What community completeness can tell about the conservation status of ecosystems: a test on vascular plants in Fagus sylvatica forests

Beech (Fagus sylvatica L.) forests are among the habitats with the highest naturalness in Europe (1). Theyare often considered as species-poor ecosystems since beech is highly competitive especially for light, butexamples of species-rich beech forests are also well-documented. It is unclear if the lack of species in manybeech forests is due to natural drivers or to anthropic management. In this study we investigated the patternsof plant community completeness (2) in the beech forests of Tuscany, central Italy. Floristic and structuralattributes were surveyed in 155 circular plots of 8 m radius in 2020-2021. The regional species pool of beechforests was retrieved from about 1,100 vegetation relevés available in the literature. We built a series ofStructural Equation Models based on Generalized Linear Modelling to assess the direct and indirect effectsof anthropogenic and environmental variables on beech forests’ community completeness. We tested if sucheffects differed according to forest types on the groups resulting from a modified TWINSPAN clusteranalysis: 1) species-rich, low-elevation and basiphilous beech forests characterized by Cardamine bulbifera,Hedera helix, Mercurialis perennis; 2) species-poor acidophilous beech forests characterized by Festucaheterophylla, Luzula pedemontana, Veronica officinalis; 3) species-poor microthermal beech forestscharacterized by Oxalis acetosella, Rubus idaeus, Dryopteris dilatata. For the whole dataset, the lack ofspecies at a given site was due to both anthropic and natural factors (Fig. 1): 1) silvicultural management(low DBH, indicating coppicing or recent cuts in high forests) that disadvantages understorey species; 2) lowslopes that naturally promote litter accumulation suppressing understory species; 3) increasing elevation,with a natural loss of species. The selected predictors had different effects on the community completenessof the three forest types, with the main causes of species absence being high elevations in type 1, none of theselected predictors in type 2, and silvicultural management and low slopes in type 3. We conclude thatcommunity completeness can be used as an indicator of the conservation status of beech forests, since testingthe role of different drivers in relation to forest type allows distinguishing natural lacks of species from thoseinduced by human influences.