The aim of the studies underlying this thesis was to quantify the genetic variability of important traits used for ranking candidate trees in northern Swedish Scots pine (Pinus sylvestris L.) breeding populations, with special focus on growth and survival and the genetic association between these traits. The thesis reports studies based on simulated data, field data from 28 progeny trials, and early test data from four artificial freezing experiments. The field and freezing experiments comprised half-sib progenies of Swedish and Finnish Scots pine plus-trees. The field trials (9–21 years old) were established in a wide range of environmental conditions. The traits analyzed were survival, tree height, spike knot frequency, branch diameter, branch angle, stem straightness, and susceptibility to infections of the fungi Phacidium infestans, Gremmeniella abietina, Melampsora pinitorqua and Lophodermella sulcigena. In the freezing experiments cold hardiness of 1-year-old seedlings was assessed after freezing in a climate chamber. In the simulation study the accuracy of single- and multiple-trait REML procedures was examined by studying estimates of within-individual genetic correlations between a categorical trait and a continuous trait with selectively deleted records. The average bias generated by multiple-trait REML was generally low, whereas single-trait REML systematically provided too moderate estimates. The variation among the correlations was generally high, showing that single-site estimates might be seriously misleading. The average within-site genetic correlation between tree height and field survival was generally positive, whereas corresponding between-site estimates were positive when the tree heights were assessed in harsh environments, but negative if the tree heights were assessed in mild environments (0.05 and –0.25, respectively). The genetic correlation between cold hardiness and field survival was on average positive (0.30), while the average correlation between cold hardiness and tree height was negative (–0.23). For the quality characters and susceptibility to infections of the pathogens, genetic associations with cold hardiness could not be verified. The most notable result was the contrasting correlation patterns across environments between tree height and field survival in the material sampled. The results show that tree heights from young trials located in harsh areas may reflect tree health and survival ability to a greater extent than growth capacity
