Based on the nature of the habitat – whether it is water (or very wet areas) or it is on very dry areas – succession of plants is called hydrach or xerarch, respectively. Hydrarch succession takes place in wetter areas and the successional series progress from hydric to the mesic conditions.
As against this, xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions. Hence, both hydrarch and xerach successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric).
The species that invade a bare area are called pioneer species. In primary succession on rocks these are usually lichens which are able to secrete acids to dissolve rock, helping in weathering and soil formation.
These later pave way to some very small plants like bryophytes, which are able to take hold in the small amount of soil. They are, with time, succeeded by bigger plants, and after several more stages, ultimately a stable climax forest community is formed.
The climax community remains stable as long as the environment remains unchanged. With time the xerophytic habitat gets converted into a mesophytic one. In primary succession in water, the pioneers are the small phytoplanktons, they are replaced with time by free-floating angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees.
The climax again would be a forest. With time the water body is converted into land. In secondary succession the species that invade depend on the condition of the soil, availability of water, the environment as also the seeds or other propagules present.
Since soil is already there, the rate of succession is much faster and hence, climax is also reached more quickly. What is important to understand is that succession, particularly primary succession, is a very slow process, taking maybe thousands of years for the climax to be reached.
Another important fact is to understand that all succession whether taking place in water or on land, proceeds to a similar climax community – the mesic.