(a) Monoclimax theory:
According to this theory given by Clements (1916), only one community exists in a particular geographical area (climate), other communi- ties present in the area are described by the terms proclimax, subclimax or post-climax and may collectively be called subor- dinate climaxes.
(b) Polyclimax theory:
This concept given by Tansley (1935) states that a number of communities may coexist in a climatc.
It is a type of plant succession that occurs in aquatic environment in which water disappears and is replaced by land (Fig 30.14). Ilydroscre starts in a new and virgin (no life) area and terminates in a forest. The different stages in a hydroscrc arc as follows.
1. Phytoplankton stage:
This is the first stage of hydroscrc in which spores or algae or bacteria enter the body of water. These organisms multiply and flourish. Such organisms called the pioneers not only add organic matter and nutrients due to their life activities, but also settle at the bottom after their death. Thus a layer of mud is formed at the bottom of the pond.
2. Submerged stage:
The mud formed at the bottom of the pond allows submerged hydrophytes to grow there. Plants like Utricu laria, Ceratophyllum, Myriophyllum, Vallisneria etc., grow in this stage, when water depth is about 10 feet. When these plants die, they get depos- ited at the bottom of the pond or lake. This, along with the eroded soil, raises the bottom of the pond or lake, making the water shallow. The shallow water habitat becomes less suitable for the submerged vegetation.
3. Floating stage:
When the depth of water of the pond or lake becomes 4 to 8 feet, the submerged vegetation gradually disappears giving way to plants like Nymphaea, Nelumbium, Typha, Pistia, and Eichhornia. Luxuriant growth of these plants prevents light penetration leading to total disappear- ance of submerged vegetation.
4. Reed swamp stage:
When the water depth of a pond or lake becomes one to three feet, the habitat is not suitable for floating plants and gives way to amphibious plants. Amphibious plants can very well survive in such con- ditions as they can successfully survive in aquatic and aerial environment. Examples of such plants are Typha, Polygonum, Marsilea, Sagittaria etc. Leaves of these plants form a cover over the submerged and floating plants.
Such covering cuts off light from submerged and floating plants that are underneath. There is a gradual disappearance of submerged plants (if any) and the floating plants. Plant debris and deposition of soil reduces the depth of water and the habitat becomes less suitable for the reed-swamp plants.
5. Sedge grass stage:
This stage which is I.lso called sedge marsh or meadow stage is the result of the formation of marshy soil due to further decrease in water level after the death of preexisting community.
In the beginning, plants belonging to the families Graminae and Cyperaccac start growing in the habitat. Examples of plants that belong to this stage arc Carex, Juncus, Mentha, Iris etc. Growth of these plants causes excessive absorption and transpiration.
This, in turn, greatly modifies the habitat and makes it unsuitable for existing plants. Accumulation of plant debris and deposit of soil par- tides along with absorption-transpiration patterns of the plants creates an environment totally unsuitable for the growth of hydrophytes. Gradually mesophytes appear and the sedge vegetation gets replaced.
6. Shrub stage:
Creation of a mesophytic habitat allows shrubs and medium sized trees to grow. These plants not only produce more shade but also absorb and transpire large quantities of wa- ter. Shade loving herbs or shrubs may grow under the trees. Examples of plant species belonging to this stage are Acacia, Cassia, and Salix etc.
7. Climax stage:
With the continuous creation of humus due to deposit of plant debris and addition of more soil, a suitable habitat is created not only for the growth of microorganisms, but also huge plants. Creation of such a community is called the climax after which further succession is not possible.
The hydroseric succession takes a very long time (thousands of years) but is responsible for the creation of a forest from a pond. It is a very slow process that cannot be observed.