Spatial changes of the treeline in the Pyrenees: understanding interactions between climate change, anthropogenic mutations and site effects – SpatialTreeP

The upper limit of the subalpine forest represents a major landscape discontinuity, whose the spatial dynamics is associated with multiple scientific issues. As a marker of environmental change, a biodiversity hotspot and a key factor in the mitigation of gravity hazards and soil erosion, this ecotone is a particularly interesting scientific object. Nevertheless, the richness of this object is reflected in a certain complexity that the current state of knowledge does not yet allow for a perfect understanding. The dynamics of the ecotone is characterised by strong spatial heterogeneity and a diversity of ecological processes (regression, progression, densification without progression, changes in form, etc.). This heterogeneity reflects the multidimensional, complex (integrating interactions) and multiscale nature of the factors at work. By focusing on the Pyrenees, a geographically heterogeneous and highly anthropised mountain range, this project aims to unravel these complex structures by favouring an integrated, interdisciplinary and spatially explicit framework. The main research hypothesis is to assume that site effects play a primary moderating role in the relationship between climate change and ecotone change, and are at the origin of a spatially differentiated hierarchy of the determinants of ecotonal dynamics, beyond the classic dichotomy confronting anthropic and climatic factors at the regional scale only. The originality of the project lies on the one hand in its structure based on the articulation of geographical scales. WP1 consists in the constitution of a regional scale database quantifying the evolution of the treeline (since the 1950s) and its factors, largely based on geomatics, from observation plots spread all along the treeline (previously digitised through automatic image classifications). The WP2, articulating regional and local scales, will consist in computing the quantitative spatial analysis of this database and will allow the identification of 20 sites of high scientific interest. These 20 sites will then be studied through in-depth analysis in WP3, allowing the collection of local data that cannot be discerned or quantified at a regional scale. These local analyses will aim to provide a reconstruction of the anthropogenic trajectories likely to have interfered with the ecotonal dynamics on these sites, as well as a series of ecological measures to characterise the habitats and their functional role in the evolution of the treeline. The other strength of the project is the multidisciplinary nature of the consortium, which was set up to meet these objectives. The team includes young and more experienced researchers, mainly in human and physical geography, environmental history and ecology. All the results of this project (evolution of the Pyrenean treeline and identification of its factors at different scales) will be disseminated outside the academic world, in particular through an online interactive mapping. This dissemination and the identification of sites of high scientific interest will make it possible to propose locally targeted recommendations to public decision-makers (national and regional parks, ONF, local authorities) for better management of these high mountain areas with strong scientific stakes.