Areas of 2,800-3,000 m a.s.l. represent the alpine-nival ecotone in the Alps. This transition zone connecting the closed swards of the alpine belt and the scattered vegetation of the nival belt may show particularly strong climate warming driven fluctuations in plant species richness compared to the nival belt. To test this hypothesis, 12 summits within this range were investigated in the canton of Grisons, Switzerland in 2004. Complete lists of vascular plant species consisting of 5-70 species were collected on each summit and the elevation of the uppermost occurrence of each species was recorded. These data were compared to historical records over 120 years in age. Within this time, vascular plant species richness increased by 11% per decade on summits in the alpine-nival ecotone. Despite this considerable change, a comparison with nival summits did not support the hypothesis that species richness increase at the alpine-nival ecotone is higher than in the nival belt. A general trend of upward migration in the range of several metres per decade could be observed. Anemochorous species were more often found to be migrating than zoochorous or autochorous species and migration was higher on calcareous than on siliceous bedrock. A comparison between the summits with the adjacent slopes in our study revealed that changes in species number could be used as an indicator for climate-induced changes-if at all-only for the narrow summit areas.

In the European GLORIA project, 12 summits (treeline to nival belt) were inventoried in three regions of Switzerland: two in the Swiss National Park Graubünden and one in Valais. Vascular plants were recorded in all three regions and bryophytes and lichens were recorded only in Valais. On each summit, vegetation and temperature data were sampled using sampling protocols for the GLORIA project (Global Observation Research Initiative in Alpine environment) on large summit sections and in clusters of four 1 9 1-m quadrats. We observed a general decrease of species richness for all three systematic groups with increasing elevation in the summit sections, but only for vascular plants in the quadrats. In Valais, there was higher species richness for vascular plants than for bryophytes and lichens on the lower summits, but as the decrease in species richness was less pronounced for cryptogams, the latter were more numerous than vascular plants on the highest summit. Vascular species showed a clear shift of the dominant life form with elevation, with chamaephytes replacing hemicryptophytes. Bryophytes and lichens showed a weak trend among the life forms at the summit section scale, but a stronger shift of the dominant forms was seen in the quadrats, with cushion replacing turf bryophytes and crustaceous replacing fruticose lichens. Altogether, these results sustain the temperature-physiographic hypothesis to explain the species richness decrease along the altitudinal gradient: the harsh climatic conditions of the alpine-nival belts act as a filter for species, but the diminishing diversity of microhabitats is also an important factor. Because cryptogams depend more on humidity than temperature and more on smaller microhabitats than vascular plants, the decrease of species richness is more gradual with elevation for bryophytes and lichens. Responsible editor: Sonja Wipf. Electronic supplementary material The online version of this article (Keywords Altitude Á Aspect Á Diversity Á Elevation range Á Life forms Á Switzerland Gradient subalpin-nival de la richesse spécifique des plantes vasculaires, bryophytes et lichens dans les Alpes internes en SuisseRésumé Dans le cadre du projet européen GLORIA, 12 sommets (de la limite de la forêt à l'étage nival) ont été inventoriés dans trois régions de Suisse: deux dans le Parc national suisse (Grisons) et une en Valais. Les plantes vasculaires ont été étudiées dans les trois régions alors que les bryophytes et lichens n'ont été considérés qu'en Valais. Le protocole du projet GLORIA (Global Observation Research Initiative in Alpine environment) a été utilisé sur tous les sommets, avec des inventaires sur de grandes sections sommitales et sur des groupes de quatre carrés de 1 9 1 m. Nous avons observé une diminution générale de la richesse spécifique dans les trois groupes systématiques pour une altitude croissante dans les sections, mais pour les plantes vasculaires seules dans les carrés. En Valais, il y avait davantage de plantes vasculaires que de lichens et de bryophytes sur les ...

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