I decided to try to answer my own question and found this:
"Lichens are often found on tree trunks, branches and twigs as the bark provides a stable place to reside to collect needed sunlight, rainwater and materials from the air. They grow on healthy trees, as well as stressed or otherwise unhealthy ones â€¦Many lichens are more evident on stressed or old tree trunks and branches giving the appearance of a "cause and effect" association with disease and decay. The primary reason for their more likely presence on those trees and branches with reduced or partial foliation is the resulting increase in available sunlight. The bark of a healthy tree continues to expand and slough off with the growth of the tree. The bark of an older or stressed tree may become more brittle with more cracks and uneven surfaces permitting lichens to attach themselves more readily. As bark ages, it changes in chemistry, texture, and ability to retain water, thereby influencing the type of lichen capable of living there."