Plant Community: Meaning and Characteristics | Biosphere | Geography

In this article we will discuss about:- 1. Meaning of Plant Community 2. Characteristics Plant Community 3. Vertical Stratification 4. Develop­ment.

Meaning of Plant Community:

Plants play very dominant role in the biosphere because these are primary producers in the biosphere and provide directly or indirectly food to all terrestrial and aquatic animals including man. The social group­ings of plant species are called plant community of which plant is the fundamental basic unit. Plants directly receive and trap solar energy (light energy) and prepare their own food with the help of sunlight through the process of photosynthesis.

Thus, solar energy converted into the food or chemical energy is transferred to differ­ent animals and micro-organisms through different trophic levels of food chain. Thus, plants are intermediary between biotic and abiotic components of the environment/ecosystems/biosphere. On the basis of importance and dominant role of plants in the biosphere the study of plants is given more significance.

The study of plants has been developed as an important branch of geography which is called as plant geography which includes the study of classification of plants, their spa­tial distribution, origin and development, dispersal and extinction and functions.

The main functions of plants are to trap solar energy and prepare their food with the help of photosynthesis and to circulate and transfer energy and nutrients among the organisms of different trophic levels of the food chain.

Characteristics Plant Community:

The group or association of plants growing together in a particular habitat is called plant commu­nity. In other words, ‘those plants which grow together in a particular habitat are referred to as a plant commu­nity, by which something more than a mere collection or assemblage is implied’.

‘A group of populations of different species living in the same local areas and interacting with one another is called ecologi­cal community’.

The plant community of a given habitat has three basic characteristics e.g.:

(i) A community consists of plants of two or more different species,

(ii) A plant community consists of such species of plants which are ecologi­cally related and can live and grow together in a particular habitat, and

(iii) A plant community is well organized i.e., it has well developed composition and structure which are the result of continuous interac­tions between different plant species and between plant ‘species and their physical environment through time.

The association or group of plant communities of any region is called vegetation. In other words, ‘all the plants which grow together in any area form its vegetation, the character of which depends not just on the different species present but on the relative propor­tions in which their members are represented’. For example, two habitats may have similar floras but their vegetation may vary from one another and two habitats having different floras may have similar vegetation.

For instance, if there are two simi­lar habitats wherein both have grasses and sal trees but there is overwhelming dominance of grasses and sparse distribution of sal trees in the first habitat whereas the second habitat is characterized by dense sal trees and sparse distribution of grasses, the vegetation of the first habitat will be grasses whereas the vegetation of the second habitat will be sal forest.

Vertical Stratification of Plant Com­munities:

Different species of plants are evolved in a habitat having favourable environmental conditions wherein different species of plant community grow together having different life-forms. The development of different species of plant community of a given region takes place through the processes of adaptation, competition and natural selection.

This results in the development of various strata or layers between the soil surface or ground surface and the tree canopy. This vertical layering pattern or vertical stratification of plants is the result of competition among various species of plant community to get sunlight because it is the primary source of energy for photosythesis through which these plants manufacture their food.

It is obvious that the availability of sunlight is mainly responsible for the development of vertical stratifica­tion of plants wherein the height of different species of plants varies significantly.

On an average, there are four vertical strata of plant community in a given region mainly in the deciduous forests of the temperate regions:

(i) Dominant layer represents the topmost layer of the plant community (fig. 43.1) which is determined by the canopy of the largest trees. This uppermost stratum is also called crown or canopy which repre­sents the highest limit of plant community in a given region. A secondary layer very often called as co- dominant layer is formed just below the crown or dominant layer by those large trees which are rela­tively shorter than the largest trees (fig. 43.1).

(ii) Second layer is located below the dominant or crown layer and is represented by plants of shrubby life-form. This is also called as shrub layer.

(iii) Third layer is formed by the herbaceous plants and is also called as herb layer.

(iv) Fourth layer represents mosses on the ground surface and is also called as moss layer or ground layer.

Develop­ment of Plant Community:

The replacement of one vegetation community in a given habitat by the other vegetation community is called biotic succession. Succession simply means the entire process of directional and sequential change of either plant community (groups of plants adapted to a particular habitat) or the whole ecosystem through time. In simpler term, an ecological succession may be defined as the process of sequential development of ecological community or ecosystem.

The sequence of the development of vegetation community (biotic suc­cession) is called sere. In other words, the transitional stages of sequential changes from one vegetation com­munity to another vegetation community are called SERE which is complete when the succession of veg­etation community after passing through different phases of changes culminates into equilibrium or mature con­dition.

This phase of development of vegetation com­munity is characterized by maximum growth and de­velopment of plants. Thus, the vegetation community developed at the end of succession is called climax vegetation, climax community, or climatic climax (due to dominant control of climate on the evolution and development of vegetation). It is obvious that the last succession in the chains of biotic successional changes of vegetation community becomes climax succession.

According to F.E. Clements (1916) biotic suc­cession of vegetation community is primarily of two types e.g., primary biotic succession and secondary biotic succession:

1. Primary Biotic Succession:

Primary succession refers to developmental se­quence of vegetation in those bare areas where there were no vegetation and animals earlier. Such areas or sites may be newly emerged sea floor, cooled and solidified basaltic surfaces due to recent lava flows, exposed lake bed due to drying of water, newly formed sand dunes, flood plains formed by recent alluvia, heaps of debris accumulated by man, the areas of exposed rocks due to melting of ice from the glacial areas, etc.

The initial sites for the primary successional development of vegetation may be of various types having varying environmental conditions as referred to above but for convenience such site is being selected which is of bare rock surface and does not have any earlier vegetation for the explanation of primary suc­cession of vegetation.

Thus, primary succession of vegetation on a bare rock surface having no prior vegetation and animals starts and is completed through the following stages:

(i) The initial plant-free site has relatively dry environment. It does not mean that the climate of this site is dry because the rocks of the concerned site are bare and are devoid of any plant and therefore the environment becomes relatively dry due to excessive evaporation though the climate may be even humid.

The pioneer plants are established upon the bare rocks of the initial plant-free site. The initial pioneer plants include mainly algae and lichens because they easily stick to the bare rocks and can easily adapt to the environmental conditions of the initial sites whether these may be hot, dry or cold.

(ii) Dust particles blown by wind settle down in the concerned habitat. These dust particles are gradu­ally deposited around algae and lichens. Some of the lichens secrete acids which react with the minerals of the rocks of that habitat resulting into dissolution of some minerals. This process starts the process of pedogenesis (soil-forming processes) and thin veneer of soil is formed in due course of time. The soil zone, though very thin in the beginning, is colonized by micro-organisms.

(iii) The formation of soils through rock weath­ering and soil-organisms continues and the thickness of soils continues to increase with time. Consequently, a few soil-living animals like mites, ants, spiders etc. are evolved. This ‘sere’ of successional development of plant community is characterized by more soil-living organisms, sporadic plants and wide open areas devoid of any plant. This type of plant community is called open community or pioneer community.

(iv) Secondary community of mosses replaces the pioneer community of algae and lichens in due course of time. The mosses spread over the soils like thin sheets and thus soils are covered by the mosses. Consequently, the moisture content of the soils is increased because the moss-cover retards evaporation.

New dense matting of mosses also provides organic matter to the soils and thus the soils are enriched by the addition of organic nutrients. Gradually and gradually seasonal and perennial grasses are developed along with new groups of animals like nematodes, spring- tails etc. which are able to obtain their food from the seasonal and perennial grasses.

The gradual develop­ment of grasses covers the whole area of the concerned habitat and thus is developed a dense vegetation cover which changes and modifies the micro-climates of the concerned habitat. The dense vegetation cover de­creases ground temperature and sunlight at the ground surface but increases moisture content of the soils because evaporation of moisture from the soil surface is effectively decreased due to shade provided by dense vegetation cover.

The open community is now changed to closed community (which means that no part of the concerned habitat remains without vegetation, in other words, whole area of the habitat is covered with vegetation). This is possible only when the environ­ment of the concerned habitat is wet but if the environ­ment of the concerned habitat is dry and the surface is of sandy desert, much area is still open and devoid of vegetation.

(v) After the development of sere of closed community there begins the competition among the plants for space, sunlight, water and nutrients.

There may be two alternative routes of competition among the plants e.g.:

(a) If the plants of the concerned habitat are of the same species, there is competition for the aforesaid elements among the different members of the same species and only the fittest plants survive during the competition. Thus, the principle of survival of the fittest becomes effective in the community develop­ment of plants. Such competition is called intraspecific competition which means survival of the strongest plants and elimination of weaker plants but the preser­vation of species is maintained.

(b) If there are more than one species in the concerned habitat, competition for getting space, sunlight, water and nutrients takes place among the individuals of different species wherein the strongest and most aggressive species establish dominance over the entire vegetation community.

But if all the species of the concerned habitat are equally powerful, there is maintained a balance of power among different species inspite of competition and the result is that all species are preserved and maintained. Such competition is called interspecific competition. This phase (sere) of community development is domi­nated by herbaceous plants and thus by herb commu­nity.

(vi) With the march of time there is developed large shrubs in the concerned habitat and the herb community is dominated and replaced by scrub com­munity. At this stage a very significant development takes place in that the seeds of flowering plants (phanerogams) are brought from the neighbouring areas to the concerned habitat by winds and given birth to trees in the otherwise shrub-dominated habitat.

The canopy of these scattered trees is much higher than the stratum of shrubs. Thus, the vegetation community upto this stage (sere) is mixed with lichens, mosses, grasses, shrubs and trees. This is called forest commu­nity and the sere of this successional development of vegetation community is called preclimax.

(vii) The final stage or ‘sere’ of the successional development of vegetation community is character­ized by the development of giant and very tall trees; the density of which increases rapidly and the whole of the concerned habitat is covered with dense and tall trees.

The roots of such tall trees penetrate far deeper in the ground. The soil zone attains its maximum depth and different horizons of the soil profiles are well devel­oped. The soil zones are colonized by various micro­organisms which decompose the organic matter and help in the process of energy transfer.

The vertical stratification of plant community is well developed. This final phase or ‘sere’ of the successional develop­ment of vegetation community is called climax commu­nity, climax succession, climatic climax vegetation etc. which represents mature ecosystem.

2. Secondary Biotic Succession:

Secondary succession refers to the develop­mental sequences of vegetation in those areas which had vegetation cover earlier but now have been ren­dered nude or bare due to destruction of vegetation (either partly or completely) either by natural proc­esses (like lava flow, prolonged drought, glaciation, natural widespread forest fires through lightning, se­vere storms, catastrophic floods etc.) or by human interferences (like intentional burning of vegetation, massive land use changes, mass felling of tress and overgrazing etc.).

It may be pointed out that such disturbed ecosystems or habitats still contain mature soils and some original vegetation and therefore the initial stage or sere of secondary succession of plant community is quite different from the initial stage or ‘sere’ of primary succession which starts on a bare rocky surface, having no earlier plants and animals.

The total time required for the development of climax vegetation or climax succession in the secondary succes­sion is much less than the time taken for the develop­ment of primary succession.

An example of secondary succession may be given from the hill areas of north-east India where jhuming or shifting cultivation is a common practice. Under this cultivation, first forest is cleared from small areas through burning and then the soil is cultivated for agricultural crops for a few years. When the soil loses its fertility, that area is left out and new areas are cleared of vegetation for cultivation.

The abandoned area or the old clearance is again colonized by vegeta­tion through various stages and it attains climax veg­etation or climax succession in a short period of time (a few years), because the sequence of secondary succes­sion is more rapid than the primary succession due to availability of mature soils.

When the vegetation community of any region is disturbed before reaching its climax sere by human interferences (through slow but long-term activities like deforestation or burning of vegetation etc.), the resultant vegetation is called sub-climax vegetation.

When the disturbances in the successional develop­ment of vegetation continue for long time, stages of normal sere of the development of vegetation do not take place but these ‘sere’ are deflected by those factors which bring in disturbances in the successional development of vegetation. The vegetation developed during the deflected sere persists so long as the factors responsible for the disturbance remain active. Such deflected climax is called plagioclimax and its various stages are called plagio sere.

After some time if the factors causing disturbances in the successional devel­opment of vegetation community cease to operate or become ineffective, then the environmental conditions of the concerned site or habitat are changed, with the result new environmental conditions of the habitat are unable to support and preserve the plagioclimax veg­etation. Thus, new vegetation develops under new changed environmental conditions in place of plagioclimax vegetation and the successional develop­ment of vegetation community takes place under nor­mal sere.