Characteristics and development of improved Pinus sylvestris in northern Sweden

Genetic differences are described between improved and unimproved Scots pine (Pinus sylvestris L.) in 36 northern Swedish field tests, covering wide geographical and climatic gradients (latitude 62.3 degrees-67.8 degrees N). Improved trees were represented by progenies from controlled crosses of first-generation, phenotypically selected plus trees, whereas unimproved trees originated from unselected natural stands. Improved trees were superior in terms of height (9.2%), stem diameter (5.4%), and stem volume (18.9%) at the age of 27.4 years. The height growth of improved trees from ages of 10.5 years to 27.4 years was similar to that of unimproved trees at a site with a higher site index. Improved trees had a 5.5% greater height/diameter ratio (i.e., were more slender) than unimproved trees, whereas differences between the tree categories in terms of survival and frequencies of ramicorns and stem breaks were minor and mostly insignificant. Little or no interaction between tree categories and site conditions for growth characters was found, implying that the results are generally applicable. No difference in response to competition between the improved and unimproved trees was detected. However, differences in their reactions to transfer were found: survival rates increased more and height growth decreased less in improved trees than in unimproved trees when grown at a site south of their geographical origin. The use of competition and height indices based on neighbouring trees to adjust for bias and site variability in single-tree plots significantly improved the estimates.