Long-term trajectories of mountain agro-ecosystems in the North-Western Alps

Past trajectories of alpine agroecosystems are legacies that we should consider to improve our understanding of current responses to global environmental changes. By integrating archaeology, history and lake sediment-derived ancient DNA records, we reconstructed the nature and intensity of agropastoral activities in the Northwestern French Alps. We investigated their spatiotemporal trajectories and interactions with the erosion dynamic and vegetation cover, between 880 and 2440 m a.s.l. Climatic conditions in the upper altitude case study site predominantly controlled plant community composition and erosion. In contrast, the other sites were influenced mainly by (agro-)pastoral activities. There, the first significant human-driven changes in plant community composition occurred earlier (Mid-Late Bronze Age or Iron Age) above ca. 2000 m a.s.l. than below (Early to High Middle Ages). This pattern mirrors the altitudinal temporal trajectory of (agro-)pastoral activities development. Regarding erosion trajectories, all sites show stable non-degraded and degraded states separated by a transition phase caused by (agro-)pastoral activities. However, this does not necessarily occur when the first significant or the most intense activities develop due to the different sensitivities of the systems. From the High Middle Ages or Late Middle Age/Modern Period, resistance or resilience phases relative to erosion are evidenced for all agroecosystems (other than the upper site). These patterns highlight adaptations of agrarian socio-ecosystems, possibly thanks to soil management strategies with soil protection and/or just being the consequence of changes in practice for other purposes. Whether in terms of plant community composition or erosion, our trajectories evidence the vulnerability of systems above ca. 2000 m a.s.l. to changes in human activities and variations in climate.