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.
It is a type of succession taking place in an aquatic environment. In a virgin pond, hydrosere starts with a colonization of phytoplanktons and finally reaches a climax forest stage. The different stages of succession are given below.
This is the initial stage of succession. Phytoplanktons, and zooplanktons are the pioneer colonizers. These organisms add large amount of organic matter and nutrients, which settle at the bottom of pond.
As a result of death and decomposition of phytoplanktons a soft mud develops at the bottom of pond. This new habitat now becomes suitable for the growth of rooted hydrophytes like Myriophyllum, Elodea, Hydrilla, Potomogetan, Vallisneria, Utricularia etc. These plants further build up the substratum. This new habitat now replaces these plants giving way to another type of plants of floating types.
In the beginning the submerged and floating plants grow intermingled but very soon the submerged plants are replaced completely. The habitat becomes changed chemically as well as physically. The dead remains of plants are deposited at the bottom. The substratum rises up in vertical direction. The important plants of this stage are Nelumbium, Trapa, Pistia, Nymphaea andLimnanthemum.
This stage is also known as 'amphibious' stage as the plants of this community are rooted but most parts of their shoots remain exposed to air, species of Scirpus, Typha, Sagittaria and Phragmites etc. are chief plants of this stage. Their rhizomes form a dense vegetation. The water level is very much reduced and becomes unsuitable for growth of these amphibious species.
Further decrease in water level changes the nature of substratum. Species of some Cyperaceae and Gramineae such as Carex, Juncus, Cyperus and Eleocharis colonise the area to form marsh or swamp. Thus mesic conditions approach the area and marshy vegetation disappear gradually.
In the beginning some shrubs and later medium sized trees form open vegetation or woodland. These plants produce more shade. They render the habitat more dry. The prominent plants of woodland community are species of Butea, Acacia, Cassia, Terminalia, Salix, Cephalanthus etc.
This is the climax community invaded by several trees. In tropical climate with heavy rainfall there develop tropical rain forests. In temperate regions, mixed forest of Almus, Acer and Quercus are formed.