(Awl Lichen) on oak trees

"Many trees don't start to accumulate mosses until they're more than 100 years old. So it's really the density of very large old trees that are draped in moss that is important at a forest stand level. The trees surveyed were estimated as being between 500 and 800 years old. Cyanobacteria in mosses on the ground were recently shown to supply nitrogen to the forest.

http://www.sciencedaily.com/releases/2011/02/110223125021.htm

http://www.ontariowildflower.com/moss.htm#moss