Effects of thinning and nitrogen fertilization on branch and foliage production in Douglas-fir.
The effects of thinning (two thirds of b.a. removed) and of fertilization with urea at 0, 224 and 448 kg/ha N on crown development of 24-yr-old Douglas fir were studied over a 5- to 7-yr period. Thinning and heavy fertilization separately increased needle mass per tree after 7 yr by 90% and when combined by 271%. Yearly needle production reached a max. 2-3 yr after fertilization, resulting from an increase in needle size, needle number per shoot, and number of shoots produced. Max. foliage mass per tree was reached 4 to 7 yr after fertilization. Thinning also increased foliage production; the effect increased throughout the study period and yearly needle production in thinned plots surpassed that of fertilized trees by yr 5. Foliage distribution in the top half of the crown was affected most by fertilization, and in the bottom half by thinning. Fertilization increased branch elongation at all crown heights, but thinning only below whorl 12. The greatest needle production was found in thinned plots fertilized at 448 kg/ha N.