Unsuspected prevalent tree occurrences in high elevation sky-islands of the western Alps

The present note intends to challenge, based on field observations, the definition of treeline and argues for considering the upper occurrences of tree-species as an integrative result of historical and contemporary processes acting on high-altitude socio-ecosystems. A field survey of Pinus cembra growing above 2800 m asl was conducted, in an ecoregion of southwestern Alps, straddling France and Italy (Queyras-Mt Viso area). Pines were described (height, density) and their habitats were contextualized (altitude, slope degree and exposure, bedrock). Individuals of Pinus cembra are commonly growing between 2800 and 3200 m asl, generally on east-facing, steep and rocky slopes, with a pine density decreasing with altitude. Pines are generally dwarf-shaped, sometimes erected or prostrated. Their needles are half the size compared to those of pines growing in subalpine forests. Pine morphology indicates harsh growing conditions, notably strong wind, aridity and frost. These habitats constitute visually emerging objects attractive for the nutcracker caching behaviour. This setting acts as a positive passive driver for pine at high altitudes, which is further favoured by intense atmospheric moisture supplied by the east from the Po valley. Steep and rocky slopes exclude livestock, likely explaining why these pines are located in grass-less habitats. These tree clusters constitute remarkable "sky-island" populations. Larix decidua and Pinus uncinata were also recorded at exceptional altitudes but P. cembra is the most frequent species in high alpine conditions. These observations altogether highlight a complex pattern of tree-species line in the western Alps, and probably beyond.