THE GENETIC RESOLUTION OF JUVENILE CANOPY STRUCTURE AND FUNCTION IN A3-GENERATION PEDIGREE OF POPULUS

A genetic approach to the understanding of tree architecture is to cro ss trees of contrasting features and to study their segregating F-2 pr ogenies. For this purpose, members of a 3-generation pedigree, combini ng Populus trichocarpa, P. deltoides, and their F-1 and F-2 offspring, were grown side by side in a clonally replicated plantation. At 2 and 3 years of growth, tree architecture was analyzed at the stem, branch , and leaf levels. In all generations, proleptic branches were more nu merous, longer, and had more and larger leaves than sylleptics initiat ed in the same year. The analysis of variance revealed significant gen otypic effects on growth, branch and leaf biometrics in the F-2 family , with broad-sense heritabilities (H-2) ranging from 0.50 to 0.80 for most traits. For branch and leaf traits, the H-2 values were found to vary among branch types and crown positions. In year 2, the degree of genetic control was stronger for sylleptics than proleptics and for up per than lower crown positions. These patterns were followed in year 3 , except that H-2 values were more a function of position within crown , as a consequence of increased competition among trees. The genetic c orrelations between branch/leaf morphology and stem growth were also a function of branch type and crown position. Generally, traits on prol eptics or at upper positions were more tightly correlated with height growth, whereas those on sylleptics or at lower positions, with basal area growth. By year 3, proleptic traits showed increased genetic corr elations with both height and radial growth. The implications of these results for the construction of ideotypes are discussed.