Environmental factors controlling the position of the actual timberline and treeline on the fells of Finnish Lapland

Air and soil temperatures, snow cover, serious snow load damage to coniferous trees, wind, topography and edaphic factors on the fells situated between 67°N and 68°N in Finnish Lapland are described and their influence on the location of the actual timberline and treeline is discussed. In addition the relation between annual climate conditions and pollen deposition in the timberline ecotone is analysed and the results of seedling density monitoring in the same environment are presented. The potential for the actual timberline and treeline to advance to a higher elevation is also discussed. The field studies were carried out on the fells of Aakenustunturi, Yllastunturi and Pyhatunturi. The average altitude of the actual timberline varies from 370 metres to 402 metres a.s. l. The actual timberline is hardly ever composed of a single tree species but featured alternating occurrences of Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and mountain birch (Betula pubescens ssp. czerpanovii). The mean tetratherms on the southern and northern slopes (+10.3°C and +10.1°C, respectively), the mean maximum tetratherm on the southern slope (+15.1°C) and the corresponding measures for the treeline (460 m a.s.l), the minimum tetratherm (+6.3°C), mean July temperature (+12.6°C), biotemperature (+3.3°C) and minimum effective temperature sum (455 d.d.), coincide closest with the results of earlier studies. The maximum altitudes of the actual timberline are dictated by many climatic factors on southern and western slopes with a gentle inclination, and the forest cover gradually becomes thinner, in which case the actual timberline does not form any easily distinguishable line. The lowest altitudes of the actual timberline are the results of an extremely high proportion of block fields, slope steepness and snow patches on the northern and eastern slopes. On the precipitous and rocky slopes trees have difficulties in taking root and in obtaining nutrients and water, while as a consequence of snow patches the growing season may be too short for tree growth at all, and if trees exist there they are suffering from low soil temperature and parasitic snow fungi. Serious snow load damage to trees evidently hampers any advance in the actual timberline, as do avalanches and mires. The location of the treeline is the result of a combination of a great number of unfavourable conditions for tree regeneration, seedling establishment and tree growth, such as inadequate snow protection, extreme soil temperatures, almost total destruction of trees by the snow load, wind pressure, an often inadequate effective temperature sum and length of the growing season, night frost in early summer, and poor, dry soil suffering from excessive evaporation. Actual timberline responses to predicted climate warming will differ greatly from site to site in relation to the local topography, edaphic features and associated ecological limitations. Any advance in the treeline to a higher elevation is likely to be slower and at least less certain than that in the actual timberline. In addition, advances in the actual timberline and treeline may even be prevented by phenomena occurring along with climate change. A potential key factor in this is serious snow load damage to the trees.