Forestlines in Italian mountains are shifting upward: detection and monitoring using satellite time-series

Abstract. The growing interest on the ecological effects of global warming and land use changes on vegetation, along with development of remote sensing techniques, fostered applied research on the successional dynamics at the upper limits of forests. The aims of this study were: i) to develop an automated methodology for mapping the current position of the uppermost italian forestlines; ii) to identify hotspots of change by the analysis of long-term greenness and wetness spectral dynamics. We carried on a Landsat-based trend analysis in buffer zones along the forestlines, testing differences between sparse and dense canopy cover classes and at different elevations and distances to the forestline. We used regional scale datasets and avoided to fix a minimum elevation threshold, in order to make the method replicable at different mountain ranges. For the spectral dynamics analyses, we used Landsat time-series of common vegetation indices for the period 1984–2023 and tested the significance of their long-term spectral trends with the Contextual Mann-Kendall test for monotonicity. We assessed that the highest forestlines are at the western sector in the Alps, and at the central one in the Apennines. We observed a common increase of the forest cover mainly close to the forestline and at lower elevations. Comparing greenness and wetness indices trends with the current canopy cover, the highest values were respectively in the sparse tree cover class, and in the dense one, particularly in the Alps.