A clustered arrangement of trees may be most appropriate in high elevation afforestation in order to take into account the variety of favourable and unfavourable microsite conditions and to create diverse stands with different tree species and a pattern of clusters and gaps. Long-term studies of the ecology of subalpine afforestations have shown that the young trees tend to be threatened over decades by fungal diseases, browsing ungulates, adverse climatic conditions, and snow movements. The intensity, duration and scale of the threats, however, varies greatly according to tree species and microsites.
Based on these ecological findings, favourable and unfavourable microsites are described. An adapted system of clustered afforestation (in German: Rottenpflanzung) has been developed and tested. On favourable microsites, 'small collectives' were densely planted with 20-30 trees of one species on the assumption that the crowns of the trees within these small collectives would close within 5-10 years. Three to six small collectives were arranged in such a distance that they would merge into a larger tree 'cluster' (in German: Rotte) after a few decades. The distance between the tree clusters was kept large enough to ensure they remained separate throughout stand development. Such stands of clusters and gaps are expected to be, when mature, more resistant to high winds and heavy snow loads and less threatened by insects than uniform stands. They should be able to contain extensive inner margins permanently, and to be pervaded by Light and warmth. Cluster planting on favourable sites can be used to minimise seedling mortality and is therefore the best strategy for afforestation at higher elevations.