Various kinds of mosses can also grow on the trunks of park trees

Mosses are primitive plants that have no roots or flowers. They reproduce via spores. 

Mosses usually grow in dark, wet locations. This is why many kinds of mosses grow on the trunks of park trees. The foliage of the tree provides the moss with a dark growing location. When it rains, the moss can gather the water that trickles down the tree's trunk.

Because they do not have roots, mosses and lichens that grow on tree trunks do not cause harm for the trees themselves.

The Hypnales mosses (on the right) can grow on rocks or on tree trunks.