Mountain ash forests of the Central and Southern Alps: Grisons-Ticino (Switzerland)-Verbano-Cusio-Ossola (North Italy)

Abstract

Mountain ash (Sorbus aucuparia) is widespread in nearly whole Europe from sea level to timberline. On north exposed peaks of some mountains of the Southern Alps (Ticino/ Switzerland and North Italy/ Verbano-Ossola) small mountain ash forests build the forest limit above 1500 m, so at Monte Lema, Monte Morissolo, I Balmit and Mottarone. Mountain ash often occurs on former forest clearings as pioneer tree as secondary plant succession. On some sites, mountain ash will be replaced later by beech as climax species. At all sites, we recognized a weak or missing generative mountain ash reproduction. One reason for that could be the dense grass cover of small-reed, Calamagrostis spec. which inhibits the establishment of rowan seedlings; other reasons could be insect and game damage, mycosis and other diseases. Vegetative propagation like root bulbils, suckers and shoots from stump are important survival strategies of mountain ash. So, at some sites have been recorded a high share of polyphyletic single young trees. At all sites, count of annual rings of 25-50 mountain ash trees revealed an average age of 40-55 years. Green alder and mountain ash often occur together within the Alnetum viridis as well as in the mountain ash forest. In the green alder bush, mountain ash occurs only as single trees, whereas Alnus viridis can dominate in both plant communities. At the southernmost sites, there is a marked weak abundance of green alder due to dryer climate. Within mountain ash forests of the Southern Alps, the typical tall perennial herbs of the green alder bush are more or less absent because of low rainfall during summer and possibly absence of green alder. At the driest sites grow more grass species, mainly small-reed (Calamagrostis spec.). Based on our vegetational surveys of mountain ash forests at the sites mentioned above, we suggest a new sub-association Alno viridi-Sorbetum aucupariae calamagrostietosum prov. of Alno viridi-Sorbetum aucupariae prov. or eventually a new association Calamagrostio-Sorbetum aucupariae prov.