“Vegetation is dynamic, an ever-changing complex, now appearing quiescent and in complete equilibrium with the habitat, now displaying an obvious evidence of change” says E. Lucy Braun (1956).
Observation of the natural changes in vegetation long ago resulted in the concept of succession.
“Plant succession is a competitive drift in which at each phase, until the climax, the constituent species render the habitat more favourable to their successors than to themselves”-Salisbury.
Under natural conditions the vegetation occupying a given habitat is called plant community.
Since the community is not stable, it passes through many developmental stages in definite sequence and in definite direction generally from simple to complex and rarely from complex to simple.
The gradual replacement of one type of plant community by the other is referred to as plant succession. According to E.P. Odum, “plant succession is an orderly process of community change in a unit area.”
Salisbury defines plant succession as follows:
“Plant succession is a competitive drift in which at each phase, until the climax, the constituent species render the habitat more favourable to their successors than to themselves.” Clements gave a very simple definition of plant succession. According to him, succession is a natural process by which the same locality becomes successively colonised by different groups or communities.”Succession is a complex universal process which begins, develops, and finally stabilizes at the climax stage. The climax is the final mature, stable, self-maintaining and self-reproducing stage of vegetational development in a climatic unit.
Succession is generally progressive and thus it brings about:
(1) Progressive changes in the soil conditions or habitats. These changes bring the habitat from extreme to optimum conditions for plant growth, and
(2) Progressive changes in the life forms or phiads.
Causes of Succession:
The main causes of succession are as follows:
(1) Climatic causes,
(2) Topographic causes, and
(3) Biotic causes.
1. Climatic causes:
Plants cannot adjust with the long range variations in the climate. The fluctuating climate sometimes leads the vegetation towards total or partial destruction and, as a result, the bare area develops which becomes occupied by such plants as are better adapted for changed climatic conditions. Drought, heavy snowfall, hails and lightning’s are some of the important factors for the destruction of vegetation. Sometimes new bare ground is formed by emersion of land from the bodies of water (ponds, rivers etc.).
2. Topographic causes:
These are concerned with the changes in the soil.
The following are two important soil factors which bring about changes in the habitat:
(i) Erosion of the soil:
Sometimes surface soil is removed by a number of agents, such as wind, water currents, and rainfall. This process is known as soil erosion. In the process of erosion new and bare area is exposed in which new plant communities begin to appear one after another.
(ii) Soil deposition:
It is one of the important causes that initiates succession. Soil deposition results owing to heavy storms, glaciers, snowfalls and landslides. If the deposition of soil takes place over an area already covered with vegetation, the plants occurring over there may be suppressed and destroyed. Deposition results in a new bare area on which succession of vegetation starts.
3. Biotic causes:
Many biological or living agencies also affect the vegetation in many respects. Grazing, cutting, clearing, cultivation, harvesting, and deforestation, all caused by living agencies, are directly responsible for vegetational change. The parasitic plants and animals also affect the vegetation and destroy it.
Succession and Climax Concept:
Plant succession is an orderly change of vegetation. It involves gradual and successive replacement of one plant population by the other. Any concrete example of plant succession taking place on a particular habitat is termed as sere, its various intermediate stages are called the seral stages and communities representing these stages are called the seral communities. Though the seral communities are not clearly distinct, yet they are recognized only because of some dominant plant species growing in them.
The first plants which appear on the bare habitat are called pioneer plants. Actually speaking, plant succession is not a series of steps or stages but is continuous and very slowly changing complex. It is dynamic process. The replacement of vegetation takes place individual by individual.
There is no jump from one dominant community to another. Dominant species of one community may persist along with some new migrants for several generations in a given area and bring about several changes in the habitat by their dense shades and leaf litter. When the habitat becomes extremely non-tolerable for the existing plants then the plants that are well suited to that habitat will come and become dominant.
After several such changes, a stage may come when the habitat becomes occupied by most tolerant species that can reproduce and perpetuate well. Thus, the process leads to establishment of climax community; a mature, dominant, self-maintaining and slow changing plant community. Climax dominants are the species best adjusted to habitat and are able to take possession of the habitat and hold it against the new invading species.
The treatments of climax and succession in Clementsian ecology imply the following assumptions about the orderliness of vegetation:
(1) The succession is an orderly growth process. Succession beginning in different environmental conditions finally reaches similar climaxes.
(2) The climax is determined only by climate, consequently climax and climatic region must correspond.
(3) The vegetations consist of climaxes and their successional conditions are clearly distinguishable.
Whittaker (1953) says that “successional processes give an impression of relative irregularities and disorderliness in detail together with a degree of orderliness in general pattern and trend.”
Clements (1928) considered the climax formation as adult organisms, the fully developed community of a region, of which all other communities are the stages of development. Since the climate alone determines the climax formation, there is one true or climatic climax in a climatic region. The climatic climax is achieved where physical conditions of the substratum are not so extreme as to modify the effects of the prevailing regional climates.
Sometime the climax is greatly modified by the physical conditions of soil, such as its topography and water content. Such a climax is known as edaphic climax. Relatively stabilized vegetation other than climatic or true climax may be produced in a given region because of distinctive soils or other habitat characteristics. These are referred to as developmental communities.
Sometimes the vegetation is prevented from reaching to the actual climax stage by the factors other than climate, as for example, fire, cutting, grazing, flooding, etc. Thus, the vegetation which is in the imperfect stage of development, is held indefinitely in stages preceding to real climax either by natural or by artificial factors. This type of imaginary climax is termed as sub- climax.
Such plant communities would tend to reach real climax state if the causative factors are removed. Sometimes many disturbances cause modification or replacement of the true climax and consequently a modified sub-climax is formed which is termed as dis-climax.
If temporary change of the climate stops the development of vegetation before it has reached the expected climax, it forms pre-climax. Sometimes the environment changes in such a way that the vegetation progresses beyond the expected climax stage. The new climax, thus formed, is termed post-climax.
Mono-climax and Poly-climax Theories:
As regards the number of climaxes in a given habitat or climatic region, there are two different schools of thought.
These are as follows:
(i) Mono-climax theory:
According to Clementsian school, there develops only one true climatic climax in a particular climatic region. This concept is generalized as mono-climax theory.
(ii) Poly-climax theory:
Tansley (1935) proposed this theory. The second school of thought holds the view which is opposite to mono-climax concept. It defines climaxes as the stabilized and self-maintaining plant communities and considers that a number of climaxes may exist in a given area and further classified them as edaphic climax, topographic climax, “biotic climax, etc.
“The essential difference between mono-climax and poly-climax approaches seems to be the relative emphasis,” says Whittaker (1963). According to him, those who support the mono-climax theory emphasize the essential unity of climax vegetations in a given area with allowance also for stabilized plant communities other than climax, but the exponents and supporters of poly-climax concept emphasize the inherent complexity of climatic climax with allowance also for prevailing climax community which characterizes the vegetation of given area and expresses its relations to climate.”
Clements’ mono-climax theory has been severely criticized in recent years on the ground that in the theory regionally prevailing undisturbed vegetation occupying the large part of the land surface was regarded as real climax and other stabilized plant communities in the same area were recognized as sub-climaxes which only theoretically could be replaced by the climax.
Braun, L.E. does not believe in mono-climax concept and comments “to me, mono-climax seems impossible.” He supports poly-climax idea but that too seems questionable to him. While working on the ‘Deciduous forests of Eastern-north America’ in 1950, Braun found many intergrading communities among which some were in the developing state and the others had reached the climax stage, some were very local in occurrence and extent (regional climaxes) and others recurred frequently over a big geographic extent (edaphic or topographic climaxes). Thus, his observations fully supported the poly-climax idea.
Again in the year 1956, in the paper “Development of Association and Climax Concept”, Braun supported the poly-climax idea and said, “the prevalence of any particular climax type may be due to control of climate; it may be related to the history of erosion cycles and past climate; it may be due to state of development of climax communities or more likely due to a combination of all.
The mono-climax theory, of course in modified form, has also been expressed by Dansereau (1954) and Walter (1954). Henry J. Costing, of Duke University in his book, “Study of plant Communities”, mentions that poly-climax theory is more practical. Nichols (1923) also expressed the same view. British ecologists under the leadership of Tansley maintained a general skepticism of mono-climax concept. Russian ecologists also have similar belief
Smithusen (1950), Whittaker (1951, 53) and many other American ecologists support climax pattern or poly-climax concept and all believe in the fact that the climax state is determined by the environments of individual plant communities and not by regional climate, the latter identifies the climax as a community steady state and substitutes ‘prevailing climax’ for climatic climax.