Feedback effects of clonal groups and tree clusters on site conditions at the treeline: Implications for treeline dynamics

Abstract

The article examines interactions between closely spaced clonal tree groups (spruce, fir, mountain birch) and dense tree clusters and their environment with special regard to Northern Europe, the Alps, and the Rocky Mountains. Modification of wind velocity and direction, trapping blowing snow and eolian dust, ground-shading and the release of sensible heat to the immediate vicinity ('black-body effect') are the main direct feedback effects of grouped trees on site conditions. Under long-lived clonal groups, soil physical and chemical properties gradually change. Clonal groups also leave their marks in geomorphic micro-structures. Compact groups may facilitate establishment of tree seedlings by providing shelter. Under global warming, the existing clonal groups above the forest limit may act as 'new' sources of viable seed that facilitate infilling of gaps in the treeline ecotone and the establishment of new tree populations above the present tree limit. On the other hand, increased deposition of snow as a result of the presence of tree groups may affect seedling establishment, and thereby overrule the positive effects of climate warming. If climate warming continues, current spatial structures, surface roughness, albedo, and snow distribution pattern, along with the resulting effects of these factors, will change across the entire ecotone. The feedback effects of closely spaced clonal groups will spatially overlap. Thus, it will be hard to distinguish between the effects of the single groups when new trees become established within the existing gaps. The northward shift of the polar treeline ecotone might contribute to heating of the lower atmosphere. On steep high-mountain slopes, however, no similar effects can be expected because of the limited size of the lower alpine area that has the potential to become covered with new forest and tree stands.