Vegetation dynamics during lateglacial and early holocene in the middle Rhone valley inferred from charcoal analysis [Dynamiques de végétation au tardiglaciaire et à l'holocène ancien en moyenne vallée du Rh?ne d'après les données anthracologiques]

摘要

Large-scale rescue excavations prior to the construction of the TGV (Train à Grande Vitesse) Mediterranean railway have considerably increased the corpus of palaeoenvironmental data concerning the middle Rhone valley (latitude 44/45° degrees North). The establishment of a regional chrono- stratigraphical framework, based on about 200 ~(14)C dates as well as on the systematic analysis of botanical remains, has greatly improved our knowledge of socio-environmental dynamics in the alluvial plain during the last fifteen millennia. The vegetal landscape of the last hunters-gatherers is documented by "off-site" data (phytoliths, charcoal from palaeofires) that complete the archaeobotanical corpus previously obtained from archaeological sites. The Postglacial forest progression is well recorded, in particular concerning its first stages. Key-species such as pine, Rhamnus/Phillyrea and sclerophyllous oak are distinguished. These pioneer taxa are notable as they also play, a few millennia latter, an important but very different part in the regressive series that characterise anthropogenic vegetation formations. On the contrary, some taxa (juniper, hazel) usually considered as typical of the early stage of Holocene forest settlement are underrepresented in the anthracological spectra of the middle Rhone valley. Deciduous oak formations settle precociously, probably already during the Late Glacial interstades. Later they react differently to climatic disturbances according to their state of maturity. They seem to regress during the younger Dryas, to the benefit of a revival of pioneer communities. On the contrary, the so-called "8.2 kyr event" has no detectable impact on the taxonomic composition of the anthracocenoses, even though it is well recorded in phytolith spectra.