Towards the old-growth forests Italian network: structural heterogeneity and plant diversity in novel candidate sites of Tuscany.

Old-growth forests play a vital role for the conservation of terrestrial biodiversity, though rare and increasingly threatened worldwide. Notable examples of these ecosystems are found in South Europe, but information about their location, structure, biodiversity and functioning is still largely incomplete. In this work, we tested the hypothesis that Tuscany, one of the most wooded region in Italy, harbors sites with old-growth characteristics in the light of the Italian ministerial guidelines and the recent EU indicators. Accordingly, we collected data on stand structural and plant diversity variables in 27 plots located in pre-selected sites across different forest types. As a result, 12 sites were inventoried in both Mediterranean and Continental biogeographic areas of Tuscany, that can be candidate for the national network. These sites are all located in publicly-owned areas (State and Regional reserves, Demanio Regionale), 10-350 hectares in surface and encompassing five main forest types across 14 Natura2000 habitats, four of which with conservation priority. 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 woody species occurring with trees ≥ 50 cm dbh. Forest specialist plants, either woody or herbaceous, were prevalent, but numerous generalists also occurred in small-scale gaps typically found in old-growth forests. Ancient forest species were also well represented, supporting the long temporal continuity of the forests. This work advances knowledge about sites with old-growth characteristics in south 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 forests to fully reach the old-growth stage in the next decades, despite the negative influence of climate change.