Climate warming accelerates forest encroachment triggered by land use change: A case study in the Italian Prealps (Triangolo Lariano, Italy)

During the past decades, the montane and subalpine belts of many European mountains experienced agricultural land abandonment followed by spontaneous recolonization of trees and shrubs on semi-natural mountain grasslands, potentially leading to severe losses in biodiversity. Here we analyse the spatial and temporal patterns of forest encroachment of a Prealpine study site in Northern Italy (Triangolo Lariano, Como) since 1954 in response to land use change and aim to assess whether this process has been accelerated by recent climate warming and atmospheric CO2 increase. To achieve a detailed reconstruction of forest encroachment dynamics, we adopted a multidisciplinary integrated approach, involving dendrochronological and phytosociological analyses, and vegetation mapping. Available maps updated with aerial photographs allowed the delineation of the forest distribution and changes of forest boundaries. Accordingly, along nine elevational transects, 60 plots were set within different forest-grassland successional stages in order to integrate vegetation mapping with phytosociological analyses and reconstruct the recruitment rates through dendrochronological analyses. Since 1954, the forest encroachment process occurred across an elevation range of 185 m, at a mean rate of 29 m per decade, although with uneven temporal and spatial trends. The forest encroachment rates increased progressively with time, from +0.8%/y (1954–1993) to +2.4%/y (1993–2015) to +4.9%/y (2015–2018). The air warming exhibited an increasing trend too, especially in the last twenty years. Betula pendula was the most abundant species responsible for this process (the only one presenting seedlings and saplings settling along the summit areas) and showed two recruitment peaks: the first in concomitance with the abandonment of the agro-silvo-pastoral practices (1950/60s), but the largest recruitment was detected with the increased air warming since early 2000s. The correlation analysis confirmed that forest encroachment was related to air warming (in particular summer warming), land use change and increase of atmospheric CO2 and that it was further accelerated in concomitance with further air warming in the recent decades. The forest encroachment process on semi-natural grasslands triggered by land use change on European mountains and its recent acceleration in response to climate warming represent an increasing threat to biodiversity, leading to potential habitat and species loss, especially of the most vulnerable habitats