You could argue that their presence show air quality to be good (a good thing obviously) but that their presence could show the tree to be in ill health, all the same.
As lichen need light to grow (they're a symbiotic union involving a fungus and a chlorophyll-containing cyanobacterium / alga), they shouldn't grow on healthy trees as trees in good health should have leaves creating too much shade for lichen to thrive.
I'm fairly sure lichens DON'T damage trees.
You could argue that their presence show air quality to be good (a good thing obviously) but that their presence could show the tree to be in ill health, all the same.
As lichen need light to grow (they're a symbiotic union involving a fungus and a chlorophyll-containing cyanobacterium / alga), they shouldn't grow on healthy trees as trees in good health should have leaves creating too much shade for lichen to thrive.