Short-Term Population Trends of Isolated Tree-Limit Stands of Pinus Sylvestris L. in Central Sweden

Abstract

The annual demographic changes and height increments of small, isolated, tree-limit stands of Pinus sylvestris L. (considered collectively as a single population) were studied during a 10-yr period (1972 to 1981). The individual vitality of specimens forming the population was also followed in detail throughout the study period. No recruitment to the population was recorded during the study period and the population size (total number of live plants) decreased by ca. 13%. The demographic changes were basically controlled by intrinsic factors but proximately executed by damage related to the annual differences in the air temperatures recorded during the initial 2 to 3 mon of the year. Long periods of radiation-controlled high temperatures during these months apparently lower the frost-hardiness, and the young pines, in particular, suffer serious damage by the subsequent onset of low temperatures. The individuals forming such isolated and small-sized tree-limit populations appear to be especially susceptible to this type of damage. Tree vitality in general, and the reproductive phase of life-cycle in particular, may suffer due to selfing and inbreeding. Such populations of pine seem therefore to be doomed to complete extinction, or to very drastic reduction in size within a rather short time after establishment (30 to 40 yr). The Holocene tree-limit history of pine in Scandinavia is briefly discussed in the light of this finding.