Shoot production in Douglas-fir branches

Abstract

Of the 3981 shoots present in 1986 on 16 Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) branches, 6–12 years old, only 1933 produced new shoots in 1987, and of these only 311 produced more than one shoot. Length of new terminal shoots was determined primarily by the length of the parent shoot (R2 = 0.57). Shoots that did not elongate had blunt buds, < 1 mm long, with no leaf primordia; shoots that elongated had pointed buds, > 2 mm long, with many leaf primordia. Buds < 1 mm long survived up to 3 years without elongating; needles on these shoots survived an additional 2 years. Parent shoots longer than 13 cm all elongated. The percentage of parent shoots < 14 cm long that did not elongate, and would have subsequently died, increased with (i) decreasing parent shoot length; (ii) increasing whorl number along the main axis of the branch (toward the branch base); (iii) decreasing total elongation of all shoots on the branch, a measure of branch vigor.