Hervé Cubizolle

Postglacial recolonization in a cold climate specialist in western Europe: patterns of genetic diversity in the adder (Vipera berus) support the central-marginal hypothesis

Understanding the impact of postglacial recolonization on genetic diversity is essential in explaining current patterns of genetic variation. The central–marginal hypothesis (CMH) predicts a reduction in genetic diversity from the core of the distribution to peripheral populations, as well as reduced connectivity between peripheral populations. While the CMH has received considerable empirical support, its broad applicability is still debated and alternative hypotheses predict different spatial patterns of genetic diversity. Using microsatellite markers, we analysed the genetic diversity of the adder (Vipera berus) in western Europe to reconstruct postglacial recolonization. Approximate Bayesian Computation (ABC) analyses suggested a postglacial recolonization from two routes: a western route from the Atlantic Coast up to Belgium and a central route from the Massif Central to the Alps. This cold‐adapted species likely used two isolated glacial refugia in southern France, in permafrost‐free areas during the last glacial maximum. Adder populations further from putative glacial refugia had lower genetic diversity and reduced connectivity; therefore, our results support the predictions of the CMH. Our study also illustrates the utility of highly variable nuclear markers, such as microsatellites, and ABC to test competing recolonization hypotheses.

DIATOM AND POLLEN EVIDENCE OF HOLOCENE VEGETATION DYNAMICS AND HUMAN IMPACT ON THE VIRENNES FENLAND (FRENCH MASSIF CENTRAL)

The main focus of this study is the effect of human activities on paleoenvironmental changes in the Virennes fens during and after the Neolithic Period as indicated by diatoms and pollen. The site, located in the Livradois Mountains, was selected because of its location within the upper catchment of the Loire Valley, where forests were common before the arrival of the first farmers. Human activity in the area dates back to the fourth or third millennium BC. At first, humans seemed to be exploiting the wooded area as there are no signs of agriculture. Starting in the Iron Age and continuing into the Middle Ages, deforestation occurred in several steps as human populations expanded agricultural activity. It was only during the Iron Age that diatoms started being preserved in the fen. This corresponded to a period of increasing moisture in the fen. At the end of the Middle Ages, the forest expanded, agriculture almost disappeared from the area and maximum water depth was recorded in the fen. During the Modern Time, a cyclic pattern of agricultural expansion and deforestation followed by reforestation and reduction in agricultural land-use was observed. Recently, the area has been converted to pasture lands while fields have been replanted with fir and beech.

Holocene evolution of two ponds on the Devès Plateau in the French Massif Central

The siliceous sub-fossil content (diatoms, chrysophyte cysts, sponges and phytoliths) of two cores was studied to determine the evolution and environmental changes that have occurred since the formation of two fenland lakes, Lac de Collanges and Lac de Freycenet, on the Devès Plateau, Massif Central, France. Cluster analyses determined eight siliceous zones, whereas a detrended correspondence analysis showed similar changes occurring in both sites, and principal component analysis identified four major shifts occurring over time at both sites, corresponding to the establishment of the fenland, the development of a pond and changes in the terrestrial environment. Four classic pollen zones previously determined for the French Massif Central were recorded in the core. The Boreal and beginning of the Old Atlantic period (9445–9250 cal bp) are marked by the development of the fenland, a decrease in pine trees and loss of grassland. The siliceous component was dominated by chrysophyte cysts indicating a cooler and lower trophic level. The Atlantic period (8365–7852 cal bp) saw maximum expansion of the fenland and the first occurrence and dominance of Aulacoseira perglabra (Østrup) Haworth, indicating higher water levels as a result of climate change. Increasing numbers of sponge spicules and the decrease in A. perglabra showed a shift to dryer conditions. During this phase, greater and more frequent droughts occurred in Collanges compared with Freycenet, most likely due to its smaller drainage basin. With the Subboreal, around 5000 cal bp, a change in conditions occurred: A. lacustris (Grunow) Krammer became the most important centric diatom and phytoliths became an important component. Pine trees increased and total herb and grasses became more important. The increase in total herb and grass pollen, along with the major increase in phytoliths, indicated an increase in human agro-pastoral activity within the area. The Subatlantic saw a decrease in water levels with periods of desiccation. A cyclic pattern of wet and dry phases was documented by a diatom increase in regularly moist and mostly moist subaerial environments. Including counts of phytoliths, sponges and chrysophyte cysts with diatoms allows better interpretation of the changes occurring over time within the area and permitted determination of the arrival of human agro-pastoral activity. The other three groups, namely phytoliths, sponges and cysts, were particularly useful when certain samples contained very few or were totally devoid of diatoms.