La forêt de chênes sur tourbe, à l'île d'Errand, dans les marais de Brière (Massif Armoricain - France), Palynologie et premiers résultats dendrochronologiques [ Growth of an oak forest over peat on Errand island in the Brière marshes (Massif armoricain, France). ]

Résumé En Fr

A great number of «fossil» oak tree trunks were embedded in upper peat on marshes - armoricain) Radiocarbon dating between 3090 ± 65 and 2830 ± 50 BP (1497 - 1171 and 1151 - 896 BC) showed that these remains correspond to an oak forest which grew over peat during a period of maximum drying out related to a regression in sea level. As the presence of this forest is not indicated in pollen diagrams, it is likely that sedimentation did not occur during the period of its growth. The forest could only have existed in such a particularly unfavorable environment when edaphic conditions of relative dryness prevented peat growth for some 400 to 500 years. The presence of this forest attests to the regression of seawater levels during the middle and late Bronze Age, subsequent to the Dunkerke O transgressive phase. Dendrochronologie study of the trunks indicated three phases in the development of the forest. The oak trees were initially pioneers man alder grove and not numerous during the first 50 to 60 years During the next 20u years, they progressively colonized the area. Finally, after another 250 years, the oak forest gradually disappeared over a little more than a century. New growth was not synchronous and occured throughout the colonization phase, thus excluding the possibility of a massive environmental change. Regeneration during the entire period would seem unlikely. It maybe considered that trees are subjected to a law of senescence governed by a negative exponential or a third-degree polynomial. However, this law can be modulated by conditions in the habitat. The results for dendrochronologie dating seem to fit with those for radiocarbon dating. As the first ring measured was dated 1342 BC and the last 998 BC, the trees would have survived to about 950 BC. The pollen study indicates that the ground water level has risen at that time, which would have asphyxiated the oak trees and allowed the alder grove to re-occupy the site.

A l'Ile d'Errand, dans les marais de Bnère (Massif armoricain), de très nombreux troncs de chênes «fossiles» gisent dans la partie supérieure de la tourbe. Ils ont été datés par le radiocarbone entre 3090 ± 65 BP et 2830 ± 50 BP (1497 à 1171 et 1151 à 896 avant J.C.) et ils correspondent à l'installation d'une futaie sur tourbe, pendant une phase d'assèchement maximum de la tourbière, liée à une régression du niveau de la mer pendant les périodes du Bronze Moyen et du Bronze Final. L'étude dendrochronologique de ces troncs permet de reconnaître trois phases dans l'évolution de la forêt. La régénération au cours de toute cette période semble difficile et il peut être admis que les arbres subissent une loi de sénescence, qui suit soit une exponentielle négative, soit un polynôme du troisième degré. Cette loi est modulée par les conditions d'habitat. Les essais de datation dendro-chronologiques pourraient dater le premier chêne mesuré de 1342 avant J.C. et le dernier de 998 avant J.C, les chênes continuant à survivre jusque vers 950 avant J G, avant de mourir asphyxiés dans un nouveau marécage en pleine évolution.