Investigating Old-Growth Forests in Tuscany (Italy): Structural Heterogeneity and Plant Diversity Across Forest Types and Novel Candidate Sites for the National Network

Old-growth forests play a vital role in the conservation of terrestrial biodiversity, though they are rare and increasingly threatened worldwide. The Mediterranean region hosts notable examples of these ecosystems, but information about their location, structure, and biodiversity is still largely incomplete. In this work, we tested the hypothesis that the region of Tuscany (Italy) harbors forest sites with old-growth characteristics in light of the EU indicators and the Italian ministerial guidelines. Accordingly, data on stand structural and plant diversity variables were collected in 27 plots located in pre-selected sites across different forest types of the region. As a result, 12 sites were inventoried that can be proposed as candidates for the national network of old-growth forests. These were largely unknown, ca. 10–300 ha in surface and encompassing five main forest types across 14 Natura2000 habitats. All stands have reached the mature or nearly senescent stage thanks to natural dynamic processes for over 70 years after the cessation of substantial anthropogenic disturbances. The structural heterogeneity index (SHI), based on living and deadwood biomass variables, was relatively high (66.2–84%). However, structural variables depended on forest type, thus on bioclimatic context and dominant tree species. Stands with beech and mountain conifers showed more pronounced old-growth characteristics than Mediterranean stands due to a faster recovery dynamic after cessation of disturbance. As many as 193 vascular plant taxa were recorded, with 16 species occurring with trees ≥ 50 cm in diameter. Forest specialist taxa, either woody or herbaceous, were prevalent, but numerous generalists also occurred in the gaps. Ancient forest species were also well represented, supporting the long temporal continuity of the forests. This work advances knowledge about forest sites with old-growth characteristics in southern Europe, contributing to the implementation of the national network and the EU Biodiversity Strategy 2030. Strict protection of these sites is necessary to allow the forest stands to fully reach the old-growth stage in the next decades, despite the negative influence of climate change.