Successional Stages of Hydrosere or Hydrarch
Hydrosere or hydrarch is a succession that starts in habitats with plenty of water such as a pond, lake, stream etc. If succession begins from a lake or any other water resource and reaches to a climax vegetation it is termed as hydrosere. On the other hand, if it begins from a dry rock or sandy habitat then the succession is termed as xerosere.
Initiation of hydrosere takes place in lakes, ponds, ditches, pools and also in running water. The following description generalizes the major stages of this type of succession.
This is the first community or pioneer community to appear in a pond or a pool of water. It is the first stage. At this stage the depth of water is nearly 4-5 metres and consists of vegetation such as green algae ( e.g., Spirogyra, Chara, Nitella etc.), blue-green algae (Cyanobacteria), diatoms, bacteria etc. Such plants are termed as the pioneers of hydrosere. The organisms of the community multiply and increase in number. But as they increase in number, they die, adding nutrients and organic matter to the habitat which becomes unsuitable for phytoplanktons to continue.
Rooted submerged stage:
The pond now becomes shallow. This habitat is now suitable for plants which remain rooted at the bottom of the pond. These plants are Hydrilla, Vallisneria, Utricularia, Potamogeton etc. When these plants grow in large numbers and cover the entire lake or pond, competition among them begins for food and space. As they grow one above the other, the plants at the bottom do not get plenty of light and air. This results in decaying of such plants. The decaying materials form the humus and settle at the bottom of the lake. As a result, the depth of the place diminishes. This creates a hindrance for the growth of these plants. But on the other hand it becomes suitable for the floating plants.
As a result of death of submerged plants more organic matter is added to the pond. The pond now becomes much shallower, being almost 2-5 feet deep. The plants growing in this stage usually remain in a floating condition. They may be either free-floating being fixed to the bottom or partially submerged with their roots fixed to the bottom such as Nelumbium, Nymphaea etc. Free floating plants such as Eichhornia, Lemna, Azolla, Salvinia, Woffia etc., also begin to appear on water surface.
When these plants completely cover up the water surface, light cannot enter into the under surface. This causes the submerged plants and submerged organs of the floating plants to decay. Due to the deposition of the decaying plants depth of water diminishes more and more. The amount of organic matter is very high. As a result, the habitat becomes unsuitable for the rooted floating and free floating plants. The pond fills with large amount of organic matter and makes it a shallow pond. Such a pond becomes unsuitable for the growth of the existing community. The plants adapted to swampy, low water content lands gradually migrate into it.
At the stage the depth of water reduces to nearly one metre or even less than that. Plants such as Polygonum, Ipomoea, Marsilea, Sagittaria etc.dominate this stage. Scirpus, Typha, Phragmites, etc also occur in such habitats and are also termed as amphibious plants.
Rhizomes of these plants are very much branched. They remain fixed to the soil where as their profusely growing branches entangle with one another. Thus forming a thick covering over the water surface. As a result the floating plants and the covered parts of reed-swamp plants do not get any light. Thus the plants gradually perish. The water level diminishes due to the decomposition of dead and decaying plants. Since the reed swamp plants need water throughout the year the condition becomes unsuitable for their further growth.
Sedge meadow stage:
In the beginning, the area shows marshy conditions. In swampy habitat generally the herbaceous plants like Cyperus, Juncus, Eleocharis, Carex etc., grow very rapidly. These plants have a high rate of transpiration causing quick water loss and hence the soil continues to become mesic. Thus the marshy vegetation disappears gradually.
As the roots and rhizomes proliferate in the soil, the plants growing previously die. Thus the area gets filled with the deposition of decaying plant materials. Herbaceous plants also help in the conservation of soil and gradually the water content of the soil gets reduced.
This stage is reached when a low land is build up to an extent where the soil still remains saturated with water. At this stage some types of shrubs proliferate and the herbs and grasses beneath them completely get shadowed. Due to heavy transpiration by the broad leaves of shrubs the soil water content further decreases. In this way after several years the swampy places gradually converts to a wetland. The prominent plants of woodland community are shrubs like Salix cornus and trees like Populus, Almus etc.
Forest stage or Climax stage:
By this time the humus content of the soil increases due to deposition of dead plants. Many bacteria, fungi and other microorganisms help in the formation of humus. This creates an ideal condition for the growth of huge plants such as Acer, Quercus etc. Trees are dominant in this stage. Herbs shrubs mosses and shade loving plants are also present in this stage.
These plants form a climax vegetation. After this no further succession of new groups of plants takes place . It is because further change of climatic condition is not possible.