This article throws light upon the transect and quadrat methods of sampling plant communities.
Method # 1. Transect Method:
When the vegetation is to be studied along an environmental gradient or eco-tone (e.g. tropical to temperate, high or low rainfall areas or precipitation gradient, adjacent areas with different types of soil, etc.) a line is laid down across a stand or several stands at right angles. This method of linear sampling of the vegetation is called transect.
Depending upon the object of study, two types of transect can be drawn:
(a) Line Transect or Line Intercept, and
(b) Belt Transect.
The extent of area determines the number and size of transects. When transects are used to sample the vertical distribution of vegetation (i.e. stratification) they are called ‘bisects’.
(a) Line Transect:
In this type of transect the vegetation is sampled only over a line (without any width). A line is laid over the vegetation with a metric steel tape or steel chain or long rope and kept fixed with the help of pegs or hooks. This line will touch some plants on its way from one point to the other. The observer will start recording these plants from one end and will gradually move towards the other end.
From this type of transect following information could be collected:
(i) The number of times each species appears along the line,
(ii) The trend of increase or decrease of distance between the individuals of a species,
(iii) The percentage of occurrence of different species in relation to the total species,
From the observations in a number of such parallel line transects, comments can be made on the habitat and other environmental conditions on different portions of the transect. Every species has its own ecological amplitude and tentatively expresses the status of available water and other edaphic conditions, atmospheric humidity, availability of light, grazing and other biological pressures.
(b) Belt Transect:
The belt is a long strip of vegetation of uniform width. The width of the belt is determined according to the type of vegetation or the stratum of vegetation under study. In close herbaceous vegetation it is usually 10 cm, but it varies from 1 to 10m in woodland. The length of the vegetation is determined according to the purpose of the study.
If a transect is essential then the lines should be marked using deep-seated wooden pegs at regular intervals. A belt could be kept isolated by installing tall wire-net fence on all its sides keeping safety-space from lines.
A belt is generally studied by dividing it into some equal sized segments. The length of each segment is generally equal to the width of the transect. These segments are sometimes called quadrats. Belt transects are used in determining and understanding the gradual change in abundance dominance, frequency and distribution of different species in the transitional region between two different types of vegetation.
The structure of vegetation with regard to the relative height, depth and lateral spread of plants in both aerial and underground parts could be determined by the use of bisects. It is essentially a line transect along which a trench has been dug to a depth greater than that of the deepest root systems. The extent of different aerial and underground parts are carefully measured and plotted to scale on coordinate graph-paper.
This method reveals the form and interrelationship of underground systems of different-species growing in the community and also their relationship to different types and / or layers of soil.
So, the bisect studies provide the following information:
(i) A rough floristic picture of the community,
(ii) Stratigraphic distribution of different species,
(iii) Utilisation of space by different species,
(iv) Underground structures of plants,
(v) Arrangement and extent of root-system, and so on.
Method # 2. Quadrat Method:
The quadrat is a square sample area of varying size marked off in the plant community for the purpose of detailed study. Generally a number of quadrats are studied to acquire reasonably faithful data to realise different analytic and synthetic characters of the plant community.
It is also effectively used to determine the exact difference or similarities in the structure and composition between two or more plant communities of related or unrelated vegetation.
Quadrats can be of four types:
(a) List Quadrat:
Enlisting the names of different species growing in the quadrat.
(b) List-Court Quadrat:
Records the number of individuals of each species represented in each quadrat.
(c) Chart Quadrat:
This records the position and areas covered by branches, mats or tufts of grasses, mosses, etc. on the coordinated or graph paper. These graphs help to compare any change in structure of community in future.
(d) Clip Quadrat:
It is used for the study of bio- mass or weight of each species, all individuals are uprooted (but when the weight of a particular organ, e.g., branch, leaf and fruit are to be determined only the concerned organ is clipped or harvested) and its fresh or dry weight is recorded.
Demarcation or laying out of different types of quadrats are basically same. Generally, an adjustable wooden frame is prepared with perforations at regular intervals on each arm. Four arms are fixed in the field with the help of long nails or surveyor’s hooks and it is ready to provide data necessary for list, list-count and clip quadrat.
But, in chart quadrat more nails or hooks are fixed to the perforations on quadrat arms at regular intervals. Nails of opposite arms are connected by threads to divide the plot into a number of smaller quadrats to facilitate the recording of the area covered by individual plants on a coordinate paper in scale. When such wooden frames are not easily available it can be replaced by long threads or ropes.
The best size of quadrat to use in a community should be determined with care It should be large enough and enough quadrats should be studied to produce reliable results.
Size of Quadrats:
The size of quadrats to be used in a given community is determined by constructing a species area curve. This is done by sampling the vegetation with nested quadrat method.
Nested quadrats are a series of quadrats, laid one over the other with gradually increasing size and can be practiced in the following way:
(i) Long thread,
(ii) Surveyor’s hooks or long nails,
(iii) Measuring tape and
Put two nails ‘O’ and ‘Y’ 5 m apart. Place the nail ‘X’ 5m away from ‘O’ nail at right angle with the OY arm. Connect YO and OX by a long thread. Place the nails A & B on OX and OY, respectively, 50 cm away from ‘O’. Using another nail make a 50 cm x 50 cm square (Quadrate No. 2) Record only newly observed species in the list.
Similarly, demarcate Quadrat Nos. 3, 4, 5 etc. increasing 50 cm arm length at every step. Continue the process so long as a recognisable number of new species is added each time (Fig. 3.4).
Selection of Quadrats:
For studying any plant community a number of quadrats should be studied. As the collected data will be processed statistically, the quadrats should be layed at random, with no bias for any particular region within the community. There are a number of methods for such random selection of quadrats.
Few such methods are given below:
(i) Collect or prepare a map of the area under study. Draw a number of vertical or horizontal lines and number them separately. The numbers of vertical and horizontal lines are to be written separately on small pieces of paper and keep these two sets of paper squares in two separate beakers.
Mix these numbers in each beaker. Draw one number from each beaker and mark the place where line representing these two numbers has crossed. Draw such number pairs repeatedly to find out the positions of a desired number of quadrats and mark the places properly.
(ii) Enter the area with blindfolded eyes and a stick in hand. Throw the stick over your shoulder at different parts of the vegetation. Each such point where the stick falls should be selected as a sample area. For experimental purposes sometimes quadrats are marked permanently with the help of deep seated wooden-pegs at four corners and studied at different times according to the need of the working programme.
To understand the biotic pressure on the vegetation like grazing, or to record its developmental history, some sample plots are needed to be kept isolated by fencing them properly with wire- nets.
It is the photographic method of recording the dynamic characters of plant community. In this technique a particular plot of the vegetation is photographed periodically by keeping the camera in the same direction and at the same height. This is done by permanently fixing three wooden pegs at a place in the vegetation so that the bases of a tripod camera-stand can be set on these pegs.
The technique is effectively used in monitoring the degradation or the recover) of rangeland, secondary succession of a denuded place, spread of a disease or even some newly introduced weed into the area. As these changes take place gradually and very slowly, it is essential to keep detailed and permanent record for comparison. A series of photographs very nicely provides that record.
(iv) Ring Counts:
The age of different types of woody plans (e.g. trees, shrubs) may be determined by counting the annual growth rings of the aerial or subterranean stems. Growth rings can also reveal the climatic history of a place chronologically like the years of high rainfall or drought, presence of some chemical in the soil or atmosphere, forest fire, heavy snowfall and so on.
The method is also important in determining the successive stages of development of a vegetation and specially the sequence of dominants and subdominants.