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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?

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A hollow center is usually/always the result of decay, usually from a wound too large to heal over, either accidental or from bad pruning. Decay goes down the center of the tree from the wound. – Ed Staub Feb 7 '12 at 18:49
I agree with @kevinsky's answer. There are a lot of myths like this. Eg. some people think woodpeckers are bad as well (if anything woodpeckers are beneficial due to all the grubs they eat) – winwaed Feb 22 '12 at 2:32
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@winwaed, are sapsuckers beneficial? They eat plenty of bugs, but pull a lot of sap out of my spruces, leaving long white stains down the bark. – jmusser Feb 22 '12 at 14:43
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The sap can attract other creatures (incl. squirrels & hummingbirds). They can also be an indication that the tree is damaged/weak. I asked MrsWinwaed (ecol.prof/ornithol.) & she replies: There's a good article at the following link: uaex.edu/Other_Areas/publications/PDF/FSA-7561.pdf The big challenge with sapsuckers is that they drill differently from other woodpeckers, and make horizontal sets of holes on the tree which may lead to girdling on small trees. As with all migratory birds they are protected by law, and you can't shoot them or get rid of them, only deter them. – winwaed Feb 22 '12 at 16:32
So yes as sapsuckers are classed as woodpeckers, you may have found an exception, although I think it is more complicated than a simple beneficial/neutral/pest classification in their case. – winwaed Feb 22 '12 at 16:34
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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".

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