Climate, not land-use, drives a recent acceleration of Larch expansion at the forest-grassland ecotone in the Southern French Alps.

Recent decades have seen significant forest expansions into treeless alpine zones across global mountain ranges, including the Alps, and this is often associated with a complex interplay of climate and land use change. The upward shift of treelines has far-reaching implications for ecosystem functioning, biodiversity, and biogeochemical cycling. However, climate variables alone explain only a fraction of treeline migration, leaving substantial research gaps regarding the influence of non-climatic factors. This study addresses these gaps by combining dendrochronological methods, high-resolution bioclimatic data, and historical land use records to examine treeline dynamics in the southern French Alps. Our results reveal a recent acceleration in tree establishment, which began in the early 2000s, and is not attributable to pastoral abandonment in the 19th century but rather to climate change. We demonstrate that historical land use changes serve as a predisposing factor for tree establishment, while recent climate change has increasingly acted as an accelerator of this dynamic. Despite identifying key climatic factors, such as thermal indicators and growing season length, that contribute to treeline shifts, our study highlights the need for further research to disentangle the specific drivers of tree recruitment and survival, particularly in the context of ongoing climate change.