Let us make an in-depth study of the units of vegetation. After reading this article you will learn about: 1. Plant Communities and 2. Plant Formations.
Plant Communities:
Individual plants (the same is true for animals, including man) growing in the open under more or less natural conditions tend always to associate together in communities—plant communities.
Each community has its special features which differentiates it from another. The edge of a river may be lined by tall grasses forming a community while in the water are other plants with floating leaves or others again, completely submerged.
All these are communities and they are essentially natural, though sometimes modified to a considerable extent by human agency. We often find plants of various degrees of heights associated together forming a stratified community, one above another. The difference in the growth form actually enables them to grow together as members of the same community.
It is also very easy to see that the various units of vegetation are not all of equal rank. In an Oakwood, as an example, in addition to the dominant species—oak, there are certain other characteristic plants. Lichens or moss covering the bark of a tree in a wood cannot really be considered as a community of the same status to the woodland itself.
Yet all these are units of vegetation and the term plant community is applied to any such unit of whatever size or rank.
A plant community is any collection of plants living together which has, as a whole, a certain distinctiveness or better, individuality. Plant communities have thus a characteristic composition as regards the species which they contain, each having their own definite structure—they are the units, which together make up the vegetation of the earth.
Transect studies are sometimes indispensable for knowing the variation of vegetation which occurs with changing environment, e.g., caused by irregularities in the topography of a soil. These studies also help us in determining correctly the succession of plant communities—how one community is replaced by another—in a particular vegetation.
A transect is a long rectangular sample plot—a continuous strip (the strip may also be divided into squares equal to their width) that gives a rough cross-section of a particular vegetation.
The belt transect is a strip of vegetation of uniform width (1-10 metres) in woodlands (largely determined by the character of vegetation) and usually of considerable length —determined by its purpose. Transects can be profitably utilised in determining primarily the biotic influence of trees upon other plants, as affected through water content and available light.
The transect method is also a commonly used one in studying forest trees.
Isolation transects are frequently used in grazing experiments. They usually consist of two strips with a protected (from cattle and rodents) strip in between, acting as a control. One of the two strips is grazed, the other left un-grazed. A history of the development of vegetation as affected by time of grazing may thus be obtained by keeping alternately grazed and un-grazed areas, open or closed.
While belt transect studies give us the structure of a vegetation in two dimensions— relative height and the lateral spread of plants (and the interrelation of one plant to the other)—the bisect studies supplement this by supplying the third—the relative position, depth and the extent of underground parts such as rhizomes or corms as well as the root system of each plant.
Bisect of profile is essentially a line transect along which a trench is due to a depth greater than that of the deepest root system. It is truly a representation of a vertical section of a community showing the roots and shoots in their natural positions.
Other methods of studying vegetation can be mentioned. A quadrat study is essential for knowledge of the structure of vegetation in a particular habitat. The quadrat (loosely, any sample plot, whether or not it is square), simply square sampling plots, is a square area of varying size (maximum about 100 sq. m.), marked off for the purpose of detailed study, which in its simplest form is the counting the individuals of each species to determine their relative abundance.
Among the kinds of quadrats are, list quadrat, (all the species in a sample unit are listed and their number counted), basal area or diameter quadrat (the measurement of basal area occupied by each species is estimated), clip quadrat (clippings are removed and dry weights, volumes and fresh weights of the tops of each individual determined), chart quadrats are those in which the position of each plant is indicated accurately in its proper place on a chart by a planimeter, a cover quadrat is one on which the actual or relative coverage or shading of these species is recorded and a count quadrat is a sample plot of which the density of plant species is determined.
Some quadrats are marked in such a way as to permit study of plants from year to year (permanent quadrat). Other quadrats are devised from which vegetation is removed artificially in order to follow critically the manner in which the plants re-enter the particular quadrat and the course and outcome of competition [denuded quadrat) among plants.
Within the last thirty years, wide and various application of mathematics have progressively invaded the study of vegetation. In order to put this study on an increasingly objective basis, hence scientifically more respectable, efforts are being made in closely re-examining certain basic assumptions with regard to the nature of the data and the modern probabilistic, statistical methods to be profitably employed.
Use of such methods eliminates personal bias and produces easily reproducible results for a given set of data, analysed by a given standard method. It will be imprudent, however, to eliminate the subjective element. In recent times, attempt are made to evaluate a given set of ecological data obtained from bisect, transect or quadrat studies, on an objective basis certainly, without losing sight of the subjective at any time.
The size of the quardrat is generally adapted to the characteristics of the community being analysed. In general, greater the floristic richness and the more heterogeneous the life forms of the community, the larger is the quadrat size.
Strictly speaking, the shape of the quadrat is square, but, for all practical purposes, sample plots can be of any regular shape—square, circular, short rectangular or even strips.
Plant communities because they possess a certain degree of individuality can be compared as a whole with a living organism in that they also have developmental history and only finally attaining a stable maturity, which is called a climax. We have seen before that plant communities are of a number of types, the nature of each being determined by the local habitat conditions. A similar combination of the same habitat factors in different areas always produces more or less similar type of plant communities.
A climatic climax is an expression of climate and is dominated by highest type of growth forms. As we have seen before, besides climate, which is probably the most stable factor, there are edaphic, physiographic, biotic and other factors which may influence the climatic factor. These factors, often termed pro-factors, help in the development of pro-climaxes.
Pro-climaxes include:
(1) Sub-climaxes where the development of typical vegetation for a particular climate type has been arrested before attainment of the final stage due to an influence other than climate,
(2) Pre-climaxes and post-climaxes where the climax formation deviates from the normal type on account of some local physiographic factor and
(3) Dis-climaxes which may result from some disturbance due to biotic modifications or change of typical climax.
Another common phenomenon seen in a tropical rain forest—stratification or storeys of vegetation—refers to the growth of shrubs, herbs, climbers, epiphytes and sometimes parasites and saprophytes as well in such forests in addition to the tall trees. These diverse life forms according to their stature and nature of existence have been called synusiae. A synusia is thus a group of plants of similar life forms and floristic composition, making similar demands on similar habitats.
The synusiae appear to have the common property of representing tightly-knit groups of species of similar ecological requirements. Thus, the species constituting a synusia may be of different taxonomical origin but can be regarded as ecologically equivalent. The total assemblage of plants living at a particular station; the community as a whole, including all its synusiae is sometimes termed Phytocoenosis.
The linean system of taxonomy appears to require existence in a group of variational unity, conforming satisfactorily in their properties with those required of the taxonomic species. These units constitute the biological species—groups of actually or potentially interbreeding populations—which are reproductively isolated from other such groups.
Plant Formations:
The largest and the most comprehensive kind of plant community, equivalent to climax and biome, is the plant formation. Plant formations may be determined primarily either by the climatic factors of the habitat—climatic plant formations—or by factors of the soil—edaphic plant formation.
There are, however, some formations which are determined partly by soil and partly by climate. In suitable soils they can only exist within a narrow range of variations in the climate. Plant formations may be determined not only by habitat factors of soil and climate but also in some cases of specific and continuous operations of man himself—anthropogenic formations.
The great climatic plan formation types of the world are determined by well- marked types of climate prevailing over wide areas of the earth’s surface. Thus a number of major vegetation types associated with particular climatic conditions can be clearly recognised such as the deciduous forests in areas, where a favourable climate alternates with an unfavourable one, due to low temperature or inadequate humidity and evergreen coniferous forests with areas where the length of the flowering season is restricted.
The tropical rain forests are conspicuous as these formation types occur all over the equatorial regions of the world. The deciduous summer forests in Europe, North America and parts of Eastern Asia and subalpine forests of coniferous trees are also among the most important climatic plant formations. Each is characterised by a specific kind of vegetation formed by the prevailing climate.
The edaphic plant formation types are determined not by climate but by the general nature of the soil. The formation types which grow in shallow water on the edges of lakes and rivers all over the world sometimes show similar growth forms, although differing in the particular species of which they consist.
Other formation types such as the mangrove formations are determined by saline soil and they are to a large extent independent of the climate.
A world formation of high latitudes (northern region) and high altitudes (high mountain side or top), cold desert, (frigorideserta) is widely known as a tundra. They are really the vegetation represented in a cold desert. The (deserta) regions are frozen for most of the year but which support a certain amount of plant life. The tundra is beyond the tree line and the plants are all small.
A few stunted creeping willows and birches are found and perhaps also a few flowering plants, which make the tundra a brilliant mass of colour for a short period each year. Most of the species are, however, mosses and lichens. Their growing season is very short and plants growing there must produce reproductive structures very quickly while the ground surface is still unfrozen.
A formation of completely treeless grasslands, especially short grasses of warm temperate or subtropical regions, is usually called steppe or terriherbosa (steppe and prairie vegetation).
Plants with which ants or termites are associated, often with special adaptive features, are referred to as myrmecophilous.
A general feature of all formation types is that the dominant species—those plants which give the formation its characteristic appearance—belonging to each are characterised by specific life forms. The following recognised climax forest formation types cover the whole of India, Bangladesh, Pakistan and Burma (Fig. 765). All these forests are determined by well-marked climate types (mainly rainfall and the accompanying humidity) prevailing over wide areas.
The tidal forest formations are, however, determined by the general nature of the soil (saline); they are edaphic formations:
1. Tropical wet evergreen—Burma, Bangladesh, Assam and a’ long belt in Western India.
2. Tropical wet semi-evergreen—Orissa and Andhra coasts, West Bengal, Assam, Bihar, Bombay, Mysore, Kerala and Bangladesh.
3. Tropical moist deciduous—Central and Northern Burma.
4. Tropical moist deciduous (Sal)—Northern India belt; Orissa extending up to Madhya Pradesh.
5. Tropical dry deciduous—almost all Central India extending up to extreme South.
6. Tropical thorn forest—Rajasthan deserts, Hyderabad, Sind and Pakistan.
7. Tropical dry evergreen—a belt on the coasts of Andhra and Madras and up to extreme South.
8. Subtropical wet forest—Mysore, Madras, North Assam, North Burma and Bangladesh.
9. Subtropical pine forest—a thin belt from Kashmir in the north to Assam and Burma.
10. Subtropical dry—Pakistan and Rajasthan (small area).
11. Wet temperate—Subtropical Himalayas from North Bihar to North Assam and North Burma.
12. Moist temperate—a subtropical Himalayan belt extending from Kashmir to North Assam.
13. Dry temperate—a Himalayan belt from Pakistan (N.W.F.P.) right up to North Assam, immediately below the Alpine forest.
14. Alpine Himalayan region, above 4800 metres.
15. Tidal—West Bengal (Sunderbans, Sagar Islands); Bangladesh; Godavari delta in Andhra; a very thin belt on the Bombay coast; Burma (Irawaddy delta); and Andaman and Nicobar Islands.
The Plant Association and Consociation:
The actual units of natural vegetation belonging to one of the plant formations are of various nature and status. The largest of these vegetation classes, apart from the climatic types dominated by, one or more species, are called the plant associations. Thus the term association refers to the major subdivisions of a formation.
A concrete community of a size smaller than minimal area and not having typical composition and structure of the associated is called fragmentary association.
A plant association has commonly more than one dominant species because it is usually composed of a number of smaller unit communities each with its own dominant. Ail association dominated by a single species of plant is called consociation. A large plant association may, therefore, consist of several consociations.
Thus an oakwood dominated by one of the species of oak or a beech-wood dominated by the common beech, is a consociation whereas deciduous summer forest though characterised by deciduous tree, is a formation but again each formation with its particular species of tree is an association.
Certain groups or pairs of moss species are habitually associated together, as for example the associations of Philonotis fontana and Bryum pseudotriquetrum of bog springs on mountains. Typha and Scirpus, although showing individually typical life forms, sometimes grow intermingled (association), but more often, separately in consociations.
If an association is characterised not by a single species but by the grouping of dominant or controlling species, it is called a faciation—the entire area of an association is thus composed of its various faciations. Each faciation corresponds to a particular regional climate—small differences in rainfall, evaporation and temperature. Temperature appears to play the leading part in the arrangement of grassland faciations.
A community dominated by shrubs and bushes is called a Scrub.
In nearly bare or recently populated areas, embankments, abandoned fields, etc., the vegetation usually consists of individuals belonging to a single species; such a community of a single species is called a family in the ecological sense. The family is usually a small unit and is typical of early stages of development, rarely being found in a stabilised vegetation.
If the individuals of a family become more numerous, adjacent families may then merge or one family may invade another. It may also be that two or more species may invade an area simultaneously. In either case, such a small initial community is generally termed a colony—a characteristic of early developmental stage of a vegetation.
There are seldom sharp lines of demarcation between the mergings of two communities of higher rank. Such transitional overlapping areas of mixed community are termed eco-tones; eco-tones contain characteristic species of each and presumably are intermediate in environment. Eco-tones may be broad and indefinite, but on mountains, or steep hill slopes or in the associations of mangroves or in associations around lakes and ponds communities may be rather strikingly delimited.
All vegetation shows more or less striking differences, very few centimetres or metres. Differences in habitat, particularly microclimate, are reflected both in number and in kinds of plants. Where these differences are continuous, usually around swamps or ponds, the structure of the vegetation is clearly shown in zonation.
A vegetation shows a definite structure with changes in temperature and available light—seasonal changes. The spring vegetation, with abundance of herbaceous growth and blossoms, changes to locally taller species with the advent of summer which again gives way to another structure in autumn.
The adjustment of various species of these seasonal changes results in what are called, aspects. Thus aspect is a seasonal impress on a community, such as the spring aspect in deciduous forests produced by perennating (geophytic) herbs or the winter leafless aspect.
Huxley (1938) introduced the word cline as a taxonomic concept indicating the existence of a variational gradient of biotypes. Cline is actually a series of form changes i.e., a gradient of biotypes along an environmental transition. Gregor (1939) used this word with prefix eco or topo to define certain terms concerned with intraspecific variation within a community of plants; a topocline is a cline which only follows a geographical gradient and an ecocline, a cline, apparently correlated with an observable ecological gradient or environmental transition.
Ecoclines can be regarded as ecological varieties of plant species. A topotype has been defined as a population of plant species in a geographical region, differing from those of another region. A topotype may be extra clinical if it does not fall within a single geographical gradient in character expression or intraclinal, if it has a reference to a particular, variable range on a geographical gradient.
An ecotype is a particular range of characters on an ecocline, related to their respective environments.
All species found in associations or consociations other than the dominants may be called subordinate species whose growths are largely determined by the presence of the dominant species (by the humus produced by the decay of the dominant species). A community of such subdominant species within an association is called a society, the lowest category in a plant community.
Subordinate species which are invariably present with an association or a consociation (may also be found outside or not) are known as constants. The exclusives and constants are the characteristic species of a plant community.
In eastern tropical Himalaya, the dominant species of plants such as Shorea, Terminalia, and Garuga, each forming its own consociation, produce what is known as Gartiga- Shorea-Terminalia association. In western Himalaya, the chief associates of Shorea robusta in the upper Doon valley are Eugenia jambolana and Terminalia tomentosa or bellerica, forming a Shorea-Eugenia-Terminalia association.