Methods of Studying Plant Community | Ecology

In this article we will discuss about:- A. Qualitative Structures of Plant Community and B. Quantitative analysis of Plant Community

A. Qualitative Structures of Plant Community:

The qualitative structure and composition of plant community can be described on the basis of visual observations without any special sampling and measurement. In the qualitative Characteristics floristic enumeration (species content), stratification, aspection, sociability, interspecific associations, life-forms and biological spectrum, etc. are studied in the field.

1. Floristic composition or species content of community:

The study of species content in a community is of paramount importance. The species content of a community can be studied by periodic collection and identification of plant species for the whole year. This will show the tolerance of each species for different environmental conditions (Hanson, 1950).

2. Stratification and aspection:

The number of strata or layers in a community can be determined by general observation of the vegetation. If one periodically observes the flora for the whole year, changes in the appearance of vegetation may be apparent with the change in the season. This is known as aspection. For this phenology of species in relation to different seasons of the year is recorded.

The term phenology was first used by shelford (1929) to mark the events associated with seasonal, succession in natural communities. Later ecological society of America defined phenology as the science dealing with the appearance of certain events during the life cycle of an organism in different seasons of the year. Lieth (1970) defined phenology as the art of observing the life cycle or periodicity of plant and animals. Phenology, thus forms a part of community periodism. It is applied to those ecosystems where seasonal changes are not well marked such as desert cosystem.

On the basis of general observations of the vegetation a number of layers have been distinguished:

• L1—ground stratum like mosses, thallophytes, lichens, etc.,
• L2—herbaceous or ground flora,
• L3—middle layer or shrubby layer, and
• L4—top layer or canopy layer of trees.

In grasslands even two to three strata may be distinguished:

3. Life-forms:

On the basis of general appearance and growth, the species of community are grouped into different life-form classes. The chief criteria for recognizing life-form classes. On the basis of percentage values of different life-form classes, real nature of habitat and community can be understood.

4. Sociability:

In a plant community, the individuals of species are not evenly distributed. Individuals of some species grow widely spaced while those of some other species are found in clumps or mats. The space relationship of plants is referred to as sociability. Individuals of some species when growing in clumps are either very weak or they tend to disappear due to hard competition and as such they cannot form big populations.

B. Quantitative analysis of Plant Community:

The structure of sociological order in any plant community cannot be studied by observing each and every individual of plant species growing in a habitat. It is rather impossible. Therefore, rough estimate of species content of a habitat is made by observing the plant species at different places or sample areas, in the habitat.

Several methods have been used by ecologists for this purpose which are as follows:

(1) Quadrat method

(2) Transect method

(3) Loop method

(4) Pointless or point method

1. Quadrat Method of Sampling the Vegetation:

The quadrat is a square sample plot or unit for a detailed analysis of vegetation. It is actually the sample-plot method of Clements (1898). It may be a single sample plot or may be divided into several subplots. In vegetational analysis, quadrats of any size, shape, number and arrangement may be used. In the study of a forest community quadrats of one-fifth acre are established to include maximum number of trees, while for studying shrubs and grass covers usually the quadrats of smaller sizes are used

For grassland and low herbaceous community, the quadrats of one square metre size or 50 cm x 50 cm size or even 20 cm x 20 cm size may serve the purpose. The shape of quadrat is usually a square (Fig. 6.5 A and B) but rectangular or even circular ones are also used. In some cases rectangular sample plots often give the best results. The ratio of breadth and length in rectangular plots is generally 1: 2 or 1: 4 or 1: 8.

Kinds of Quadrats:

Quadrats are named according to the use. These are of the following types:

(i) List quadrats:

When the organisms encountered in the sample plot are listed by their names, the quadrat is called list quadrat. It includes all the species botanically identified or otherwise. A series of list quadrats gives floristic analysis of the community. This is used for studying the frequency of different species.

(ii) Count quadrat or list-count quadrat:

When the species name and the number of individuals of each species found in the sample area are recorded, the sample plot is called count or list-count quadrat. This type of plot is usually used in forest survey work.

(iii) Cover quadrat:

When the actual or relative coverage is recorded usually as percentage of ground area covered or shaded by vegetation, the sample area is known as cover quadrat.

(iv) Chart quadrat:

Quadrats that are mapped to scales to show the location of individuals of species are called chart quadrats. Individuals plants are recorded on miniature quadrat on a graph paper often with the help of an instrument called pantograph. This is very tedious work but where long range studies of vegetational changes are being made, this method provides a big picture. So far as the distribution of quadrats in the study area is concerned, statistically reliable estimates are obtained by randomized plots.

2. Transect Method:

A cross-section of an area used as a sample for recording, mapping or studying vegetation is called transect. It may be a strip, belt or a line across the area of study. The species occurring along these strips or lines are recorded. Because transect is continuous through the study area, it can be applied in studying the gradual and continuous changes in the vegetation along the line or strip with the change in environment. On the sloppy area, the transect is laid between two points at different altitudes.

The transects are of two types:

(i) Belt transect

(ii) Line transect

(i) Belt transect:

It may be established as follows:

(a) The total area of the site to be studied is divided by 5 or 10 to obtain the total number of sample areas.

(b) A series of belt transects of predetermined width and length are laid and the belts are divided into equal sized segments (Fig. 6.8). These segments are sometimes called quadrats, but they differ from true quadrats in that each of them represents one observation point.

Each segment within a belt is a part and the belt as a whole is one sampling unit.

(c) Names of species and number of individuals of each species in each unit are recorded.

The belt transect method is used to estimate abundance, frequency, and distribution of species in the community.

(ii) Line transect:

It is one dimensional transect. In this method, observation is taken on lines that are laid randomly or systematically over the study area (Fig. 6.8).

The procedure is as follows:

(a) A metric steel tape or steel chain is stretched between two stalks 33.5 metres or one chain apart.

(b) The line is considered to be a one centimeter wide belt extending along one side of the tape or chain.

(c) The observer moves along the lines and records plant species and the distance they cover along the line transect. For grasses, rosettes and dicot herbs, the distance covered is measured along the line at ground level. For shrubs and tall herbs, the shadow or foliage cover is measured.

(d) Twenty or thirty randomly placed lines under most conditions adequately sample the community.

The following information’s can be obtained by this method:

(i) The number of times each individual species appears along the transect.

(ii) The occurrence percentage for each species in relation to the total species.

(iii) The total linear distance in cm of each species along the line.

(iv) The total distance of intercept by all species per 30 m line.

3. The Loop Method:

This is a simple, accurate and quick method for sampling of only grassland and low herbaceous communities. It is used for determining community composition, species frequency and range condition. In this method equally spaced 100 small circles or loops located along a stretched line are used as observation points.

The procedure of loop method of sampling is as follows:

(a) A small wire loop of 2 cm diameter is made.

(b) A point is located at random in the community and from the point 33.5 m long steel tape is stretched out. The observation points are marked in each metre at 33 cm, 66 cm, and 100 cm mark. In this way 99 observation points are marked in 33 metre distance. Near the end of the tape at 33.33 metre mark an additional point is marked. This brings the total number of observation points to 100.

(c) At each observation point wire loop is dropped to the right side of the tape and species encountered in the circle is recorded.

In this method 20 to 30 transects under most conditions adequately sample the community. Since 100 observation points are used in each line, the sum-totals are read as percentage. By this method species contents and cover are easily computed.

4. Pointless or Point Method:

In this method of sampling observations are taken on the point in the study area where a nail or set of nails touch the ground on grid lines or at random places.

There are several point methods of sampling but here only the following two methods will be discussed:

(i) Point frame method

(ii) Point centre method

(i) Point frame method:

This method was introduced by Levy and Maiden (1933). It is done with the help of point frame. This consists of a scale like frame, supported by a pair of legs. The frame bears 10 equidistant holes having 60 cm long pointers or pins

It is placed one after another at several observation points in the study area and the plant species that are hit by the pointed end of the Pointers or nails are recorded. Besides this, the number of times the species are hit and the total number of points taken are noted. From these values the quantitative structure of community is explained. This method is employed in the study of grassland and low herbaceous communities even on uneven ground.

(ii) Point Centre or Quarter method:

In this method of sampling four measurements are taken at each observation point. The observation points can be mechanically placed along a straight line or they can be located at random. Quarter method was first described by Cotton and Curtis (1956). In this method an easy instrument is used which consists of a brass needle or a nail fitted with rubber cork and compass on the top

The procedure of sampling is as follows:

(a) At each observation point the needle is fixed in the ground. This is central point.

(b) The working area is divided into four quarters or quadrats by visualizing two grid lines predetermined by the compass at right angles. Both the lines should cross each other at the central point.

(c) Now in each of the four quarters plant nearest to the central point is spotted and species recorded. The distance of each plant from the central point and also the basal diameter of the plant are measured.

(d) Tally at least 50 such points.