Quick Notes about the process of plant succession!
A plant community can be compared to an individual living organism with a developmental history this developmental history is called plant succession. Among the plant groups some are more stable than the others.
Some plant groups remain essentially the same for a long time, perhaps for centuries or even more, others are changeable, giving place, in course of a short time, to other communities of different composition.
But generally there is a definite trend in one particular direction, towards a position of equilibrium; all such progressive change is referred to as plant succession which is really the developmental process of plant communities from birth to maturity.
If an area of ground is bare of vegetation or if new bare ground is formed by emergence of land from the sea or river, by drying up of a lake or by a deposit of sand blown by wind or by landslide or by biotic agency, plants will begin to appear in it sooner or later. The first vegetation to occupy the area as pioneer colonisers are few and soon give way to others and these again to others, until a stable and intimate relationship between the vegetation and the habitat is established. This type of development of vegetation is called—autogenic succession.
Change in habitat may also be brought about by continually acting outside factors such as a gradual change in climate, filling up of a lake by deposition of silt, increasing concentration of solutes in soil water, leaching or gradual washing out of nutrients from surface layers of soils, etc.
This may result in the habitat becoming less suitable for the first colonisers and more suitable for others. Plant has nothing to do with the changes in the habitat which thus takes place and in this type of succession the first colonisers pack up, being unable to cope with changes in the habitat and others more adapted to the new circumstances take their place. A change in vegetation is thus brought about—allogenic succession.
During the first stage of the formation of plant communities on bare rock areas denuded of all vegetation, individuals which appear from seeds and spores blown in by winds, with fewer demands on the soil, have more or less wide unoccupied spaces between them—open community. Later the spaces become occupied either by new arrivals from outside the area or by the next generation seedlings of the original colonizers. Eventually all available soil surface becomes occupied by plants forming a closed community.
When a community of plants becomes closed, the habitat conditions change due to the reaction of the plants forming the community upon the habitat. As the individuals die, the products of their decay accumulate in the surface layers of the soil as humus, thereby altering physical structure as also the chemical nature of the soil leading to an increase of its water-holding capacity.
Thus the inhospitable soil now becomes more favourable and allows plant life, much more complex and more suited to the changed habitat conditions than the original pioneers, as the regenerated soil can now satisfactorily meet the higher demands made on the soil by these more evolved newcomers. Thus bigger plants progressively replace smaller ones, the original dominants arc replaced by successful newcomers and ultimately woody plants appear on the scene and occupy the area.
With the arrival of subordinate species, a highly complex climax forest vegetation is finally established. A stage of equilibrium is reached in time between the plant community and the climatic as well as edaphic factors of the locality. Thus the plant community becomes permanent.
Any particular and concrete example of plant succession is termed a sere and the various changes, a particular sere undergoes are termed seral stages or seral communities.
The early course of initiation of vegetation on bare grounds, wet or dry, differs entirely according to the nature of the initial habitat, for the plants which can colonise such different habitats differ very widely indeed. If the habitat be wet, on submerged ground on the edge of a lake or a pond, the pioneer stages in succession will consist of species of hydrophytic nature, from submerged aquatic plants (i.e., Hydrilla, Ceratophyllum, etc. ) to aquatics with floating leaves (e.g., water-lily).
As succession proceeds the soil level is built up by accumulation of decaying plant remains till it reaches the surface of water. Shrubs and trees which can tolerate waterlogged soil around their roots—the helophytes (marsh plants, like Typha)—often follow and as the level of soil is built higher above the water-level by accumulation of decomposing plant remains (humus), the habitat becomes drier by the addition of organic matter and is now certainly better aerated. The trees can now come in and the series of communities may be ultimately completed by climatic forest formation; such a sere beginning on a wet area is called hydro sere.
On dry bare habitats with extreme deficiency of water such as exposed rock surface, rocky slopes, wind-blown sand and sometimes pumice and volcanic ash, the early stages of plant succession are totally different. Lichens, (the algal partner is very often a member of the group, Cyanophyta) both crustose and foliaceous and terrestrial algae, predominantly the blue-green Cyanophyta (Nostocaceae) together with certain xerophytic rock mosses are the pioneer colonisers on such areas. Blue-green algae seem to be pre-eminently suited as the most impressive plant-pioneering community for all types of dry, inhospitable soils.
Their success as pioneering colonisers is, no doubt, due to their marked ability to withstand adverse conditions of the habitat such as desiccation, extreme temperatures, ability to hold rainwater, etc., but a capacity for considerable nitrogen fixation, as shown by many species of their group of algae in addition to photosynthesis, cannot be discounted as a possible factor determining their high status among all pioneer colonisers. The lichens, mosses and the algae disintegrate the surface of the substratum which with decayed parts of their bodies (humus) form a thin soil.
Further colonisation by other mosses follows which form thicker cushions and the amount of soil gradually increases enough to support xerophytic herbs followed by xerophilous shrubs and trees and eventually with the formation of reasonably thick layer of soil and humus, the climatic forest formations. Successions initiated on such areas where there is an extreme deficiency of water are termed xerach and such a sere, i.e., different stages of the developmental process, is a xerosere. If the sere takes its origin only on rock surface, it is termed lithosere.
A sere taking its origin on saline soil can be termed halosere. The pioneer communities in early stages of plant succession in a halosere in the Ganges and Godavari delta are the true salt-resisting mangroves such as Avicennia followed by other true mangroves, e.g., Bruguiera, Rhizophora, Ceriops, etc.
These are the plants which are best adapted to those inhospitable surroundings of high salt conditions of the soil which is largely unsuitable for other plants. As the soil becomes less saline, other species which are not really true mangroves, such as Excoecaria, Agriceras, Acanthus ilicifolius and succulents like Suaeda arrive and adapt themselves ultimately forming mixed mangrove forests. With the arrival of salt tolerant perennial grasses, halophytic herbs and shrubs come in, and the series is completed and a new climax forest formed.
A sere starting on sand is a psammosere. The pioneer communities to begin succession on such a sere are generally sand-binding grasses such as Spinifex, Ipomea, etc. Varieties of shrubs and trees follow in due succession and thereby the sand is fixed and stabilised. The organic content of the sand increases with the simultaneous increase in the moisture-holding capacity. The sandy soil may now be fit to support a climax type of forest formation.
Any seral community below the climax association is termed associes.
A miniature of vegetative succession of microscopic communities such as bacteria and fungi and followed occasionally by terrestrial algae on fallen logs, twigs, etc., has been often termed a serule.
The development of vegetation in an era is called an eosere, the major developmental process of climatic climax forest formations in a particular geological era. The term geosere has also been sometimes used to designate the total plant succession in time of the geological past.
A series of climaxes, following one another in any given area as a result of climatic change, is sometimes referred to as clisere. Contiguous climaxes move together in a common direction because of widespread climatic change that induces regional parallelism.
If the development of a vegetation is initiated not on a new ground, but on an area whose pre-existing vegetation has been destroyed by fire, earthquake or by other means, it is known as secondary succession (subseres). The classic case of re-colonisation of Krakatoa group of islands in the Pacific which was completely destroyed by a volcanic eruption denuding the islands of all traces of visible plant life is an example of secondary succession.
The pioneering was started by a few members of the family Nostocaceae (Cyanophyta) within a few years which appeared first as a dark-green gelatinous layer on the volcanic ash. Members of this group of algae are equally important pioneers in secondary plant successions under less inhospitable circumstances, e.g., eroded soil. The course of a subsere is necessarily different from either a hydrosere or xerosere on new ground because the point of commencement is different and the time taken to complete it is also less.
An enumeration of the various stages of succession which are characterised by similar series of stages, whether in primary or in secondary, hydrosere, xerosere or halosere, may be advantageous:
(a) Nudation—the formation of the bare areas by natural causes or artificially.
(b) Colonisation and aggregation—after the establishment of the first scattered invaders, the individuals come to be grouped as a result of propagation.
(c) Ecesis—adjustment, establishment and final attainment of maturity of the colonising species of plants that have migrated from neighbouring areas to new situations. It essentially consists of three well-known physiological processes, e.g., germination, growth and reproduction. Ecesis is the most decisive factor in invasion, and migration without it, is totally ineffective.
(d) Reaction—between the colonising plant species and the habitat causing change in habitat conditions of atmosphere and the soil, e.g., area once fully lighted becomes more or less shaded (temperature becomes lower and the air is more humid); a wet area becomes drier for the migrants absorb large amounts of water from the soil which is lost through transpiration; water-retaining capacity of a dry soil increases and it becomes richer by the accumulation of humus caused by the decay of dead roots, stems and leaves; the dry area gradually becomes more moist and certainly more favourable to plant growth.
(e) Competition—as a result of ecesis, competition among different colonising species ultimately leading to the survival of the species, fittest or luckiest for that particular habitat.
(f) Stabilization—achievement of complete emancipation and stability of the dominant species.
(g) Climax—final formation of climax-type forests.