Succession on bare rock is called Xerosere. The lichens are the pioneer species and invade the bare rocks. They secrete acids which dissolve the rocks, help in their weathering and formation of soil.  Mosses grow in with the lichens to form a mat over the substratum. Bryophytes or annual grasses succeed the lichens. They are able to hold small amount of soil.  Bigger trees appear in the later stages and are succeeded by other species untill a climax community forms an equilibrium with the environment. It leasds to the formation of forest ecosystem.