GIS-analysis of tree-line elevation in the Swiss Alps suggests no exposure effect

Counter intuition, an analysis of tree-line position across the Swiss Alps based on a geographical information system (GIS) with a spatial resolution of 100 to (2.5 million points) revealed no difference in climatic tree-line altitude with slope exposure. Through step wise discrimination procedures our analysis accounts for anthropogenic tree-line depression. Any land cover bias affects the frequency of GIS-points corresponding to tree-line forests rather than the mean elevation of such points, captured by our analysis. We explain this phenomenon (1) by the absence of significant drought effects in the Alps (no disadvantages for southwest slopes), (2) by the fact that tree tops, unlike low stature vegetation, do not profit from greater radiation warming on south slopes during the growing season but are thermally coupled to free air circulation, and (3) by preliminary data for root zone temperatures during the growing season, which do not differ between south and north slopes, as long as the soil is screened by a closed forest canopy. The overall difference in season length and snow cover, often seen between south and north slopes, does not seem to affect tree-line position but explains greater natural forest fragmentation on north slopes. It is this greater fragmentation and patchiness (avalanche tracks, snow beds etc.) which seem to have nourished the idea of a generally lower limit of tree growth and tree lines at northern slopes. These results are in line with a recently developed theory, which suggests that tree-line elevations in humid climates correspond to similar isotherms, irrespective of latitude and thus, season length.