This work reconstructs the history of local landscape at two sites located in the Champsaur Valley (French Alps), namely Lac de Faudon (1577 m asl) and Laus des Combettes (1175 m asl), during the last 3400 years. Here we propose a multidisciplinary approach to explain complex human/climate relationships and their effects on the evolution of biodiversity over time scales that go beyond human life span. Modern pollen data and vegetation surveys from 49 sites, selected within different environmental and land-use contexts in the Champsaur Valley, were used to create a pollen-based transfer-function to quantify pasture pressure. Its application to the two well-dated pollen sequences, covering the last 3400 and 2000 years respectively, allowed reconstructing the evolution of pasture-pressure through time. The pollen-based reconstructions were compared with changes in percentages of palynological and NPP pastoral indicators like spores of coprophilous Fungi, showing a good correlation but differences in the inferred intensity of the pastoral pressure. Palaeoecological data, in consistence with archaeological and historical evidences, underline a continuous human presence surrounding the two sites since the Antiquity. Pollen diversity reaches its highest value during the Medieval Warm Period (850-550 cal BP), when demography increase and stability of human presence are reported. An unprecedented fall of diversity is recorded at the beginning of the climate deterioration of the Little Ice Age (650 and 380 cal BP), coinciding with invasions from neighbouring human groups and wars and with a reduced pastoral activity at both sites. Data suggest that the interaction between human activities and climate changes produced important transformations in the composition of the local flora, resulting in a weakened ecosystem highly dependent on Human cares and more sensitive to climate variability. Keywords: palynology, late Holocene vegetation history, human/environment relationships, pasture pressure modeling, diversity indexes.

Abstract of an Oral Presentation or a Poster