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The following points highlight the eight main stages of hydrosere in plants. The stages are:- 1. Phytoplankton Stage 2. Submerged Stage 3. Floating Stage 4. Reed Swamp Stage or Swamp5. Sedge-Meadow Stage or Marsh 6. Alder or Willow Carr Stage 7. Wood Land Stage 8. Climax Woodland.
Stage # 1. Phytoplankton Stage:
Planktonic stage includes minute autotrophic diatoms, phytoflagellates, cyanobacteria etc. Their dead and decomposed organic matter mixes with silt and forms soft mud at the bottom of the pond. Unicellular floating algal plants such as diatoms, members of chlorophyceae are pioneer species of a water body like lake, pond. Their spores are carried by air to the pond.
The phytoplankton is followed by zooplankton. They settle down to the bottom of the pond after death, and decay into humus that mixes with silt and clay particles brought into the basin by runoff water and wave action and form soil. As soil build up, the pond becomes shallower and further environmental changes follow.
Stage # 2. Submerged Stage:
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As the water body becomes shallower, more submerged rooted species are able to become established due to increasing light penetration in the shallower water. This is suitable for growth of rooted submerged species such as Myriophyllum, Vallisneria, Elodea, Hydrilla, Nymphoides peltata and Ceratophyllum. Deep freshwater will not support rooted, submerged plants because there is not enough light for photosynthesis in the depths.
These plants root themselves in mud. Once submerged species colonize the successional changes are more rapid and are mainly autogenic as organic matter accumulates. Inorganic sediment is still entering the lake and is trapped more quickly by the net of plant roots and rhizomes growing on the pond floor.
The pond becomes sufficiently shallow (2-5 ft) for floating species and less suitable for rooted submerged plants. Over time, sediments will be transported into the lake (or pond) by streams or rainwater draining into it from the land. Large amounts of sediment can be deposited in this way. Buried older plants form good humus for next seral stage.
Stage # 3. Floating Stage:
The floating plants are rooted in the mud, but some or all their leaves float on the surface of the water. The water depth will gradually decrease, allowing rooted, submerged plants, such as pondweed to grow. Water-lilies, which are rooted, but with floating leaves may also, become established.
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These include species like Nymphaea, Nelumbo and Potamogeton., which help the water become rich in mineral and organic matter. Later free floating species like Azolla, Lemna, Pistia, Eichornia, etc. appear. This rapid growth of plants builds up the pond bottom and makes the water shallower. Some free-floating species also become associated with root plants. Floating plants, such as duckweed, may also be present.
The large and broad leaves of floating plants shade the water surface and conditions become unsuitable for growth of submerged species which start disappearing. The plants decay to form organic mud which makes the pond shallower yet (1-3 ft.) organisms and floating plankton in the water. The vegetation traps and holds more and more of the incoming sediment, so that the water becomes shallower.
Decomposing dead plant and animal matter provides food for detritivores and increases the nutrients in the water. This promotes plant growth.
Stage # 4. Reed Swamp Stage or Swamp:
These plants tend to have tall, flexible spear-shaped leaves. This allows the plants to cope with large fluctuations in water level; always retaining some portion of the leaves above the water for effective photosynthesis. Also called amphibious stage and plants like Typha, Sagittaria, Scripus, etc., replace the floating plants.
These plants produce abundant amount of organic wastes and lose huge amounts of water by transpiration. Addition of organic matter raises the substratum of the pond and becomes unsuitable for growth of amphibious plants. The water in swamp areas teems with invertebrate life. It also provides an ideal place for rooted plants.
By this stage, the water may be too shallow to support fully submerged plants. Iris pseudacorus, Sparganium erectum and Typha latifolia grow rapidly. Besides these plants such as Phragmites, Typha and Zizania to form a reed-swamp .These plants have creeping rhizomes which knit the mud together to produce large quantities of leaf litter. This litter is resistant to decay and reed peat builds up, accelerating the autogenic change. The surface of the pond is converted into water-saturated marshy land
The leaf bases of the swamp plants are extremely effective at retaining incoming silt, particularly in the winter when the leaves have died back. Rotting plant material progressively builds up, which also raises the ground level.
Stage # 5. Sedge-Meadow Stage or Marsh:
These damp areas provide a wonderful transition habitat for newly emerged from the water. At this point they are very small and extremely vulnerable to drying out. The lush wet vegetation provides an ideal hiding place. Successive decreases in water level and changes in substratum help members of Cyperaceae and Poaceae such as Carex spp. and Juncus to establish them-selves.
They form a mat of vegetation extending towards the center of the pond. The above water leaves transpire water to lower the water level further and add additional leaf litter to the soil. Eventually the sedge peat accumulates above the water level and soil is no longer totally waterlogged. Plants which grow equally well in swamp or marsh conditions, will continue to grow.
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The habitat becomes suitable for invasion of herbs (secondary species) such as Mentha aquatica, Caltha, Iris, and Galium which grow luxuriantly and bring further changes to the environment. Tree seedlings, such as willow, which favour wet soil conditions, will become well-established and begin to grow up. Swamp plants which are adapted to grow in partially submerged conditions, will gradually die out as the marsh floor progressively rises above the water level.
Sedge-meadowhasa very high transpiration rate, transferring large quantities of water from the sediment into the atmosphere. Together with the silt-trapping effect of the marsh plants, this greatly increases the rate at which the marsh dries out.
Stage # 6. Alder or Willow Carr Stage:
The soil is still wet, but no longer completely water-logged and anaerobic. The willow has grown up and dominates the ground. Many of the marsh plants will have been shaded out by the trees.
They are replaced by a variety of woodland floor plants including sedges, rushes, ferns and small flowering herbs which are adapted to low light levels and which flourish in wet conditions. Plants like Carex echinata and Chrysosplenium oppositifolium grow rapidly. Along with the seeded plant fungi like Mitrula palludosa also grow from the soil like miniature lollipops. Fruiting Bodies of Mitrula palludosa in drier areas, increasingly aerobic decomposition will accelerate nutrient recycling, elevating the humus and nutrient content of the soil.
Stage # 7. Wood Land Stage:
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First the peripheral part of the area is invaded by some shrubby plants, which can tolerate bright sunlight and water logged conditions. Plants that grow are Cornus, Cephlanthus etc. Further fall in the water table, along with mineralization and soil buildup favours the arrival of plants for next seral community.
The soil now remains drier for most of the year and becomes suitable for development Xerophyte vegetation or of wet woodland. It is invaded by shrubs and trees such as Salix (willow), Alnus and Populus etc. These plants react upon the habitat by producing shade, lower the water table still further by transpiration, build up the soil, and lead to the accumulation of humus with associated microorganisms. This type of wet woodland is also known as carr.
Stage # 8. Climax Woodland:
Climax tree species include Oak, Ash and Beech. They are slow-growing, but because they are also tall and long-lived, in time, they will come to dominate an area. The particular climax tree species which eventually dominates the woodland will depend on soil type and other environmental conditions.
The number of species making up the woodland community is very dependent on the structure of the wood and on how much light gets through to the woodland floor. Where the ground is not too heavily shaded, herbs such as Wood Anemone and Wild Garlic will flourish. The living trees are also hosts for a variety of mycorrhizal fungi, such as the Fly Agaric, which is associated with birch.
