Variable Holocene climate, treeline dynamics and changes in natural environments in northern Finnish Lapland

Abstract

The forest-tundra ecotone in northern Fennoscandia is a sensitive marginal area where climatic forcing mechanisms change environmental conditions. In this area there is a very distinct zonation of vegetation, including the northern limits of Norway spruce, Scots pine and mountain birch. The palaeoclimatological studies in Finnish Lapland focused on the limit of pine and its changes through time. Subfossil trunks of pines have been found beyond the present occurrences of this species and those findings give concrete evidence of its former wide mid-Holocene distribution area. The pine limit has retreated during the past 5000 yr indicating a 2°C lowering in temperatures. Trunks of old pines have been preserved in large quantities in small lakes of northern Lapland and they constitute a valuable source of information of past climate. Discs cut from the subfossil logs are well suited for dendrochronological studies. It is shown by several studies that the radial growth of present forest-limit pine is strongly determined by the summer temperatures. Thus a long pine chronology can be used to reconstruct time series of past summer temperature variations. A 7600 yr long tree-ring chronology has been constructed, but it still has uncertain correlation between the ring-widths around 400–100 B.C. However, there is a 2000 yr long “absolute” chronology, which will eventually be connected to a 5000 yr long “floating” chronology. The tree rings show the interannual variability of temperatures, but the distinction of the low-frequency variability is problematic. Tree-rings indicate that the interannual variability has increased during the latter part of the Holocene, presumably as a consequence of increased instability of the climatic system. Correlations are made with other climatic proxy data, but they have not allowed a consistent picture of the Holocene short-term temperature variations. It is clear that the humidity has increased in northern Fennoscandia during the past 5000–4000 yr. A period of harsh climatic conditions occurred 2500–2000 yr ago. It was an unfavourable time for the growth of forest-limit pine and in that sense it seems to have been a more severe climatic disturbance than the “Little Ice Age” in historical times a few hundred years ago.