Fine-scale tree cover changes in Switzerland in 40 years and their impacts on plant biodiversity

To understand the future of forests, we must have a detailed understanding of past changes due to climate and land use change. And woody encroachment can have important consequences for carbon stocks and mountain biodiversity. The abandonment of agricultural activities results in woody encroachment, converting species rich mosaic habitats like dry meadows, bogs, and fens into expanding forested areas. These habitats, which are particularly important for biodiversity conservation in Switzerland, face imminent threats. Woody vegetation encroachment not only alters habitat structure but also can drive declines in vascular plant species due to associated effects, such as increased shading or altered competition. We quantify cover and height of woody plants in Switzerland during a 40-year period. Our data are based on a high-resolution Vegetation Height Model (VHM) with 1 meter resolution for four time steps between 1979 and 2020. Our research addresses this complex relationship by utilizing 40 years of high-resolution vegetation height data (1 meter) across Switzerland. For plant biodiversity data, we use a comprehensive dataset comprising over 4,000 vegetation surveys from the effectiveness of habitat conservation monitoring (WBS) in Switzerland. We find that although the greatest increases of woody shrub encroachment are occurring along the southern Alps in Ticino and Graubünden, it is also occurring in habitats in the central and northern Alps at a slower pace. We see both infilling with trees in already existing semi-open forest landscapes as well as upward tree and shrub migration at and above treeline. We found this has significant consequences on habitats of conservation concern, where plant species diversity decreases even at the initial stages of encroachment.  When plots contained tree species even less than 0.5 m tall, plant species richness was 50% lower than in plots without tree species. By analyzing the drivers and consequences of agricultural abandonment, our project aims to offer scientific insights for mitigating the adverse impacts on biodiversity associated with land use changes.