Effects of thinning and nitrogen fertilization on branch and foliage production in Douglas-fir

Abstract

The effect of two levels of thinning (zero and [Formula: see text] of basal area removed) and three levels of nitrogen fertilization (0,224, and 448 kg N/ha) on crown development of codominant, 24-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) trees was studied over a 5- to 7-year period. Thinning and heavy ferilization separately increased needle mass per tree after 7 years by 90% and, when combined, by 271%. Yearly needle production peaked 2–3 years after fertilization and resulted from an increase in needle size, needle number per shoot, and number of shoots produced. Maximum foliage mass per tree was reached 4–7 years after fertilization. Thinning effect on needle production was lower initially, but increased throughout the study period. A continuing foliage production in branches low in the crown contributed to thinning effect on foliage mass and crown size. Foliage distribution was affected most in the top half of the crown by fertilization and in the bottom half of the crown by thinning. Fertilization increased branch elongation at all crown heights, but thinning alone had no effect on crown width down to whorl 12.