A location with luxuriant growth of lichens on the trees indicates that the:
trees are very healthy
trees are heavily infested
location is highly polluted
location is not polluted
Symbiotic association between algae (phycobiont) and fungi (mycobiont).
They are good pollution indicator.
Lichens are composite organisms showing symbiotic association between algae and fungi. They are very sensitive to pollution and their luxuriant growth indicates pure and natural environment at the given location.