Genetic architecture, genecology and phenotypic plasticity in seed and seedling traits of yellow-cedar : (Chamaecyparis nootkatensis (D.Don) Spach)

The overall objective of the thesis was to estimate the amount and distribution of genetic and environmental variation, and correlations between genetic variability and seed source origin, of yellow-cedar (Chamaecyparisnootkatensis (D. Don) Spach). Variation was measured for traits of seed, and for morphological and physiological traits of seedlings grown in a common garden, and in differing greenhouse environments. The study focused on traits that sampled the developmental sequence of events that influence a population’s adaptation to its environment. These included growth rate, phenology, drought resistance, cold acclimation, and dormancy. Significant variability was evident at both the population and family within population level in most traits measured. Substantially more genetic variability (2 to 16 times) was found at the family within population level as opposed to the population level in all but two traits. Narrow-sense heritabilities varied from 0.16 for growth during third-year shoot initiation to 0.64 for first-year height in the nursery bed. There was little evidence of adaptive variation for seed and germination traits: however, growth traits and cold-hardiness were moderately to strongly correlated with latitude and elevation of seed origin. Seedlings from more southerly and high elevation populations were taller, had greater diameter, grew later into the growing season, and were more susceptible to cold injury during acclimation and at maximum hardiness, than more northern populations. The above trends were not apparent if southern populations (Oregon) were excluded. Environments had a large effect on growth and morphology of yellow-cedar. Shoot elongation was extremely plastic, responding to both decreased photoperiod and water-stress through decreased shoot growth. Upon release of the stress treatments, growth increased to relative rates greater than the non-stressed trees. In all growth and morphology traits, there was minimal evidence for significant genotype by environment interactions at either the population or family within population level, with both photoperiod treatments and water regimes. Genetic variation in gas exchange, water relation parameters, and morphological traits, in response to a drought, was evident with 2-year-old yellow-cedar seedlings among and within populations. Seedlings from Coquihalla, a xeric habitat, had less shoot and lateral branch extension, and less biomass allocated to branches and more to roots, as compared to mesic sources, under both well-watered and drought conditions. As well, these seedlings maintained greater rates of net photosynthesis and higher levels of stomatal conductance under both well-watered and droughty conditions. Yellow-cedar populations at the extremes of environment for the species, i.e. southern and continental populations, have responded to environmental selection pressures by changes in gene frequencies. The changes most likely have been aided by reduced gene flow due to spatial isolation and poor sexual reproduction (Russell et al. 1990). At the same time, however, the species has maintained a substantial amount of both genetic variation and phenotypic plasticity within populations. Yellow-cedar seems to have evolved an intermediate mode of adaptation with less genetic differentiation associated with geography than Douglas-fir, Sitka spruce, and western hemlock, but more genetic differentiation than western white pine and western red cedar.Forestry, Faculty ofGraduat