadratic mean diameter than did narrowly spaced stands. The densest stands initially had the greatest overall stand basal area and volume, but accumulation rate in the dense stands had been declining with time. After 12 years, the less dense stands had met or exceeded the basal area periodic annual increment of dense stands. Across all densities, the periodic annual increments of quadratic mean diameter, basal area, and volume in fertilized plots were significantly greater than in unfertilized plots following the first and second urea applications. However, the first fertilization was insufficient to produce a significant increase in stand yield and the significant fertilization increases in yield were found following the second and the third urea applications. This study showed neither significant fertilization effect nor density effect on dominant height. In addition, no significant interactions were found for any stand growth and yield variables considered, but fertilization responses showed different trends among density treatments over time.