In California, I saw a sign on a tree that was covered with lichens and surrounded by dead trees. It seemed a plea for help. But according to the Hastings Biological Field Station of the University of California, lichens can capture airborne nutrients and return them to the soil. That is considered good. Another website paints a more complex picture, suggesting lichens may sometimes be associated with the death of trees in California. Read it here.
Suzanne's Mom's Blog 
The sign saying “please,” all covered in lichens like that, is so evocative!
Thanks, Asakiyume! You wouldn’t believe how many discussions we had on this end about whether it was OK to be evocative in a way that might add to the misinformation on the web! I think I found a balance.