Let us make an in-depth study of the introduction, process and types of plant succession.
The process of plant succession consists of nine steps. The nine steps are: (1) Nudation (2) Migration (3) Germination (4) Ecesis (5) Colonisation and Aggregation (6) Competition and Co-action (7) Invasion (8) Reaction and (9) Stabilisation.
Introduction:
Like an organism every plant community has a developmental history; this developmental history is called plant succession. A plant community first comes into existence with the colonization of a bare area by spore-bearing or seed-bearing plants. The bare area may be a rock, open soil surface or a shallow pool or lake; it is successively occupied by different plant communities.
According to Odum, “plant succession may be defined as an orderly process of community change”.
In the words of Salisbury, “Plant succession is a competitive drift in which at each phase until the climax the constituent species render the habitat more favorable to their successors than to themselves.”
Most plant associations are not static and constant in extent and character but tend to expand their range to cover the entire environment to which they are well suited. Some particular species, on account to its size, abundance and ability to compete successfully with its associates, usually becomes dominant.
The presence of a particular dominant species, which is usually called as pioneer community, may in time change conditions so that a very different group of plants invade the community and eventually replace the original colonizers.
Climatic and topographic changes of various kinds may in time modify conditions so that there are marked changes in the distribution of plant associations. In a given locality one group of plants may thus encroach upon another. This kind of succession may be rapid, or it may extend over many years and it will continue until stability is attained through the establishment of vegetation which is essentially permanent. This is called as climax association for the region in question.
The great plant formations, such as forest, grassland, and tundra are climax formations. The process of succession may start at places like bare rocks, exposed soil surface, shallow water such as a small pool, silting up rivers and banks of a lake etc.
Plant succession may be of two kinds:
(i) Primary succession:
It begins in areas which have previously been unoccupied by plants, such as open water, bare rock, or sand.
(ii) Secondary succession:
This kind of succession begins wherever the existing vegetation has been destroyed without denuding the area of soil. It usually starts after forest fires, cutting of the trees, flood and erosions. It is also of common occurrence in abandoned agricultural lands. A single case of plant succession at a particular kind of habitat is usually referred to as a sere, and the various stages of a sere are called seral stages.
Depending upon the nature of the habitat on which the plant succession begins seven types of seres may be distinguished:
1. Hydrosere:
When succession starts in aquatic habitat (Fig. 4.1).
2. Xerosere:
When succession initiates on a dry, bare land.
3. Lithosere:
It starts on a bare rock surface.
4. Psammosere:
Initiating on sandy habitats. Here the pioneer community comprises sand-binding grasses with runners, e.g. Spinifex and Ipomoea biloba.
5. Halosere:
It starts in saline soil or water. Here the pioneer plants usually have succulent leaves and stem e.g., Suaeda maritima, Acanthus ilicifolius, Chenopodium, Basella and some species of Asclapias.
6. Senile:
It is the succession of micro-organisms and lower plants on dead plant parts and bodies.
7. Eosere or Geosere:
It is the development of vegetation in an era.
Process of Plant Succession:
Major steps in a autotrophic succession are as follows:
1. Nudation:
An area is exposed.
2. Migration:
The process of dispersal of seeds, spores and other structures of propagation of the species to bare area is known as migration.
3. Germination:
It occurs when conditions are favourable.
4. Ecesis:
Successful germination of propagules and their establishment in a bare area is known as ecesis.
5. Colonisation and Aggregation:
After ecesis, the individuals of the species increase in number as the result of reproduction.
6. Competition and Co-action:
Due to limited resources, species show both inter and intraspecific competition. This results into elimination of unsuitable and weaker plants.
7. Invasion:
Various other types of plants try to establish in the spaces left by the elimination of plants due to competition.
8. Reaction:
The newly arrived plants interrupt with the existing ones. As a result of reaction, environment is modified and becomes unsuitable for the existing community which sooner or later is replaced by another community.
9. Stabilisation:
Finally, there occurs a stage in the process when the climax community becomes more or less stabilized for a longer period of time and it can maintain itself in equilibrium with the climate of the area. As compared to seral stage community, the climax community has larger size of individuals, complex organization, complex food chains and food webs, more efficient energy use and more nutrient conservation.
Major Trends during Succession:
1. There is an increase in structural complexity.
2. Diversity of species tends to increase.
3. Biomass and standing crop increase.
4. There is a decrease in net community production.
5. Increase in non-living matter.
6. Food chain relationship becomes complex.
7. Niche becomes special and narrower.
8. Energy use and nutrient conservation efficiency increases.
9. Stability increases.
Types of Seres:
(A) Hydrosere:
A sere beginning on a wet area is often referred to as a hydrosere. It may proceed in open bodies of water, such as ponds, lakes, and marshes etc.
Hydrosere consists following six seral stages (Fig. 4.1):
(1) Submerged stage:
In this initial seral stage, a number of submerged aquatic plants, such as Hydrilla, Elodea, Potamogeton, Ceratophyllum, Najas, Vallisnaria, Utricularia, Ranunculus and several algae occupy the shallow pond or lake, which, accumulating after death and decay, gradually raise the bottom of the pond or lake. Silting may also be associated with this accumulation. The inadequate oxidation of flora and fauna remains of the lake results in the formation of humus-which makes the bottom of the lake firmer.
(2) Floating stage:
As the bottom of the lake is raised, a second, or floating, stage follows, characterized by plants like Nymphaea, Polygonum, Limnanthemum and Castalia etc. These plants are rooted in the mud, and their broad leaves float on the surface of the water shading the submerged plants below. Besides these, free floating plants like Azolla, Eichornia and Lemna may also make their appearance. The death and decay of the submerged and free floating plants further raise the level of the lake bottom and contribute further to the soil-building process. This initiates the next reed-swamp stage.
(3) Reed-Swamp stage:
This stage is initiated in extremely shallow waters (i.e., hardly one to four feet deep). The area is invaded by amphibious plants like Scirpus, Typha, Phragmites etc. These plants remain only partly submerged in water.
Their rhizomes are profusely branched and they are rooted in the bottom of the lake. These plants prevent light to reach submerged and floating plants which consequently die, and their dead remains settle down on the lake bottom raising its level further.
Now a second group of plants, such as Sagittaria, Alisma and Acorus etc., invades the area. Eventually the habitat is made unfit for the growth of the plants of reed-swamp stage. The soil becomes dry enough to afford a foothold for terrestrial species.
(4) Sedge meadow stage:
Reed-swamp stage is followed by sedge-meadow stage which is characterized by plants like Carex, Juncus and Eleocharis. The soil level continues to rise and soil organic matter continues to increase. More competent and dominant plants, such as Mentha, Caltha, Iris, Galium, Campanula and Teuricum etc., invade the area. By excessive transpiration and soil binding, these species make the area too dry for any hydrophytic plant. This eventually leads to other-sub-climax vegetation.
(5) Woodland stage:
The sedge-meadow stage leads to the formation of heath land which remains saturated with water in spring and early summer. A new sub-climax vegetation dominated by shrubs and small trees make their appearance in this area. Important among these plants are Salix, Cornus, Cephalanthus, Alnus and Populus etc. Due to shade of these plants grasses and sedges disappear from the area. Shrubs and trees further lower the water table and bind the soil.
(6) Climax forest stage:
As more and more plants appear in the area, competition among these plants also intensify and soil organic matter further increases, soil becomes more fertile and consequently the area is invaded by larger trees. Competition then becomes less intense as the community becomes stable and a climax state is reached. It may also be pointed out here that succession in water always does not necessarily lead to land community. When succession starts in deep and large open water it may lead to a stable aquatic vegetation.
(B) Xerosere:
When succession starts on a dry, bare area, it is usually referred to as xerosere. When the bare area is dry, the pioneers may be more or less xerophytic, the degree of xerophytism depending on local climate and physiographic factors.
A xerosere usually includes the following six seral stages (Fig. 4.2):
(1) Crustose lichen stage:
Succession on the bare rock surfaces begins with crustose lichens as pioneers. These lichens migrate to the rocks by means of wind-borne spores and soredia. The lichens grow only when enough moisture is available, but they can withstand drought conditions for long.
The lichens release carbon dioxide during respiration which after combining with water forms a weak acid. The mechanical and chemical action of the lichens on the underlying rock, loosens particles, which, together with decaying lichen remains form a thin layer of soil on the soil surface. The requisite nitrogen is brought in by rain and wind-blown dust. These lichen form pioneer community.
(2) Foliose lichen stage:
Simple crustose lichens may be followed by larger, leafy forms, such as Parmelia, Dermatocarpon. Umbilicaria, which grow on the slight accumulation of soil and humus Foliose lichens further loosen the rock particles. They overshadow the crustose lichens which eventually die and decay thus increasing the amount of humus in the soil.
(3) Moss stage:
Lichens are succeeded by mosses, which, like lichens, are able to survive in dry environment. These mosses are xerophytic in nature and important among these are the species of Polytrichum and Tortula.These mosses form an open community connected with a dense rhizoid system which passes through and binds together a few millimeters of soil particles. Among the shoots of these mosses wind and water borne soil continues to accumulate. The primary role of these mosses is to stabilize the soil surface and to increase its water-holding capacity.
(4) Herbaceous stage:
The moss plants increase in number until a close carpet of moss is formed over the soil. The mosses shade the lichens and successfully compote with them for water and nutrients which eventually result in the death of the lichens. The death and decay of the lichens and old mosses add to the amount of organic matter in the soil and still further increases its water- holding capacity.
In his way the habitat is rendered suitable for the growth of higher plants and consequently a new community of herbaceous plants, such as Festuca, Verbascum, Poa, Potentilla and Solidago etc., invade the area. The herbaceous plants over shadow the mosses, compete successfully with them for space, water and nutrients. The soil increases in thickness by disintegration of the rock and the decay of the various plant parts, more nutrients become available and next higher community, dominated by shrubs, appear.
(5) Shrub stage:
Shruby plants, such as Rhus, Physocarpus, Symphocicarpous, invade the area, erstwhile dominated by herbaceous plants, by means of seeds and underground rhizomes. The herbaceous plants of the preceding stage, now shaded, tend to disappear. The death and decay of the herbaceous plants further enrich the soil. As the shrubs grow in size and number, they continue to modify the soil and make the habitat more and more suitable for the support of still higher plants i.e., trees.
(6) Climax forest stage:
The first tree species to invade the area are usually xerophytic in character, but as the soil moisture increases, these are gradually replaced by mesophytic ones. The mesophytic species compete successfully and become dominant because their seedlings are much more shade-tolerant. Competition gradually becomes less intense as the community becomes stable and a climax state is reached.
From the foregone discussion it may be concluded that the succession, whether hydrosere or xerosere, is characterized by a more or less similar series of stages which may be counted as:
(i) Nudation, (i.e., formation of bare soil surface)
(ii) Colonization,
(iii) Ecesis (i.e., establishment and attainment of maturity)
(iv) Reaction,
(v) Competition,
(vi) Stabilization and
(vii) Climax.
The plants of climax community propagate and maintain the character of the community unless the climate changes or community is disturbed by man, fire or some other agency.