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Studies of variation in hemlock Tsuga populations and individuals from southern British Columbia Meagher, M. D.

Abstract

In view of western hemlock's importance to British Columbia's forest economy, and the lack of knowledge of its variation patterns, modes of inheritance and heritabilities, studies were begun in 1968 on mountain and western hemlocks collected from 128 parents in 17 locations in southern British Columbia. The purpose was to describe the variation in cone morphology, to detect hybrids and determine the species' ability to form artificial hybrids, to screen western hemlock populations for differences in nursery seedling performance due to altitude and source area within a narrow latitudinal belt, and to determine the importance of cone and pollen parentage to western hemlock seedling growth and characteristics. Morphometric examination of bagged and open-grown western hemlock cones revealed the most stable features: scale width and distance between scale tip and widest point. These were used to compare western and mountain hemlocks, and to test for intra-specific variation in western hemlock. Material for the seedling population studies was reared in two nurseries: a container nursery of the Canadian Forestry Service, Victoria, and a bare-root nursery at the University of British Columbia (U.B.C). Family identity was maintained in both trials. Measures of seedling dimensions and frost damage were taken from the former trial after one growing season, while the latter was assessed for germinative rate, "mutants", bud set, bud burst, frost damage and survival after one and two seasons. Substantial differences between western and mountain hemlocks were found for both cone and seedling features. Mountain hemlock cones are longer, are composed of more scales, produce rounder scales and larger, more regularly-shaped, more mucronate bracts than western hemlock. No cones indicating a true hybrid were found. Seeds of mountain hemlock germinated faster, and the seedlings set and burst buds earlier, were more frost-hardy and shorter than western hemlock from comparable elevations. These differences, plus the unsuccessful hybridisation attempt on Mount Seymour, indicate that the species are genetically distinct. Western hemlock population studies were conducted with open-pollinated seeds collected in five areas near the 49th parallel; sampling in three areas extended over the species' altitudinal range. Height, height:diameter ratio, branch number and frost damage to plug-grown seedlings were inversely correlated with elevation, whereas maximum branch length was not. Sensitive adaptation to local conditions was inferred from differences in the trend of seedling mean height with elevation of source between two Coastal localities. In the bare-root nursery, seed stratification accelerated germination of western hemlock, but still it was slower than mountain hemlock. Chlorophyll- deficient or virescent seedlings were found in western hemlock only, but in no pattern related to origin. Other "mutant" seedling types were noted. A possible adaptation of "bluish" seedlings to high-altitude conditions is discussed. Bud set was associated with location and altitude of western hemlock seed origin, but bud burst was not correlated with either. Strong positive correlations occurred between mid-autumn bud set percent and elevation, yet differences in percent bud set were found among populations from comparable altitudes. Bud set was negatively, though weakly, correlated with family mean height. Frost damage in the bare-root nursery was correlated with that found in the plug-grown stock, was independent of family height, and was negatively correlated with elevation of seed origin and mid-autumn bud set. Clinal adaptation with elevation was apparent for seedling height, bud set and frost hardiness. A complete diallel cross was conducted between three western hemlock trees (A, D and E) at U.B.C. to allow detection of any genetic control over some of the features analysed in the population studies and of the genetic nature of the species. Filled-seed yield was reduced by selfing in all cases, perhaps due to the presence of deleterious recessive alleles in each parent. Parental combination affected germinative rate and the frequency and type of abnormal germinants, but not total germination. Quantitative and qualitative parameters were derived from germinants, and from seedlings after they were transplanted to plugs in a growth chamber. Cotyledon number and hypocotyl colour appear to be controlled by many genes, whereas chlorophyll-deficient types appear to be homozygous recessives involving one or two genes. Analysis of height growth beginning six weeks after transplanting indicated relative family height at six months. Estimated narrow-sense heritability at six months was 0.63 for height, but only 0.01 for branch length. Selfing generally reduced seedling height, except for Tree D, which displayed high general combining ability. Genetic control was found also for most dimension and weight features analysed, however, evidence of maternal control was found for some features. Pronounced environmental modification of all growth parameters was found. Comparing the population and controlled-cross results, it is concluded that mountain and western hemlocks are sufficiently distinct genetically that interspecific hybridization would be an unlikely source of genes in a western hemlock breeding program, that western hemlock populations differentiate rapidly with locality and elevation, particularly in features regulating bud set and frost hardiness, that open-pollinated families represent the general combining ability of the seed parent in the features assessed here, and that the heterogeneity noted between and within families is in part genetically caused, mostly by the seed parent. The adaptive role of this heterogeneity is discussed. The preliminary suggestions for reforestation and breeding programs that can be made from these studies are that seedlings should be planted within approximately 1000 feet of the elevation of seed source, and that single-tree selection and intraspecific crossing, rather than only provenance selection, be pursued in Southern British Columbia. More detailed and long-term studies are required to determine the validity of these recommendations.

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