Drivers and patterns of community completeness suggest that Tuscan Fagus sylvatica forests can naturally have a low plant diversity

European beech (Fagus sylvatica L.) forests can have a high variability in plant species richness and abundance, from monospecific stands to highly species-rich communities. To understand what causes the low plant diversity observed in some beech forests, we analyzed the drivers of plant community completeness in 155 vegetation plots. Data were collected in mature, closed-canopy beech forests in Tuscany, central Italy. Site-specific species pools were estimated based on species co-occurrences. We used Generalized Least Squares linear modeling to assess the effects of anthropogenic and environmental drivers on the community completeness of whole communities and on the set of specialist species of beech forests. We also tested the response of the total cover of the herb layer to the selected predictors and related both the predictive and response variables to species composition in a Non-metric Multidimensional Scaling ordination. The community completeness of whole communities and that of beech forest specialists were negatively affected by total beech cover and positively influenced by slope. Moreover, the community completeness of whole communities was negatively impacted by elevation and positively influenced by disturbance frequency. The cover of the herb layer decreased with increasing beech cover, elevation, and precipitation. High community completeness and high cover of the herb layer were associated with the presence of thermophilic species of mixed deciduous woods in low-elevation beech forests. Our results suggest that a low plant community completeness and a low cover of the herb layer are mainly due to the competition by beech itself when it forms pure forests in its ecological optimum. Such competition is better exerted at upper elevations and in sites with low slopes, where beech litter accumulation is a limiting factor for understory species. Such evidence suggests that species absence in mature beech forests is mainly due to natural drivers and should therefore not be considered an indicator of ecological degradation of the forest.