I've always thought lichens were neutral to tree health, and I think they are attractive, but someone recently told me to get rid of them, because they will cause rot and harbor disease. Do they really do that? Is that why so many trees have hollow centers?
Lichens are considered to be neutral to tree health. They are not symbiotes or parasites. They just hang around..... ;)
However a tree in poor health may not be able to grow new bark faster than lichens can encrust it. Normally no control measures are necessary for lichen. Stimulating the tree by feeding, mulching, watering and applying a foliar feed is usually enough.
In regards to the common sight of a tree with a hollow centre this is due to the different way trees grow than people. A person with a hollow centre would be....hungry or dead but a tree only uses the centre of it's trunk for mechanical support and food storage. All the living tissue is contained within a cylinder running up the trunk. The tissue is surrounded by protective bark and backs onto the storage area in the interior. If the interior rots out it is like not having an attic or basement. You can still live in your "home".
Lichens are autotrophic species, that means that they produce their own food. It is possible thanks to a symbiotic association of a fungus (a lot of species exists) and an algae (four main groups exist, among which green and cyanobacteria (photosynthetic in fact).
That means that lichens don't take energy or nutrients from their host. So no worry for your trees.
An abundance and diversity of lichen indicates good air quality, so this species is useful to assess the air quality of your surroundings!