Pines beyond the polar circle: Adaptation to stress conditions

Summary

Plant populations in extreme conditions differ from those in more favorable conditions in many respects. They have developed special physiological adaptations for withstanding e.g. extreme temperatures or drought. On the other hand, low survival and fertility may cause smaller effective population sizes, which in turn influences patterns of genetic variation. Scots pine has the widest range of all pine species, extending in the north far beyond the arctic circle. Close to the tree limit, the growing season is short and survival is low. Very northern populations also have lower and more irregular male and female flowering and seed production than southern ones. The consequences of these conditions on genetic variation have been examined. All Finnish populations have similar patterns of variation at allozyme loci and nuclear DNA markers: high variation but no differentiation between populations. Northern populations seem to have slightly fewer recessive deleterious alleles than southern ones. There is, however, strong differentiation between populations with respect to adaptation to the very short growing season. In common garden experiments, northern populations cease growth and set buds much earlier and develop frost tolerance earlier than southern populations. The genetic basis for this high differentiation in bud set can be explored by using dense maps of molecular markers. The implications of these patterns of variations on conservation of genetic resources and tree breeding will be discussed.