QUANTITATIVE GENETICS OF YIELD BREEDING FOR POPULUS SHORT-ROTATION CULTURE .3. EFFICIENCY OF INDIRECT SELECTION ON TREE GEOMETRY

Quantitative analysis of genetic covariances was used to identify the critical morphological components of wood productivity and to evaluate the efficiency of indirect selection for these components at the four levels of biological organization, (1) leaf, (2) branch, (3) main ste m, and (4) whole-tree, in 4-year Populus deltoides x P. simonii and P. deltoides x P. nigra F-1 progeny. A total of 44 morphometric traits m easured at the four organizational levels showed varying genetic assoc iations with productivity variations being dependent on traits, develo pmental processes (current terminal, sylleptics, and proleptics), and hybridization combinations. Most of the leaf and branch traits on the current terminal and/or sylleptic branches had higher genetic correlat ions with stem-wood volume than those on proleptics, which resulted in larger indirect selection responses in volume, especially in D x S pr ogeny. Indirect clonal selection on leaf size and area, branching capa city, and branch angle at age 4 years was expected to generate 10-35% more genetic gain per year in 6-year volume than direct selection on 6 -year volume in the D x S progeny. The efficiency of indirect selectio n on the numbers of different order branches and bifurcation ratio was greater than 1.0 relative to that for direct selection for stemwood v olume in the D x N progeny. Under the pressure of artificial selection for superior volume production, with the proportion selected=15%, the two F-1 progeny populations exhibited distinct evolutionary divergenc e in tree geometry. The high-yielding D x S clones displayed a decurre nt-like crown with strong apical dominance, whereas the crown ideotype for the high-yielding D x N clones was found to be excurrent-like and surrounded by dense foliage and branches.