Genetic variation in juvenile growth of tamarack (Larixlaricina) in northwestern Ontario

Nested designs incorporating provenances and families and clones within provenances were used in two common garden tests near Thunder Bay, Ont. (48°N, 84°W) to evaluate genetic variation in growth and sylleptic branching among and within populations of tamarack (Larixlaricina (Du Roi) K. Koch) from 46°N, 80°W to 53°N, 93°W. At 7 and 8 years there was a north-south trend of increasing height among provenances. Genetic variation in height was related to both rate and duration of shoot elongation, but latitudinal variation in shoot growth mainly resulted from differences in late-season elongation. There was substantial genetic variance in height within populations, and broad-sense heritability based on pooled variances was 0.23 for one test population at 8 years and 0.11 for the second at 7 years. The more vigorous southern provenances had a greater potential for sylleptic branching in the test area than northern sources, and wide within-population variation in syllepsis was under stronger genetic control than growth (broad-sense heritability = 0.47 in year 6). Patterns of genetic variation observed in the study were not congruent with patterns of morphological and allozyme variation previously reported for the sampled populations.