Mountain ash (Sorbus aucuparia) forests of the Central and Southern Alps (Grisons and Ticino, Switzerland – Prov. Verbano-Cusio-Ossola, N-Italy): Plant ecological and phytosociological aspects

Mountain ash (Sorbus aucuparia) is widespread in Europe from sea level to the timberline and reaches its most northern range in N-Norway at 71 °N. Sorbus aucupariaoccurs on acid, dry to moist and mesotrophic to oligotrophic soils. The species often grows in secondary forests together with Alnus viridis, Sambucus racemosa, Betula pendula, Frangula alnus and some Salix spp. In most climax forests mountain ash grows only in the shrub layer. In the subalpine belt, the tree is present in green alder scrub and European larch-Swiss stone pine forests. On some N-exposed mountain slopes of the S-Alps, small mountain ash forests with green alder build the upper forest limit above 1500 m. In this study, ecological and phytosociological aspects of this particular forest community along a N-S-transect from the Swiss Central Alps (Grisons) to the South Alps (Ticino/ N-Italy) were investigated. We consider site conditions, local spread, rejuvenation, tree age structure, infestation and phytosociological aspects of S. aucuparia. The occurrence of A. viridis in the N-S-transect and the role of S. aucupariain the secondary and climax forests have been investigated. While in the green alder scrub (Alnetum viridis) of the North single mountain ash trees are present, alder-mountain ash forests have been found at the S-Alpine sites with S. aucupariacovers 25–80%. Here, the Alnetum viridis is absent, and A.viridis is element of the mountain ash forests. In the montane and lower subalpine zone, S. aucupariawill often be replaced later by beech or other climax trees. Above the upper beech limit, we consider the green alder-mountain ash forests as climax forests. At the driest forest sites, grass species are pre-dominant – mainly Calamagrostis spp. and Avenella flexuosa. Based on the earlier provisional phytosociological name of S-Alpine green alder-mountain ash forests, we suggest the name Alno viridi-Sorbetum aucupariae Hari, Leisinger et Zysset 1993 – according to the first description by HARI et al. (1993). Based on our vegetation records of green alder-mountain ash forests, we propose a new sub-association Alno viridi-Sorbetum aucupariae calamagrostietosum subass. nov.