The impact of elevation, topography and snow load damage of trees on the position of the actual timberline on the fells in central Finnish Lapland

Abstract

This study examines the impact of selected environmental variables on the position of the limit of continuous forest (actual timberline) on three fells situated in central Finnish Lapland, namely Aakenustunturi, Yllästunturi and Pyhätunturi. The factors studied are elevation, topography and snow load damage to coniferous trees. The potential for ascending timberlines in the region is discussed. The limit of continuous forest in the study area is hardly ever composed of a single species but rather features alternating occurrences of spruce, pine and mountain
birch. The average altitudinal position of the actual timberline is highest on the southern and western slopes (366–428 m a.s.l.), while the lowest altitudes are recorded on the northern and eastern slopes (336–403 m a.s.l.). Prevalence of block fields and slope gradient are the most significant and visible factors controlling the position of the actual timberline. On the studied fells, from 10% to more than 50% of the total length of the actual timberline is controlled by block fields. The lowest occurrence of the limit of continuous forest, at 270 m a.s.l.,
results from an extremely high proportion of block fields and steep slopes. Snow patches and delayed snow melting also hamper the upward advancement of the forest. Continuous forest reaches its highest altitudes at 440 m a.s.l. on slopes
with gentle inclination and land cover other than block fields, and 460 m a.s.l. in sheltered furrows. Average snow load damage to spruce and pine at the actual timberline varies in the ranges 15–58% and 30–76%, respectively. Snow load damages on trees undoubtedly impede the advance of the actual timberline to higher elevation in all exposures. Seedling density (number/ha) on south-facing slopes near the actual timberline is ca. 5 times greater than on the north-facing slopes. Recent regeneration and seedling establishment is almost absent at
the treeline near the fell tops. Results suggest that there is potential for actual timberline advance on south-facing slopes with gentle inclination and missing block fields. It is uncertain, however, whether natural regeneration will take place on felltops with few or no trees.