USE OF SELECTION INDEXES TO INCREASE TREE HEIGHT AND TO CONTROL DAMAGING AGENTS IN 2-YEAR-OLD BALSAM POPLAR

Cuttings were collected from balsam poplars (Populus balsamifera L.) representing 21 populations in Minnesota, Wisconsin, and Michigan, and a replicated nursery trial containing 153 clones was established. Tree height, phenology, leaf morphology, and resistance to damaging agents were evaluated during the second growing season, and genetic and phenotypic variances and covariances among traits were estimated. Clones differed significantly in all traits. However, populations differed only in tree height, number of leaves produced after August 31, bud-set date, number of sylleptic branches (i.e., branches that arise from buds the same year that the buds formed), and severity of Septoria leaf spot symptoms. Tree height was positively correlated with number of leaves, bud-set date, most measures of leaf size, and resistance to Melampsora leaf rust. Predicted responses to index selection for 2nd-year tree height using leaf area or leaf length as secondary selection criteria were not significantly greater than predicted response to univariate selection for tree height alone. However, results supported use of restricted selection indices to curb increased susceptibility to damaging agents that would otherwise result from selection for tree height.