Tree bark offers lichen a good place to grow and which type of lichen that grows on certain trees depends on the tree species as what grows there is dictated by the PH level of the tree.
Lichen are made up of 2 or more organisms, most commonly fungi and algae. The lichen do not harm the tree as they draw on the water and nutrients from the surrounding atmosphere which means they tend to grow in areas that have good air quality. If you are as lucky as me to live here in The New Forest UK the trees in our woodlands are draped in lichen and is more obvious on the bare winter branches.
Lichens are very slow growers and can be hundreds of years old this is why loss of our ancient woodlands are a major threat to the species.