Are Lichens Tree Parasites?

Never say never.

The dogmatic answer to that frequently posed question is a comforting “no” echoed throughout garden publications.  Here is a typical statement from the New Mexico State University Horticultural Guide H-167:

“Lichens may rest directly on the bark or be attached shallowly to it, but they do not enter the inner bark where food is transported, and hence do not rob the tree of nourishment. They are not parasites.”

Lichen with the emerging side-branch kaput.

A trouble with dogmas is that while they spread and simplify, occasionally somebody goes back and checks them out.   Not that finding parasitism in lichens is new. One of the links below dates from the 80s.    And, because lichens are not a single “thing,” but rather many unrelated species in diverse symbiotic relationships,  ixnay on big sweeping generalizations.

I’m not posting this to shock treehuggers.   Seems that under most circumstances most lichens are…