Lichen is supposed to be an indicator of good air quality. This is because of the fragile cooperation between two components of the lichen, fungi and the actual lichen. They cooperate in their mutual way of receiving and dealing with air borne nutrients, which makes them vulnerable to air pollution.
This wild apple tree in the outskirts of the forest near me should be proof that the air where I'm living is very good and I can safely keep on breathing. I',m glad about that.