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Comptes Rendus Biologies
Volume 333, n° 5
pages 424-428 (mai 2010)
Doi : 10.1016/j.crvi.2010.01.002
Received : 20 October 2009 ;  accepted : 6 January 2010
Écologie / Ecology
Expansion of green alder (Alnus alnobetula [Ehrh] K. Koch) in the northern French Alps: A palaeoecological point of view
 

Fernand David
Centre européen de recherche et d’enseignement en géosciences de l’environnement, Cerege (UMR6635 CNRS), europôle de l’Arbois, université Paul-Cézanne, BP 80, 13545 Aix-en-Provence, France 

Abstract

Pollen analyses were undertaken on a small peat bog (Ecuelles 06° 49′ 41″E, 45° 58′ 49″N, 1855m asl ), located on the Anterne mountain (Upper-Arve Valley, French north-western Alps). The study highlights the role of green alder (Alnus alnobetula [Ehrh] K. Koch) in Holocene vegetation dynamics of the nowadays treeless subalpine belt. At this place, the onset of human perturbation caused a retreat of fir and arolla-pine stands and an expansion of green alder, which consequently dominated the landscape from 3700 up to 1965 cal. BP. After 1965 cal. BP, the clearings led to the present grasslands with few ligneous species (spruce, larch) on inaccessible cliffs or green alder on the edges of torrents or in avalanche corridors. Picea percentages have increased after 3900 cal. BP, but, due to human activities, spruce has never constituted large stands in the study area. The present general expansion of green alder is due to the decreasing human impact and it constitutes the first step of re-afforestation that should lead to mixed stands of spruce and arolla-pine. The study gives a new evidence of the past diversity of the vegetation cover and do not support the idea that green alder colonization at the subalpine belt constitutes a long-term risk for the vegetation diversity.


Keywords : French North-Western Alps, Holocene vegetation history, Green alder, Human impact, Deforestation, Bronze-Iron Age


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Introduction
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