Plant traits and ecological strategies in the dark diversity of forest vegetation in the province of Varese (Lombardy) = Tratti funzionali e strategie ecologiche nella dark diversity della vegetazione forestale in provincia di Varese (Lombardia)

Biodiversity has long been at the center of ecological research, however there is still no clear understanding of why some species are absent in a given location, although they could potentially be present based on ecological factors. This undetectable component of biodiversity is defined as "dark diversity", and can only be estimated through indirect measurement methods, one of which is the use of co-occurrence plots. In this study, we used the standardized methodological protocol proposed by the Dark Diversity Network (DarkDivNet), to survey species assemblages in co-occurrence plots distributed randomly within a study area of 10 km radius, located in the province of Varese (Lombardy, Northern Italy). We carried out 42 floristic-vegetational surveys representative of seven types of forest vegetation: acidophilic beech forests, species-poor acidophilic beech forests, basophilic beech forests, neutrophilic beech forests, oak and birch forests, Tilio-Acerion forests and Corylo-Fraxinetalia forests. For each of the species detected, we measured three plant functional traits (LA, LDMC and SLA) used to calculate Grime’s CSR ecological strategies. Specifically, the purpose of this study was to: 1) estimate the dark diversity and the level of completeness of the community for each type of vegetation; 2) test the possibility of calculating the weighted average of the dark diversity (CWM, Community Weighted Mean) of plant traits and of the ecological strategies of plants (C-, S-, R- scores), weighted by the probability of each species belonging to the dark diversity; 3) compare the composition of the CWMs of traits and CSR strategies of dark diversity with the CWM of the observed diversity (weighted by the observed abundance of species in each single survey). The highest dark diversity was identified for acidophilic beech woods, in particular those subject to greater management intensity (i.e. species poor beech forests), while the lowest was evident in recolonization forests (Corylo-Fraxinetalia). The application of CWM to dark diversity has provided promising results for both traits and CSR strategies, and the latter exhibit great predictive power. Dark diversity exhibited a high degree of ruderality (R), as opposed to strong selection towards stress-tolerant (S) strategies in the observed diversity; a greater degree of competitiveness (C) for dark diversity was evident only for acidophilic beech woods, suggesting that the latter may be subject to the entry of alien competitive species if suitable spaces are created. Finally, it is possible to hypothesize that the balance between stress tolerance and ruderality could be the basis of the properties of the plant communities respectively linked to resistance and resilience.