Classification of Plants Community

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Plant communities have been classified in several ways according to particular need or viewpoints.

The following criteria have been widely used in the classification of communities:

(1) Physiognomy,

(2) Habitat,

(3) Species composition and dominance.

Physiognomy refers to the general appearance of plant community. Major plant communities of large area are classified into component communities on the basis of physiognomy. Component communities recognized on the basis of physiognomy are named after the dominant life forms as for example, forest, grass land, desert community, etc. Major communities are sometimes divided into smaller segments on the basis of habitat, mainly on the basis of water contents in the habitats.

Sometime, five different types of component communities can be delineated in a major community on the habitat basis:

(1) Wet land community,

(2) Wet-mesic community,

(3) Mesophytic community,

(4) Dry- mesic community, and

(5) Dry land community.

Major plant communities are often divided into smaller divisions on the basis of species composition and dominance. Such a classification requires the knowledge of species content of the community, frequency (the regularity with which a species is distributed throughout community), dominance, and fidelity (faithfulness of species to their community). If two areas have similar communities, the communities are named after dominant organisms or those that show high frequency, e.g., Betula-Rhododendron-Magnolia association, Oak-hickory forest.

Clements recognized dynamic nature of community and he developed flonstic classification with emphasis on succession, dominance, constancy, and diagnostic species vegetation can be analyzed into the following classificatory units in descending sequence.

1. Formation:

According to Clements, the major unit of vegetation is plant formation. Plant formation is a great vegetational unit in a region determined by several dominant growth forms as for example, the forests which are characterised by trees. Plant formation is a product of macroclimate and is controlled and delimited by climate alone. In other words, ‘formation occurs in a natural area of essential climatic unity’ (Weaver and Clements, 1938).

Whittaker is of the opinion that plant formations are not distinct or concrete vegetational units determined solely by climate, but they are abstract groupings of communities of similar physiognomy (outwar appearance of community) and environmental relations. The formation, according to Clements (1938), is a complex organism and as such it arises, grows, matures and reproduces. Matuie vegetational unit which shows no sign of change in its climatic region is termed as climax community and changing vegetational units are termed as seral communities.

Dansereau (1958) considers the following basic formation types:

(i) Forests:

These are characterised by trees. The forests may be evergreen and deciduous, high and low (mostly above 8 metres) tangled with lianas, heavy with epiphytes or sparse, long lived and some rather short lived. They may harbour little undergrowth or a dense carpet of mosses. Forests of 6 types have been recognized by Dansereau (1958).

These are:

(a) Tropical rain forest,

(b) Temperate rain forest,

(c) Tropical deciduous forest,

(d) Evergreen deciduous forest,

(e) Needle leaf evergreen forest,

(f) Evergreen hard-woods.

(ii) Woodland:

It is open stand of trees which are as tall as trees growing in forest but scattered rather than clumped.

(iii) Savanna:

It contains low, branched, often flat topped small trees (less than 10 m in height) and many other woody plants scattered regularly or assembled in small groves. The intervening spaces are often occupied by seasonal grasses (in tropics).

(iv) Scrub:

It is an essentially continuous stand of medium sized, compact growing, busy plants separated by bare ground or herbaceous patches.

(v) Prairie:

It is continuous stand of tall dense grasses which are more than 50 cm high. They are usually seasonal in distribution. Besides grasses, very few shrubs may be present there.

(vi) Meadow:

It is continuous stand of herbaceous plants, the majority of them being graminoids. Meadows are devoid of woody plants.

(vii) Steppe:

It is an open stand containing bunch of grasses interspersed with low shrubs Steppe and scrubs or steppe and meadow or even prairie differ each other in respect to the plant coverage.

(viii) Desert:

It is characterised by extremely low permanent coverage. The perennial vegetation (mostly woody or succulent and evergreen) is very sparse and during rainy season the ephemerals may cover the area.

(ix) Tundra:

It consists of a very low woody vegetation, trailing shrubs and cushion plants intermingled with mosses.

(x) Crusts:

These are formed by algae, fungi, lichens on the rocky surface or soils.

2. Association:

Every climax formation consists of two or more subdivisions which are known as associations, each being marked by more than one dominant species that are peculiar to It. Association is regional vegetation in the formation. It is climax of sub climate within the general climate of the formation. Each association is similar throughout its extent in physiognomy in its ecological structure and in general floristic composition (Weaver and Clements 1938)’ Example of association is Oak-Beech association. Developing counterpart of association is called associes. Now the association concept has fallen in disuse and in its place community continuum is gaming popularity.

The vegetation is continuous, though differing from place to place. It cannot be categorized into well defined units because it is in changing state. Whittaker (1951, 56) asserts that association is not a concrete natural community. L.E. Braun too criticized association concept in a symposium organised by Ecological Society of America in August, 1956.

The main points of objections are:

(i) That the communities do not have definite limits but intergrade with one another.

(ii) That the species which seem to characterize them may extend to other communities, although, probably in different proportions.

(iii) That two communities are not exactly alike.

(iv) That the vegetation (leaving aside only those places where habitats change abruptly) is continuous, though differing from place to place.

3. Faciations:

Each association which fundamentally includes many dominant species consists of two or more subunits. These are called faciations. Each faciation may be dominated by two or more dominants, but the total number of dominants in faciation will be lesser than that of association. Seral or developing counterpart of faciation is termed as fades. Another local variation of the association is location which varies particularly in the composition of subdominants and influence.

4. Consociation:

When there is only one dominant in climax community that is known as consociation. Consociations are smaller unit communities whose single dominants still have the life-forms characterizing the formation. Such eca (vegetational units) are modified greatly by edaphic conditions, example—Oak or Beech consociation in Oak—Beech association. The developing counterpart of consociation dominated by single species is termed as conscious.

5. Societies:

Association and consociations can further be analyzed into several minor communities which are under the direct influence of local variations of habitat. These minor communities are dominated by one or more species other than the dominants of associations and consociations. These smaller units are called societies.

The dominants of societies are sub- dominants of higher econ. Society thus shows dominance within dominance whose dominant species is (or are) subordinate when we consider consociation as a whole. Developing societies are called socies. If socies have got two or more invading species without evident association, they may be called colonies.

6. Clans:

In each society there may be found two or more smallest units. These are called clans.” Each clan is a small aggregation of single but very locally and overwhelmingly dominant species. The seral equivalent is the family derived from multiplication and greganous growth of single immigrant.