The actual recording of the plant community is done by the use of small sampling units. These units may be in the form of area, line, or point, as has been employed. In the quadrate, transect, and point sampling methods, respectively as follows:
(a) Quadrate method:
It is a technique which is used when only a part of a large area is sampled. On the basis of this information, the total population of the area is estimated. For example, if we want to know the number of pine trees in a forest, we can make a total count, but this may be too much time consuming, difficult and expensive.
Instead, if we count the trees in several blocks or squares (quadrates) of the forest and by extrapolating these results for the whole forest area, we can make an estimate of the total number of trees. The quadrate method is used to measure the population density of organisms such as plants, planktons, earthworms, insects and also blood cells in the blood.
Quadrate sampling method for population estimation the pine forest has been marked off into 30 squares. Thirteen trees are counted in four corner squares and two central squares. Thus, if 13 trees are in 6 squares, we would estimate 65 pine trees to be present in 30 squares (after Herreid II, 1977)
Shape and size of quadrates:
A quadrate is a sample unit or plot which is an area of a definite size. In shape, it may be rectangular, square or circular. The size of the quadrate is determined according to the characteristics of the community. The richer the flora, the larger or more numerous the quadrates must be.
To sample forest trees the fifth-acre plot is a popular size, but it may be too large if trees are numerous of if many species are involved. Smaller quadrates are used to study shrubs and understory trees. For grass and herbaceous plants, 1 m2 is the usual size.
Kinds of quadrates:
Quadrates are often labelled according to the uses of or data derived from them.
(1) List quadrate:
In list quadrate, the organisms found in the area are listed by name.
(2) List-count quadrate:
In list-count quadrate, in addition to listing out the species, numerical counts of individuals of each species are also made for estimating the abundance and density. This method is widely used in forest survey work.
(3) Chart quadrate:
It is a quadrate that is mapped to scale to show the location of individual plants. In this method, the individuals are recorded on a miniature quadrate on graph paper with the aid of an instrument called pantograph at intervals of months or years.
(4) Experimental or permanent quadrate:
It is a modified chart quadrate which is used for successional studies in an area. In this method, the chart quadrate after charting the vegetation is left undisturbed, and the same site is studied for vegetation changes over a long period of time.
(b) Transect method:
A transect is a cross section of an area used as a sample for recording, mapping, or studying vegetation. Because of its continuity through an area, the transect can be used to relate changes in the environment. Line transect method is commonly used for sampling shrub stands and woody understory of the forest and it consists of taking observations on a line or lines laid out randomly or systematically over the study area. For this purpose a measuring tape or calibrated string is used.
(c) Point method:
In commonly used point-frame method, the sampling unit is a point. The point frame apparatus is made up of a wooden frame. Ten movable pins, each 50 cm long, are inserted in the frame. The apparatus is placed at randomly in the area under study at several places. Each pin is considered as one sampling unit.
When one records the different species present in the community or vegetation of a given area by above mentioned three common methods, then analytical parameters like frequency, density, abundance, etc. of each species are then calculated by quantitative methods to be discussed later.
The way in which plants of different species are arranged in vertical strata in order to make full use of available ecological requirements like light intensity, temperature, moisture contents, organic contents of soil, etc., is known as layering or stratification.
The data on stratification are obtained by the use of bisect, where by constructing the vertical projection one may plot the stature of plant species, in order to find out the distribution pattern of stem and roots of different species.
Phaenology is the study of organisms as affected by climate especially dates of seasonal phenomena, as seed germination, flowering, fruiting, leaf-fall, seed and fruit dispersal. Data on phenology of each species in the community are recorded. Phenology of different species may differ and this is influenced by the environment.
Vitality means the condition of a plant and its capacity to complete the life cycle. Some species are weak and their seeds fail to germinate. The health of a species is determined by the weight of the plants, stem height, root length, leaf area, leaf number, number and weight of flower, fruit and seed, etc.
A life-form is the sum of the adaptation of the plant to climate. Rannkaler (1934) considered that the way in which different species overcome the adverse environmental conditions, determines their limit of distribution. Thus, the plant’s climate can be expressed by the statistical distribution of life forms in the flora of a particular region.