Characteristics of Plant Communities | Ecology

In this article we will discuss about the analytical and synthetic characteristics of plant communities.

Generally, analysis of community characters is being done for two specific purposes:

(i) To record variation within and between communities and

(ii) For naming and classifying communities.

Analytical characteristics are those features of community which can be observed or measured directly in each aspect. It involves measurements of various characters in sample plots, commonly known as quadrats. Measurements made in sample plots (quadrats) are scientifically processed to reflect the characteristics of the entire community.

Two sets of characters, viz.:

(i) Analytical, and

(ii) Synthetic are studied in a community at the same time.

1. Analytic Characters:

They are directly observed or measured in sample plots. They include kinds and number of species, distribution of individuals, number of individuals, height of plants, etc.

2. Synthetic Characters:

They are derived from the measurements of analytic characters and utilise data obtained in the analysis of a number of stands.

Analytical characters are of two types:

(i) Qualitative:

They are based on non-quantitative observations, e.g., species composition and stratification of vegetation. They are expressed only in qualitative way.

(ii) Quantitative:

They are expressed in quantitative terms. They are measured. The major quantitative characters include frequency, diversity, cover, biomass, leaf size, abundance, dominance, etc.

They are as follows:

Frequency:

This is based on percentage of sample plots in which a species is present, indicating its dispersion in space.

This frequency of each species is calculated as follows:

Frequency percentage = number of sampling units in which that species occurred / number of sampling units studied Χ 100

Diversity:

This is denoted by number of individuals per unit area, indicating the relative abundance of a species.

Cover and Basal Area:

This is percentage land area occupied by a species, indicating the influence zone of a species. Although sometimes used in general sense for the area occupied by a plant, (which may be the herbage cover or the cover of basal area), it is generally used for above ground parts.

Thus, cover or herbage cover signifies primarily the area of the ground occupied by the above ground parts of plants, such as leaves, stems and inflorescences as viewed from above.

However, basal area refers to the ground actually penetrated by the stems and is readily seen when the leaves and stems are clipped at the ground surface. It is one of the chief characteristics to determine dominance. It is measured either at 2.5 cm above ground or actually on the ground level.

Biomass:

This expresses quantity of living materials per unit area, indicating the growth of a species. Thus, biomass is the standing crop expressed in terms of weight (i.e., organism mass) of the living matter present. The amount of living material, present in a component population at any time, is known as the standing crop, which may be expressed in terms of weight per unit area.

Leaf Area:

The percentages of species having different leaf sizes, indicating the adaptation of the vegetation to the prevailing environment. As the leaves are essential part and are very much affected by climate condition, their shapes and sizes have been taken as important criteria in determination of quantitative characters.

Density:

Density represents the numerical strength of a species in the community. The number of individuals of that species in any unit area is its density. This gives an idea of degree of competition.

It is calculated as follows:

Density = Number of individuals of the species in all the sampling unit/Total number of sampling units studied

The value thus obtained is then expressed as number of individuals per unit area.

Abundance:

This is the number of individuals of any species per sampling unit of occurrence.

It is calculated as follows:

Abundance =Total number of individuals of the species in all the sampling units/Number of sampling units studied

Synthetic Characters:

These are determined after computing the data on the quantitative and quantitative characters of the community. For comparing the vegetation of different areas, community comparison needs the calculation of their synthetic characters. These are determined in terms of presence and Constance, fidelity, etc.

Presence and Constance:

It expresses the extent of occurrence of the individuals of a particular species in the community.

Fidelity:

This is the degree with which a species is restricted in distribution to one kind of community. Such species are sometimes known as indicators.

Dominance:

Here, the dominance is expressed in synthetic form. On the basis of density, frequency and dominance (cover) values; there has been proposed idea of Importance Value Index (IVI). IVI of a species in the community give the idea of its relative importance. For IVI, values of Relative density.

Relative frequency and Relative dominance (cover basis) are obtained as follows:

Relative density = Density of the species x 100/Total density of all the species

Relative Frequency = Frequency of the species x 100/Total frequency of all the species

Relative dominance (cover) = Dominance (cover) of the species x 100/Total dominance (cover) of all the species

Now for IVI, three values are added. IVI values of different species are then arranged in decreasing order.

Other Synthetic Characters:

In addition to above mentioned characters, there are some other synthetic characters. They are quite useful in comparative studies on communities. Such characters include, interspecific association and association index, index of similarity, dominance index, species diversity and diversity index, etc.