Old trees can be so overgrown with moss that the bark is barely visible. Among the moss One tends to find many species of lichen growing. Some of the lichen species are large and easy to see, while others are barely visible. Many lichens and mosses have special requirements and grow almost exclusively on just tree species with nutrient-rich bark, like elm and ash. These species will scarcely ever grow on trees with nutrient-depleted bark. Birch and pine belong to this latter category.
In north-facing lees of the outer Åkra Fjord, one finds a special lichen flora on the many pollarded trees. Two of the species are only visible under a microscope and are found here as the only place in the whole of Scandinavia. In addition, one finds several rare species of skin lichen, which also are known individually from other places in Sunnhordland. The distinctive lichen flora containing many species seems for several reasons to be associated with pollarded trees. The tree has for a long time stood in an open cultural landscape, under better light conditions than in a dense forest. In addition, the pollarded trees of elm and ash have been able to grow old because they have been harvested and looked after for many generations. On old, large trees the bark has a more uneven surface than on younger trees. The older bark is more supple and has more cracks where moss and lichen can get a foothold.