Oaks often have real mosses, often growing as a carpet on the upper surfaces of large branches that are well shaded. Ramalina needs sunlight and water to grow. It can sit in the hot, dry summers of California, or in a museum drawer, for very long periodds of time, and appear dead. But as soon is it is wet and in the light, it springs back to life. We don't know why it grows more abundantly on some trees and not others, given an equal amount of open branches without leaves in the winter on otherwise similar trees.
Dead branches occur on almost all trees, even healthy ones.
These dead branches often have large clumps of Ramamlina because there is so much more light on a dead branch, as the leaves are gone. Sometimes heavy rains and strong winds will make the clumps of lichen heavy and exert a great deal of force, often breaking off the weakest dead branches in winter storms. But clearly, the lichens persist and thrive in otherwise healthy trees for decades. Lace lichen, like the green lichens on rocks in the tundra, does not hurt the host. In this case, Ramalina can provide a great benefit to the host oak, by capturing wind-borne nutrients.