The term lichen is used to describe a symbiotic relationship, a relationship in which both parties/organisms benefit. This relationship has to be between a fungus and a photosynthetic plant. This basically means that a fungus and a tree put together are considered to be a lichen relationship. There are many examples of this relationship in nature. Whenever a person sees dark green or brown mulch on a tree this is an example of this. The reason that this is considered a symbiotic relationship and not a parasitic relationship is because the mulch gains shelter and support while the trees just have added vegetation that aids it in its development.