Lichens are amazing. They’re a fusion of multiple life forms: a fungus plus an alga, cyanobacterium, or both, growing intertwined. The fungus eats the sugars that its partner makes from sunlight. Greenshield Lichen (Flavoparmelia caperata) is comfortable growing on city trees so long as the pollution isn’t too bad. It doesn’t hurt the trees, and it’s pretty, too… what’s not to lich(en)?
Learn more about them in Lichens of the North Woods by Joe Walewski.