The following points highlight the top two types of succession in plant communities. The types are: 1. Succession on a Bare Rock (Xerarch) 2. Succession in Aquatic Environment (Hydrarch).
Type # 1. Succession on a Bare Rock (Xerarch):
For example, in the succession from bare rock, there must first be the long process of rebuilding the soil. Organisms such as lichens can survive on bare rock. Gradually, the effects of the lichens together with wind, rain, ice and temperature changes begin to break rocks into soil.
The lichens also trap soil that is transported by wind and water. As soil formation takes place, other plants such as mosses, grasses and flowering herbs are able to survive and reproduce. After hundreds of years such succession from bare rocks may result in a climax forest in the area.
The pioneer colonisers on the bare rock are crustose lichens which occur on the rock surface in the form of membranous crusts. Their spongy nature enables them absorb excess amount of water and minerals. The lichens secrete carbonic acid (H2CO3) in excess, which reacts with the rocky materials and loosens the rock particles.
The corroded rock particles together with decaying lichens make the first thin layer of the soil on the rock surface. After accumulation of little soil and humus the rock surface previously occupied by crustose lichens, now gets covered with foliose and fruticose lichens.
Now because of enough moisture mosses also invade, and a lichen-moss carpet is formed which provides suitable substratum for the germination of seeds of herbaceous plants which are dispersed in it.
The seeds of higher plants germinate and grow successfully in pockets of newly formed soil on the rock. When the soil increases in thickness, the herbaceous vegetation, which consists mainly of annual and perennial herbs, develops very quickly.
The roots of these plants penetrate deep in the crevices of the rock, where they secrete acids and accelerate the process of rock disintegration. Decaying roots, stems, leaves and other parts of the plants become deposited on the soil surface in the form of humus.
This further increases the water holding capacity of the soil. These reactions make the habitat more suited for woody plants than to the existing herbs. Now herbaceous species make way for the invasion of shrubs followed by the trees. Decaying roots, leaves, twigs, etc., of these shrubs also enrich the soil with humus.
These important reactions bring about such conditions in the habitat, that are most suited to the trees rather than to shrubs or herbs. Now the trees invade the area which has been occupied previously by shrubs.
The first trees growing in such areas are dwarf and widely spaced. With the increase in the water holding capacity of the soil these trees gradually disappear and subsequently the mesophytic large trees are developed. Passing through seral stages in course of time, climax community gets established, which is generally dominated by trees.
Dominance of Species:
This refers to the fact that not all the species in the community are equally important in determining the nature of the community out of hundreds of species present in the community, relatively only a few exert a major controlling influence due to their large size.
The stability of a community depends upon the large number of species of a community, the larger the number of species that interact, the more stable that community will be. For example, if a biotic community is represented by a single species of eucalyptus or casuarina (i.e., monoculture), it may be completely wiped out if it is infested by fungal, viral or bacterial disease.
However, when a mixture of many species of trees is raised up, only one species would be destroyed by a particular disease, and the rest species will survive.
Various studies and records have revealed that development of a climax forest from bare rocks or sand dunes in primary succession takes about 1000 years or more, whereas a secondary succession requires a shorter period of time.
A destroyed grassland recovers in 50 to 100 years, while a destroyed forest in more than 200 years. When a man destroys a forest by felling the trees, or denudes a grassland by overgrazing his cattle, the process of community succession reverses.
Type # 2. Succession in Aquatic Environment (Hydrarch):
Hydrarch succession occurs in the aquatic environment. Such succession is recognisable only if the colonisation of plant communities takes place in small shallow ponds, lakes, etc., where wave action speeds up the process by allowing the erosion of soil towards edge regions.
Water bodies are prove to sitting as a result of soil erosion from surrounding areas. On the other hand, blockage of rivers by landslides and construction of dams lead to formation of new lakes, where hydrarch succession proceeds in due to invasion of aquatic species. Thus, the filling process speeds up quickly and consequently the body of water disappears within few years-time.
In a new and virgin pond hydrarch starts with the colonisation of phytoplankton and zooplankton which constitute the pioneer community. The simple forms of life, such as bacteria, algae and many other aquatic forms of plants (phytoplankton) and minute animals (zooplankton) floating in water are the pioneer colonisers.
All these organisms add large amount of organic matter and nutrients due to their various life activities and after their death, they settle in the bottom of pond to form a layer of silt.
The phytoplankton stage is followed by submerged plant stage. The submerged plants, with their roots anchored in the mud are next to colonise the pond. When these plants die their remains are deposited at the bottom of the ponds or lakes.
This process gradually raises the bottom of the ponds and lakes up. As this process progresses, the body of water becomes more and more shallow, consequently the habitat becomes less suited for the submerged plants but more favourable for other plants.
Now the floating plant species invade the pond. In the beginning the submerged and floating plants grow intermingled but in the course of time the submerged plants are replaced completely.
The most tolerant species in the area are able to reproduce and perpetuate. Due to continuous interaction between plant communities and aquatic environment, the habitat becomes changed chemically as well as physically.
More water and air borne soil and dead remains of plants are deposited at the bottom. Thus, the substratum rises up in vertical direction. Important floating plants that replace the submerged vegetation are: Nelumbo, Trapa, Nymphaea, Pistia, Limnanthemum, etc.
With the continued siltation, the pond bottom is gradually raised and water layer becomes shallow and rich in nutrients. The habitat is changed so much that it becomes less suiting to the floating plants, some other plants, such as reeds, are able to colonise the pond. This is accompanied by the invasion of dragon flies, crustaceans and more rooted species, such as Typha, Phragmites, etc.
The foliage leaves of such plants are exposed much above the surface of water and roots are generally found either in mud or submerged in water. The foliage leaves form a cover over a submerged and floating plants and thus they cut off light from the plants underneath them. Under such condition neither submerged nor floating plants can survive.
Further deposition of soil and plant debris at the bottom reduces the depth of water and makes the habitat less suitable for the pre-existing plants. When the bottom reaches very close to the water surface many secondary species of plants, such as Polygonum, Sagittaria, etc., make their appearance. Thus, the species composition of the pond keeps changing with time.
The filling process finally results in a marshy soil which may be too dry for the plants of pre-existing community. Now the plants well adapted to this new habitat begin to appear in the pre-existing community in mixed state.
As most of the plant species are sedges and rushes, and therefore, this is called sedge marsh stage. With increased setting of silt and deposition of dead organic matter derived from pre-existing community of plants, the pond becomes shallower until it gets transformed into a terrestrial habitat.
Gradually grasses, bushes and other mesophytes start appearing and after sometime the sedge and marshy vegetation is totally replaced by them.
In the beginning some shrubs and later medium sized trees form open vegetation or woodland stage. These plants produce more shade and absorb and transpire very large quantity of water. Ultimately these terrestrial species colonise the pond area and a climax community is established. In the climax forest, all types of plants are met with.
Trees are dominant and they have control over the entire vegetation. During hydrarch succession, the colonisation by land plants usually progresses from margins toward the centre of the pond area. At the climax stage, a complete harmony is established among plant community and habitat.