In an attempt to better understand adaptation to north-temperate and boreal environments, I have studied variation in phenology, ecophysiology and single nucleotide polymorphisms in an extensive range-wide collection of Populus balsamifera L. (balsam poplar) populations. Based on three years of observation, I infer that the differences in phenology between two common garden sites, with similar photoperiodic regimes but dramatically different climates, is based on differences in spring start date resulting in different dates of photoperiodic competency for height growth cessation and leaf senescence. Autumn phenophases in balsam poplar are primarily cued by the absolute photoperiod and do not respond to direction of change or climate warming. Interactions between photoperiod, climate and genotype can have large, heretofore unreported effects on root:shoot ratio. By comparison to P. tremula L. (European aspen) and published data for P. trichocarpa Torr. & Gray (black cottonwood), I found, in common garden conditions, a global tendency towards increasing photosynthetic rates with latitude. Height growth, being under photoperiodic control, follows the opposite pattern. When photoperiodic limitations were removed in a greenhouse experiment, higher photosynthesis in high latitude genotypes of balsam poplar was associated with greater height increment. Mesophyll conductance also varied clinally and accounts, in part, for higher photosynthesis in the northern balsam poplar genotypes. Phenotypic data presented in this thesis will ultimately be used for large-scale association genetics in the hopes of identifying candidate genes controlling adaptation to growing season length. As a step in this direction, we examined the comparative nucleotide diversity of the three above Populus species. We confirm that the closely related North American species (i.e., both within the section Tacamahaca) have lower nucleotide diversity than the more distantly related European aspen (section Populus). Divergence between the sections is estimated at about five million years ago, whereas P. balsamifera and P. trichocarpa diverged more recently (~0.8 million years ago). Linkage disequilibrium in balsam poplar decayed rapidly (within 400 bp).Forestry, Faculty ofGraduat
