Trees in the subalpine belt since 11 700 cal. BP: origin,expansion and alteration of the modern forest

High altitude alpine ecosystems are likely to be highly sensitive to future climate change. Understanding long-term tree stand dynamics may be a key requirement for forecasting such changes. Here, we present a high resolution record of paleobotanical macroremains covering the last 11 700 years, from a small subalpine pond situated in the inner French Alps, at 2035 m a.s.l. The early presence of larch (Larix decidua), arolla pine (Pinus cembra) and birch (Betula) at this elevation, just after the end of the Younger Dryas cold transition, suggests the occurrence of either glacial tree-refugia located nearby in the northwestern Alps, or a previously unrecorded early and rapid tree migration. The 8200 cal. BP cooling event is characterized by a rapid and limited expansion of mountain pine (Pinus mugo/uncinata type). Mixed stands of larch, birch and arolla pine established at 8300 cal. BP and were present through the mid Holocene. After the Holocene climatic optimum, at 5600 cal. BP, recurrent fires led to the development of highly dynamic and more diversified forests, with larch, birch, arolla pine, mountain pine and fir (Abies alba). Natural and anthropogenic disturbances, e.g., fires, avalanches, slash-and-burn and other agricultural practices, influenced subsequent vegetation until the last millennium when tree-pasture established around the lake. The data indicate that the vegetation was progressively dominated by open larch woodland from 4000 years ago, and was clearly established during the Middle Ages (1250 cal. BP) up to the nineteenth century, when land began to be abandoned. The modern vegetation, dominated by larch and arolla pine and resulting from land abandonment, tends to resemble the communities that occurred from 8300 to 4000 cal. BP, before the postulated anthropogenic alteration of subalpine forest ecosystems. The plant macroremains analysis provides a unique and precise record of stand-to-local vegetation composition and dynamics that can bridge paleoecology and forest management.