Peltigera collina aka tree pelt lichen.
image by Ed Uebel/commons.wikimedia.org
Tree pelt lichen, known botanically as peltigera collina, is a slow-growing life form made up of a fungus paired with either blue or green algae. It grows in a two-sided leaf form on the trunks of hardwood and coniferous trees in areas that meet its requirements for sunlight, humidity, nutrients and air circulation. The tree pelt lichen does not use the tree for sustenance. Instead its algae photosynthesize to provide food for the fungi. Lichens are supported by drawing in everything in the air that surrounds them, both the good and the bad, hence they serve an important environmental role as natural air cleaners.
Provide a hardwood or coniferous tree trunk as the preferred growing substrate for the tree pelt lichen. High ambient humidity helps to encourage lichen growth. Periodic rainfall will fulfill the same need of the lichen just in a different manner. Shade to filtered sunlight is the preferred growing exposure, though some sun is tolerated.
Clear away any invasive weeds that are growing near the tree pelt lichen and may threaten to overtake it. Pull them out by the roots without disturbing the lichen itself if at all possible. Make any alterations to the tree canopy environment with extreme care. Sudden increases in sunlight from very heavy canopy trimming can stress the lichen or accidentally knock it off of its substrate. Make changes slowly allowing the lichen to acclimate with each change to its environment.
Refrain from attempting to harvest or relocate tree pelt lichen from the substrate where it is established. Any disruption to the home location to which the lichen is acclimated will damage or possible kill it. Tree pelt lichen appears most frequently in entirely undisturbed areas.