Altitudinal variation in the response of the vegetation to Late-glacial climatic events in the northern French Alps.

Pollen and macroscopic plant remains were studied at eight neighbouring sites, in order to define Late-glacial vegetational history in the northern French Alps. Local pollen assemblage zones are described and indicate that the main elements of vegetational history combine the features of adjacent regions. Herb pollen assemblages are similar to those recorded in the Swiss Late-glacial, whereas the early occurrence of shrubs and trees is typical of the southern French Alps. An early effect of altitude on vegetation distribution is demonstrated at the end of the Oldest Dryas. At that time the landscape consisted of a patchwork of Arteimisia-Chenopodiaceae steppe, with shrubs in hollows and valleys. Betula was recorded up lo 700 in and Juniperus up to 1300 m. This altitudinal variation became more pronounced with the arrival of new species. During the Alleröd, mixed stands of Betula and Pinus were present up to an altitude of about 1000-1300 m, with Betula reaching 1700 m, this limit depending on factors such as exposure and continentality. The Younger Dryas oscillation is clearly recorded, as at more oceanic N.W. European coastal margins. Nevertheless, the record of the vegetation response to climatic change depends on the elevation of the studied sites. The limit of Pinus stands fell to 700-1000 m. At a lower altitude the climatic signal is weaker and alders [Alnus glutinosa (L.) Gaertn, Alnus incana (L.) Moench] began to spread. Continuous occurrences of alder owing to regional pollen transport are detected up to the highest sites. The empirical limit of Alnus is therefore proposed as a biostratigraphic marker of the Younger Dryas in the area. Finally. The possibility of a climatic deterioration that started during the late Alleröd is discussed on the basis of absolute chronology.