Abstract
Mountain ecosystems, and in particular European Alpine ecosystems, have a rich biodiversity, as they represent complex associations controlled by elevation, soils and rocks, and climatic conditions following latitude, longitude, slope orientation and aspect. Vegetation belts are organized with respect to altitude according to a concomitant decrease in temperature and increase in precipitation, and are defined by their dominant plant elements. Plant diversity within the Alps also results from these mountainsÕ location at the transition point between cold"temperate and warm"temperate climates, as well as from their historical biogeography. In order to explain how past environmental changes have shaped the modern plant diversity and the organization of vegetation in altitudinal belts, a temporal dimension may be provided by paleovegetation data. Abundant micro" (pollen) and macro"remains (leaves, fruits, seeds) show that European Cenozoic vegetation exhibits a similar latitudinal and altitudinal organization to the vegetation belts observed today in south"eastern China, where most of the taxa!Ð!missing in Europe since the late Neogene!Ð!now occur. Late Eocene to Pliocene pollen floras of the south"western and Eastern Alps are characterized by the presence of megatherm plants and an abundance of mega"mesotherm plants. These are typical of moist evergreen low"altitude forests, and are characterized by the presence of Cathaya, a conifer now restricted to subtropical China at mid to high elevations. Pollen data indicate the presence of Abies/Picea forests at high altitudes since at least the Oligocene. Since then, several thermophilous taxa have declined, and some have disappeared from the Alps due to natural or human" related processes that are responsible for the modern plant diversity pattern there. These processes include the uplift of the mountain range, which began ca. 35 Ma, late Neogene global cooling, Pleistocene glacials/interglacials and more recent human impact.