Identifying mechanisms shaping lichen functional diversity in a primeval forest

Functional diversity is considered an important tool for the identification of ecological mechanisms shaping the coexistence patterns of species communities along different environmental gradients. However, there is a huge gap in knowledge regarding the importance of ecological mechanisms in shaping the composition of lichen communities. We analyzed the functional diversity patterns of the lichen species in a European lowland primeval forest ecosystem, taking into consideration levels of forest communities, phorophyte species and substrates. The greatest functional diversity of lichens at the community level (functional richness and functional dispersion) was found in mixed deciduous forests, where Fraxinus excelsior and Carpinus betulus revealed the highest values of functional dispersion. The most functionally diverse lichen communities were found on the bark of trees, the branches of trees, and the bark on trunks or branches of fallen trees. The lowest functional diversity was found in coniferous forests, evidenced by the low functional dispersion of lichen assemblages, indicating lower heterogeneity of possible niches and harsher environmental conditions comparing with deciduous forests. In coniferous forests, lichens produced small ascomata and ascospores, while in deciduous forests, ascomata and spores of lichens were larger. We concluded that two contrasting mechanisms shaped the functional diversity of lichens in the primeval forest ecosystem: (i) niche-partitioning in deciduous forests, and (ii) environmental filtering in coniferous forests. Our study has substantially broadened and enhanced the existing knowledge concerning the assembly rules of lichen communities and can be considered as a model for the distribution of lichen functional traits in the natural lowland forests of Europe. This approach may also be used to model future changes taking place in this type of ecosystem. Our results are particularly important for forest managers responsible for protection, as they highlight the fundamental importance of certain tree species and specific forest substrates for maintaining high lichen functional diversity. Focus on the protection of all features of diverse forest structures and of diverse forests providing high variety of niches is essential for the successful conservation of forest lichen biota.