Mountain treeline ecotones are transition zones between the closed forest and the upper tree-less vegetation and are considered sentinels of global warming effects on terrestrial ecosystems. Despite their widely recognized relevance in the scientific literature, only 5% of treeline ecology research deals with the spatial component of treeline patterns and processes. Treeline ecotones are constrained by multiple factors acting at different spatial and temporal scales. For this reason, the project OLYMPUS is based on a multi-scale approach suitable for treelines of the Italian Alps and the Apennines, the two major peninsular mountain ranges. OLYMPUS is specifically conceived to assess treeline patterns and processes by comparing their past and current distribution at different spatio-temporal scales. The specific goals are: 1) to detect current forestline and assess its drivers and status; 2) to assess human pressure and land abandonment effects on the treeline ecotone; 3) to classify past and current forms of treelines; 4) to evaluate the role of biotic and abiotic factors on tree establishment and survival; 5) to appraise the influence of microsite and species interaction on tree growth patterns. Current forestline position will be detected through the tree cover density (TCD) European dataset. This polyline will be used for defining the buffer area where the assessment of treeline spatio-temporal changes at a regional-scale will be made. Landsat images of the last 4 decades will be used to infer long-term trends in vegetation dynamics (e.g., photosynthetic activity). The regional-scale
assessment of forestline position and medium resolution treeline spectral changes will allow to select 12 mountain landscapes for a detailed land use/land cover changes (LULCC) and spatial pattern analysis. At each mountain landscape (10-20 km2) we will measure anthropogenic changes occurred in the last 70 years using landscape metrics from aerial images. At each landscape, we will outline the past and current treeline physiognomy and measure its relative shift. A fine-scale assessment of treeline ecotones will include a selection of 6 treeline ecotones (3 in the Alps and 3 in the Apennines) adopting a UAV-based structure-from-motion (SfM) photogrammetric approach. A local land cover and a seedlings position map will be produced using both digital surface model and RGB orthomosaic derived from UAV acquisition at each site. Biotic and abiotic drivers of seedling establishment at the treeline will be analyzed through spatially-explicit statistical analysis. Tree growth performance and dynamics will be assessed by tree-ring analysis, comparing newly established trees/forests against former ones, as detected from previous LULCC analyses. The overall project contribution is its spatially explicit approach crossing different disciplines (remote sensing, landscape ecology, forest ecology, dendroecology, etc…) and at different spatial scales (region-landscape-tree).