Very good article on various Lichen which are good to eat (some preparation is NECESSARY) and two which are NOT GOOD TO EAT.
http://www.offthegridnews.com/off-grid-foods/its-1000-years-old-its-edible-and-its-on-your-property/This might prove to be a lifesaving food source.
It is also good because not many people will see lichens as food and will pass them by when foraging.
DO NOTE the suggestions on preparation. Lichen are naturally bitter (acidic) and need either long soaking/cooking times and/or the use of a neutralizing agent such as Sodium Bicarbonate...
Lichen grows on living and dead trees and some rocks.
General rule, avoid yellow or orange colored lichens, stick with the green or gray varieties.
[edit] butter spelink