Histoire végétale d'une tourbière littorale : le Marais des Bourbes (Olonne-sur-Mer, Vendée)

Abstract

To the north of the town of Sables d'Olonne, the palaeo-environmental study of the Marais des Bourbes (located 46°33′N, 1°49′W, +4 m NGF) by means of paleopalynology, enabled us to reveal a number of different ecosystems (maritime, continental and anthropic) in this one small peat bog. A study of the salt and fresh water plants shows the alternate influence of marine and fresh water cycles. Successive transgression and regression of the sea levels were revealed demonstrating clearly the oscillating nature of the Flandrian transgression. The regressive phase, begun towards 4800 BP, ended at the end of the Neolithic Age. The Marais des Bourbes was a fresh water wetland, probably consisting of one or several basins of deep, stagnant water, surrounded by sparse alder woodland. It would appear that the beginning of the Bronze Age saw a return of the transgression, a phenomenon causing numerous changes to the landscape. As to the Iron Age, the negative oscillation of the Hallstatt period was followed by a positive pulsation during the la Tène period. The first phase featured the disappearance from the pollen spectrum of all halophytic plants and the second featured the development of species developing in brackish water. The transgression accelerated during Gallo-Roman times and the coastline reached a level close to that of the present day. Peat formation stopped and the depression of les Bourbes was largely occupied by mud flats and brackish wetland. The significant event of the Late Roman Empire and the beginning of the Early Middle Ages was the marked desalination of the environment, which was connected, with the final phase of negative oscillation. The freshwater bog blossomed once again over the peat and progressively took on the aspect of a reed marsh. Lastly, a final positive pulsation began at the end of the early Middle Ages. It would appear that the present level was reached around the year 1000 AD during this new transgression. During the Late Middle Ages, a significant phase of silting took place corresponding to the development of the dune edifice, as it exists today. This silting was the cause of significant changes in the circulation of waters, both fresh and salt. The bog then evolved into peaty grasslands. During the 19th century, the grasslands were progressively invaded by alder groves and willow stands. At the end of the Neolithic and during the Bronze Age, the forest system underwent clearances but these were not accompanied by development of agricultural and pastoral activities. On the other hand, during the Iron Age an almost total eradication of the forest was followed by cultivation of the lands around the peat bog: cereals, probably Brassicaceae, chestnut and walnut. Despite the persistence of elements indicating that cereals, hemp, vines, walnut and chestnut were still cultivated, the advance of the shoreline during the period of the Roman Empire certainly did not favour expansion of agricultural practices around les Bourbes. The Late Roman Empire saw an expansion of all human activities. This upsurge in agricultural activity was probably connected with a drop in maritime influence. But the expansion was only short-lived. Throughout the Middle Ages, numerous phases of clearing led to the formation of a dense network of hedges and trees (bocage). Although at the beginning of the Middle Ages human influence waned, the founding of the port at Sables d'Olonne in 1218 and the implementation of feudal authority along with the fallow land system boosted the economy and agriculture of the region (trade, fishing, production of salt and cultivation of cereals, hemp, buckwheat and vine). The Modern period was first of all marked by considerable pressure on the forest ecosystem, leading to the formation of a loose bocage system. However, from the middle of the 16th century, the agricultural economy slowed down, probably related to the unrest in the 16th and 17th centuries. From the 18th century, economic and agricultural recovery in the Bourbes region materialised in the form of small clearances; the development of grain production and diversification in the types of plants cultivated (buckwheat, hemp, vines, market gardening). The passage to the sub-actual period corresponds to the development of a pine forest. Indeed, the dune formed in the Middle Ages advanced inland. To halt erosion and fix it in place, the Vendée sub-prefecture ordered that it be planted in pines in 1836. The land surrounding the Bourbes is still farmed (buckwheat, hemp, cereals, market gardening).