GENETIC-VARIATION IN TREE STRUCTURE AND ITS RELATION TO SIZE IN DOUGLAS-FIR .2. CROWN FORM, BRANCH CHARACTERS, AND FOLIAGE CHARACTERS

Genetic variation and covariation among traits of tree size and struct ure were assessed in an 18-year-old Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) genetic test in the Coast Range of Ore gon. Considerable genetic variation was found for relative crown width ; stem increment per crown projection area; leaf area and branch weigh t relative to crown size; branch diameter and length adjusted for stem size; branch stoutness; cross-sectional area of branches per crown le ngth; and needle size. Little genetic variation was found for branch n umbers per whorl, branch angle, and specific leaf area. At both the ph enotypic and genetic level, large trees growing well relative to growi ng space had tall, narrow crowns, high leaf areas per crown projection area or branch length, greater partitioning to leaves versus branches , and stouter branches. Thus, large, efficient trees were those that i nvested more in the photosynthetic machinery of leaf area and the bran ch biomass necessary to support that leaf area, but distributed that l eaf area over a greater vertical distance. Unfortunately, these traits also were associated with increased branchiness, and selection for th ese traits would be accompanied by reductions in harvest index and woo d quality.