Succession of plants in Water or Hydrosere or Hydrarch
Hydrosere is a termed used for succession of plants occurring in water. It can be well understood by the process of succession occurring in pond. Hydrosere, originating in a pond, starts with the colonisation of some phytoplanktons which form the pioneer plant community, and finally terminates into a forest, which is a climax community together with their chief components of vegetation.
Just like other autotrophic successions, in a hydrosere too, successive changes take place in plants as well as animals life. Different stages of succession in plants are based upon the dominant forms of plants in that community, as phytoplankton stage, rooted submerged stage, etc. and so on. These various stages together with their chief components of plant species of a hydrosere which are as follows:
- Phytoplankton Stage: They constitute the pioneer community. Some blue-green algae, green algae, diatoms and bacteria etc., are the first organisms to colonise the primitive medium of the pond. The soils are very much reduced with a pH value less than 5.00. They multiply and grow for sometime.
- Rooted Submerged Stage: As a result of death and decomposition of phytoplanktons and their mixing with the silt, brought from the surrounding land by rain water, there develops a soft mud at the bottom of pond. This new habitat which tends to be a bit shallower and where light penetration may now occur easily becomes suitable for the growth of rooted submerged hydrophytes like Myriophyllum, Hydrilla, Potamogeton, Vallisneria, Utricularia These plants bring about further build up of the substratum as a result of death and decay. The water level also decreases making the pond more shallower. This new habitat now replaces these plants giving way to another type of plants which are of floating-leaved type.
- Rooted Floating Stage: By now the water depth is almost 2-5 feet. These plants colonise the habitat with their rhizomes. They all are rooted hydrophytes with their large leaves floating on the water surface. These are the species of plants such as Nelumbo, Nymphaea, Trapa, Some free floatation species such as Azolla, Lemna, Wolffia, Pistia Salvinia, Spirodella etc. also become associated with the rooted plants, due to availability with the rooted plants, due to availability of salts and other minerals in abundance. The water level by now becomes very much decreased, making the pond more shallower. The decomposing organic matter formed due to death of these plants brings about further build up of the substratum. Thus floating species sooner or later disappear from the area.
- Reed-swamp Stage: This stage is also known as amphibious stage as the plants of community are rooted but most parts of their shoots remain exposed to air. Species of Scirpus, Typha, Sagittaria and Phragmites are the chief plants of this stage. They have well-developed rhizomes and form a very dense vegetation. The water level is by now very much reduced and finally becomes unsuitable for the growth of these amphibious species.
- Sedge-meadow Stage: Due to successive decrease in water level and further changes in the substratum, species of some Cyperaceae and Gramineae, such as Carex, Juncus. Cyperus and Eleocharis colonise the area. They form a mat-like vegetation towards the centre of the pond with the help of their much branched rhizomatous systems. As a result of high rate of transpiration, there is much rapid loss of water, and sooner or later the mud is exposed to air as a result of which nutrients like ammonia, sulphides etc. become oxidised to nitrates and sulphates. Thus mesic conditions approach the area and marshy vegetation disappears gradually and gradually.
- Woodland Stage: By the time of disappearance of marshy vegetation, soil becomes drier for most time of the year. This area is now invaded by terrestrial plants, which are some shrubs (Salix, Cornus) and trees (Populus, Alnus). By this time, there is much accumulation of humus with rich flora of microorganisms. Thus, mineralisation of the soil favours the arrival of new tree species in the area.
- Forest Stage: This is the climax community. The woodland community is rapidly invaded by several trees. In tropical climates with heavy rainfall, there develop tropical rain forests, whereas temperate regions, there develop mixed forests of Ulmus, Acer and In regions of moderate rainfall, there develop tropical deciduous forests of monsoon forests.
Thus, in hydrosere, stage 1 is the pioneer community, stage 7 is the climax community, and stages 2 (rooted submerged stage) to 6 (woodland stage) as the seral communities (seral stages).
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