[HIGHLIGHTS] Is lichen on trees dangerous for its host?

Lichen is a symbiosis of fungi and chlorophyll partners, most often green algae, sometimes cyanobacteria. The fungus provides the algae with shelter which protects it against the sun's rays.

In return, the algae provides the fungus with the organic molecules it produces through photosynthesis, as well as vitamins that stimulate its growth. Lichen is not a threat to the survival of the tree or shrub. It certainly attaches to the tree but it only remains on the surface, without penetrating the tissues or drawing sap.