Genetic parameters for seasonal height and height growth curves of white spruce seedlings and their implications to early selection

The logistic growth curves were fitted to seasonal cumulative height of white spruce seedlings taken at 14-day intervals to study genetic variation in seedling growth trajectories and their relationship to height growth in the field. To do this, seedlings were raised and measured in the greenhouse for 18 weeks. Observed heights were then used to fit growth curves. The observed heights, predicted height values, and growth curve parameters were subjected to statistical and genetic analysis. Height of 11-year-old trees from the same seed collection growing at two field sites in Alberta provided data for comparison with greenhouse traits. Analysis of variances and covariances showed that individual-tree heritability ( h i 2) and heritability of family means ( h f 2) for 18-week predicted seedling height (H18) were, respectively, 0.863±0.135 and 0.921±0.231. Heritabilities for the upper asymptotic height parameter ( k) were 0.866±0.132 ( h i 2) and 0.921±0.232 ( h f 2). This shows that either H18 or k could be used to study genetic variation in the test material, since they both produced the same results. The rate of growth ( r) and age at the point of inflection ( t 0.5) parameters had h i 2 of 0.242±0.062 ( r) and 0.232±0.061 ( t 0.5), and h f 2 of 0.717±0.107 ( r) and 0.712±0.105 ( t 0.5). Selection for H18 and k would, respectively, yield 50.8 and 51.7% of the genetic gain expected from direct selection for 11-year height at site A. The corresponding selection efficiencies for site B were 53.8 and 54.4%. These selection efficiencies show that selection for H18 and k in the greenhouse would be more efficient in improving gain per year than selection for 11-year field height. The use of seasonal seedling height growth curves in genetic studies and early selection is discussed.