They are indicators of fresh and clean air.
I did not know that!! Very interesting.That central bunch of lichens (or moss or ?) would be nice in a close-up too. Lichens make for some interesting 'minimalist' abstractions. Well not really abstraction, per se, because the subject is real, but a viewer may not always know what they are seeing in a close foto of lichens on rocks or trees. Some lichens are also very interesting when photographed in IR.