Belgeo (Sep 2006)
Variabilité des enregistrements polliniques en montagne et reconstitutions paléoécologiques
Abstract
Pollen study of five sedimentary sequences cored in the vicinity of Annecy (Lac d’Annecy 450 m, Poisy 490 m, Les Rondets 940 m, Bouttecul 1200 m, Plateau des Glières 1500 m) shows the variability of pollen records in a small geographical area. Two comparisons display the multiple parameters affecting pollen records. The comparison Lac d’Annecy/Poisy concerns a great site with tributary and a small endoreic site at the same altitudinal belt, whereas the comparison Poisy, Rondets, Bouttecul, Glières concerns similar endoreic sites at different altitudinal belts.The first comparison illustrates the best record of easily wind dispersed taxa (Pinus, Quercus, Abies, Juglans) in the great site. The best record of Fraxinus, Ulmus, even Alnus glutinosa/incana after 6000 BP in the small site reflects the location of those taxa around the site. In the two sites, high Tilia values, a poorly wind dispersed taxon, indicate the large regional distribution of the trees. The difference in the pollen source area recruitment linked to basin characteristics is confirmed by Abies curve which shows in the great site a second peak not recorded at the small one. This constitutes a first impediment to the climatic translation of pollen record in montanous areas characterized by high local variations.The second comparison of small basin at different elevation illustrates the vegetation cover expansion out of phase at higher altitudes. At the first step of the Holocene, Betula and Pinus colonized belt below 1500 m. Around and after 9000 BP Corylus, Quercus, Ulmus, Tilia extended up to 1500 m. An altitudinal gradient is clearly evidenced by Corylus values decreasing with the altitude. Pinus is then present above 1500 m. On and after 8000 BP Abies is recorded at each site. Abies spread has been stopped at 5900 BP at the lowest site where a vegetation cover similar to the present one has developed. At higher altitude Abies expanded until the large development of Picea around 4000 BP. Fraxinus and Alnus percentages are higher at hill belt and Tilia at montane belt up to 1200 m. From about 6000 BP the effects of the different human uses at different altitudes have been added to those altitudinal gradients.
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