Evolution of vegetation landscapes during the Holocene in the central and downstream Loire basin (Western France)

Abstract

Nine sites along the downstream and middle section of the Loire river at Cordemais (Loire Atlantique, France) situated in the estuary to Saint-Nicolas-de-Bourgueil (Indre et Loire, France) were investigated. Interdisciplinary studies combined palynology, geography, archaeology, sedimentology and history, thus enabling us to reconstruct the evolution and the long-term response of the Loire valley ecosystem to natural variations and anthropogenic pressure from the Preboreal to the present in an integrated manner. The Atlantic marine transgression (between 7000 and 5000 B.P.) caused the level of brackish water to increase at Oudon between 6740+205/−200 and 5010+115/−100 B.P. This phenomenon, which was the first of its kind to be detected near the central Loire region (approximately 80 km from the current mouth of the Loire river), was accompanied by the development of subhalophile vegetation (Chenopodiaceae) and the appearance of dinoflagellate cysts. A regressive phase occurred during the Subboreal, about 4500 B.P., and led to the erosion of most of the estuarian sediments and to the disappearance of plant species linked to salinity at Oudon: peat deposits built up at most other sites. Human activities had an early effect; moderate deforestation took place at Champtoce about 6600 B.P. as farming was already orientated towards rearing animals. However, possibly cultivated plants were present towards the middle and the end of the Neolithic period (wheat, rye, buckwheat, flax) at about 5600 B.P. and chestnut and walnut were probably exploited in the Loire valley region at about 4600 B.P. The Bronze Age seemed to mark a phase when societies settled down (planting of vineyards) and deforestation peaked from the Gallo-Roman period onwards. The textile industry (flax, hemp), in the context of crop rotation set up during the Iron Age, developed rapidly during the Middle Ages, whereas nowadays the rearing of animals is the dominant activity in the Loire Valley, following the introduction of maize into the region in 1950.