Lichens

Lichens are entities made up of two main organisms living in symbiosis: the fungus (the mycobiont) and the photobiont, which is either a green alga or a cyanobacterium. They can therefore be considered as chimeric organisms or, due to the presence of other organisms cohabiting in the thallus (bacteria, yeasts, etc.), as complex autonomous ecosystems.

The name of the lichen is that of the main fungus, referred to as ‘lichenised.’ Lichenisation has appeared several times independently in the fungal kingdom, in both ascomycetes and basidiomycetes, but over 95% of species belong to the ascomycetes.

Nearly 19,500 species of lichenised fungi are currently known worldwide. In tropical forests, lichens are most abundant on bark (corticolous species) and leaves (epiphyllous species), and to a lesser extent on rocks (saxicolous species).

Another group are the lichenicolous fungi, which are highly specialised organisms that live exclusively on lichens, usually as host-specific parasites, but also as broad-spectrum pathogens, saprotrophs or commensals. Almost 2,000 species are currently known worldwide. These lichenicolous fungi were also taken into account in the Life On Trees programme surveys.

Photobionts and other lichen cohabitants have not been studied because they require their own molecular studies to estimate their diversity due to the absence of morphological characteristics to distinguish species.

The distribution of the diversity of lichenised fungi in the three trees in the Life On Trees programme was studied using quadrat sampling. These quadrats are placed parallel to the axis of the trunk and branches and distributed as evenly as possible in the canopy and on the different sides of the trunk (facing north, east, south and west). In addition, random samples of leaves from epiphytic plants were taken in order to search for other species of lichen carried by the trees. Some of the terminal branches of the trees were brought back to the field laboratory for detailed study using the quadrat method.

The lichen samples are dried and preserved for later identification. Small pieces of lichen are also placed in vials containing fixative to allow DNA sequencing to help identify specimens belonging to groups with a complex taxonomy or to identify sterile lichens.