The vegetation occupying a given habitat is called plant community. The community passes through several developmental stages in a definite sequence from simple to complex. The gradual replacement of one type of plant community by another is known as plant succession. E.P. Odum defines plant succession as an orderly process of community change in an unit area.
According to Clements succession is a natural process by which the same locality becomes successfully colonized by different groups of communities. Such an orderly and progressive replacement of one community by another until a stable community called climax community occupies their area is called ecological succession.
The different stages of plant succession taking place at a particular habitat is known as sere. The first plants which appear on the bare habitat are called pioneer plants. After several changes, a habitat becomes occupied by most tolerant species which forms a climax community.
The Xerosere originates on rock surfaces which is in an unweathered state. The pioneers to colonize these primitive type of substratum are lichens. In a Xerosere successive changes take place in both plants and also in animals. The various stages are described below.
The soil is absent for the complete penetration of roots. Blue-green algae and lichens are the pioneer species. In cooler climates, crustose lichens like Rhizocarpon, Rinodina and Lecanora are the common pioneers. They produce acids which bring about weathering of rocks. The dead organic matter of algae and lichens become mixed with the small particles of rocks to form a thin layer of moist soil on the rocks.
They appear on the substratum partially built up by the crustose lichens. It includes species of Parmelia, Dermatocarpon which have large leaf-like thalli. The weathering of rocks mixed with humus results in the development of a fine thin soil layer on rock surface and thus there is a change in the habitat.
The development of thin humus-rich soil layer on rock surface favours the growth of certain xerophytic mosses such as Grimmia, Tortula, Polytrichum, Bryum, Barbula and Funaria.
Due to the extensive growth of mosses, more soil accumulates. Minerals are added to it due to leaching. This favours the growth of some herbaceous plants like Aristida, Festuca, Justicia, Tridax etc.
Due to much accumulation of soil, the habitat becomes suitable for shrubs. Species of Rhus, Phytocarpus, Zizyphus and Capparisdominate this stage. The shrubs overshadow the herbaceous vegetation and produce more organic matter.
It represents the climax community. Due to the weathering of rocks, thin layer of soil is formed, which supports small trees likeAcacia, Prosopis, Boswellia etc. Plants require high rainfall to reach climax stage. In moist and wet climates and also in temperate climates dense climax forest is developed.