The plant community can be studied for its different analytic and synthetic characters.
I. Analytical Characters:
The following qualitative and quantitative characteristics of a plant community constitute its analytic character:
A. Qualitative Structure of Plant Community:
The qualitative composition and structure of the plant community can be studied and described by visual observations only without using any sampling or measuring technique.
1. Kinds of Species or Floristic Composition of the Community:
Knowing the flora of a plant community is of paramount importance. The flora can be prepared by periodic collection and identification of different plant species throughout the year.
The floristic study on survey provides the information about the qualitative richness of the vegetation, and also an idea of the habitat structure because each species has its own range of tolerance to the environmental conditions, i.e., the ecological amplitude of a species.
The abundance or sparseness of certain species (indicator species) indicates the prevailing favourable or adverse conditions in relation to their ecological amplitude. These information are fundamental to the study of communities.
2. Abundance:
The relative distribution of a species within the community is known as its abundance. It is related to density but should be estimated in a qualitative way.
The abundance is expressed in an arbitrary scale of five degrees:
1. r = rare or very sparse (1 to 4)
2. o = occasional or sparse (5 to 14)
3. f = frequent or not numerous (15 to 29)
4. a = abundant or numerous (30 to 90)
5. va = very abundant or very numerous (100 +)
3. Stratification in the Vegetation:
All the plants in a community are not of the same size and do not occupy the same strata in multi-storeyed structure. This distribution of vertical space of the community is known as stratification. The stratification could be studied in both aerial and underground organs in a plant community. The number of strata will be more with the increase of floristic richness.
With general observation, the following strata could be recognized in a plant community (Fig. 1.2):
L1 = ground-covering plants like moss, prostrate herbs, lichen, etc.
L2 = ground flora or herb layer of considerable thickness.
L3 = shrub layer or middle layer.
L4 = canopy of small and medium trees.
L5 = top canopy of tall trees and woody climbers on them.
4. Periodicity or Phenology:
The various processes of plant life like germination, growth, leaf-flash, flowering, fruiting etc. occur in different seasons in different species of plants. This is called the periodicity or the phenology. Phenological information about different plants are very important because it has direct bearings on flora and community structure.
5. Vitality:
Study of vitality or the reproductive success of plants in a community is very important and normally done as per following classification:
V1 = seeds germinate but plants die soon without reproducing.
V2 = plants survive after germination but cannot gain the ability to reproduce.
V3 = feeble plants that never complete their life cycles but reproduce vegetatively.
V4 = plants reproducing sexually but rather feebly.
V5 = plants sexually reproducing very well.
6. Life-Forms:
Any plant community has its own characteristic structural composition of life-forms e.g. trees, shrubs, herbs, annuals, epiphytes, etc. This community structure is largely determined by the set of climatic conditions with a selected set of plant species. In other words, the life-forms of the vegetation are to a certain extent indicators of the regional climate.
The Danish botanist Raunkiaer (1909, 1928, 1934) has extensively developed this idea and considered that it is the un-favourable environmental conditions which exert the controlling power over the growth form. He stressed the significance of the adaptations of buds and shoot-tips for overcoming adverse temperatures and surviving the drought.
Raunkiaer recognized five life-form groups (Fig. 1.3):
Braun Blanquet’s System:
Braun Blanquet (1951) modified the Raunkiaerian classification of life-forms and proposed a new scheme:
Dansereau’s Symbolic Classification:
For the physiognomic description of a plant community Dansereau (1951, 1957, 1958) has developed an ecological classification using different types of symbols and alphabets depicting its structure and function (Fig. 1.4).
Dansereau’s system can be used comparing the vegetation of different places, vegetation type at one place in different seasons, etc. (Fig. 1.5).
Biological Spectrum:
The representation or the percentage distribution of different life-forms in a vegetation is expressed as biological spectrum. Plant communities widely separated geographically can be effectively compared on the basis of their detailed biological spectrum.
The life-form classifications are based primarily on the water relation of different plants and can be correlated with the influence of climatic conditions over the vegetation as:
1. Phanerophytes — warm and moist
2. Chamaephytes — extremely cold climate
3. Hemicryptophytes — conditions suitable for grassland
4. Cryptophytes — warm and dry
5. Therophytes — hot and dry
Raunkiaer prepared a normal spectrum, based on sampling of world flora, using one thousand entities. The normal spectrum provides the base line and the biological spectrum of any flora can be compared with it to understand the deviation from the normal climatic conditions.
The normal spectrum is:
A. Phanerophytes 46%
B. Chamaephytes 9%
C. Hemicryptophytes 26%
D. Cryptophytes 6%
E. Therophytes 13%
The biological spectrum can be presented graphically by drawing separate columns for each life- form (Fig.1.6):
Exercise 1:
Determine the Biological Spectrum of a Piece of Vegetation Depicting Raunkiaer’s Life-Forms:
Plant communities are generally composed of a number of species with different types of Life-Forms exhibiting their adaptations to pass over the un-favourable season of the year. The percentage distribution and the graphical expression of different life-forms is known as Biological Spectrum of a vegetation.
Requirements:
(i) Long ropes,
(ii) Surveyor’s hook or wooden pegs,
(iii) A digger,
(iv) Measuring tape; and
(v) Graph paper.
Procedure:
Demarcate the area under study with the help of hooks or wooden pegs and ropes. Record all species growing within the demarcated area, find out the position of their perennating buds and determine their life-forms using following criteria:
Results:
Record different species of plants in Table 1 and summarise them in Table 2:
The percentage distribution could be compared on the graph paper by drawing separate columns for each life-form (Fig. 1.7).
Inference:
From the percentage distribution of different life-forms one can inculcate the nature of vegetation (like grassland, halophytic or marsh land vegetation, shrub-land, desert, forest, etc.), environment of the area (temperature, precipitation, availability of light, with velocity, etc.) and other features relating to soil or other factors like proximity to a water source, pollution, etc.
7. Sociability:
Individuals of a species are not evenly distributed in the plant community. This distribution may be random, regular or grouped (contagious). The space relationship of plants is referred to as sociability which expresses the relation of individuals to each other and indicates their closeness.
Blanquet (1932) recognized five arbitrary classes of sociability:
Class S1 — plants growing singly Class
S2 — plants growing in small groups Class
S3 — plants occurring in small patches Class
S4 — plants growing in large patches or broken mats Class
S5 — plants covering the entire area forming nearly a pure population
Plants producing large number of seeds with high rate of germination generally show a high degree of sociability. So, the optimumness of environmental conditions to the adaptive capabilities of the species is expressed in its sociability.
8. Inter-Specific Associations:
Sometimes two or more species — with nearly the same ecological amplitude — grow together in the community and form the inter-specific association.
This type of association is possible if:
(i) The species can live in similar environmental conditions,
(ii) The species have similar geographical distribution,
(iii) They can reduce the competition due to the different life-forms, and
(iv) Their relationship is obligatory for one or both the species.
B. Quantitative Structures of Plant Community:
In the plant community different species are represented by few or a large number of individuals aggregating in different vegetational units. It is essential to know the quantitative structure of the community, specially the numerical distribution and the space occupied by the individuals of different species.
After extracting the essential data, following structures of the community could be determined:
1. Population Density:
The density of a species is the numerical representation of its individuals in an unit area or volume. The density of a species refers to the adequacy of its different requirements and the availability of space.
Density is determined by the following formula:
Density (D) = No. of individuals of the species in all the sample plots/Total no. of sample plots studied
For phytosociological purposes it is generally expressed as:
Relative Density (R.D.) = No. of individuals of the species/No. of individuals of all the species × 100
Qualitatively, Density of a species tells us its abundance in the vegetation or community and could be expressed to one of the abundance classes as has already been discussed.
2. Cover and Dominance:
The area of the land or ground covered by the aerial portion of the plant is referred to as cover. In a multi-storeyed community such a study is conducted for every stratum of vegetation separately as there is overlapping of plants. For the trees, cover is generally studied as basal area which refers to the area of ground actually penetrated by the stem.
The mean basal area of trees is calculated by the following formula:
Basal area per tree (or, mean basal area) = Total basal area/Number of trees
The basal area can be only a fraction of the total land area of the community but the canopy of a tree species may cover (canopy cover) several times of this land area due to the overlapping of canopies (Fig. 1.8).
The basal coverage or the area covered by a species is used to express its dominance. The higher the coverage the greater is the dominance. At first the average or mean basal area is calculated and then it is multiplied by the density of the species to determine the basal cover per unit area.
The Relative Dominance (R. Dm.) of a species can be calculated with the help of the following formula:
R. Dm. = Total basal area of the species/Total basal area of all the species
3. Height of Plants:
Though the height of plants is a genetic character but it is greatly modified depending upon the prevailing environmental conditions. In fact, the height of the plant is a good index of the suitability of the place for growing certain species of plants. It is also convenient to prepare growth curve based on height instead of weight, as the method does not require harvesting of the individuals under study.
4. Weight of Plants:
Growth of a plant is also measured on the basis of its dry weight as it expresses the total biomass of the vegetation (also see Clip Quadrat).
5. Volume Occupied by Plants:
The structure of the vegetation can also be studied by measuring the volume of space occupied by the aerial parts of the plants.
6. Frequency:
Individuals of a species are not evenly distributed within the Community. While individuals of some species are found to grow in clumps or in continuous mats, individuals of different species indicate their adaptability to the local environment and also their success in reproduction. Thus, the frequency of a species is expressed as the percentage occurrence of its individuals in a number of observations.
The frequency of different species growing in a community can be determined by the formula:
Frequency (F) = No. of sample plots in which the species occurs/Total no. of plots sampled
For phytosociological purpose it is generally expressed as Relative Frequency and is determined by the formula:
Relative Frequency (R.F.) = No. of occurrences of the species/No. of occurrences of all the species × 100
Raunkiaer (1934) recognized five frequency classes in the form of Presence and Constance:
7. Importance Value Index:
In order to express the ecological success of any species, values for its density, frequency, abundance, cover and any other such criteria do not yield the total picture. The overall picture of the importance of a species to its heterogeneous community can be obtained by adding the values of relative density (R.D.), relative dominance (R. Dm.) and relative frequency (R.F.).
The score out of the total value 300 is called the Importance Value Index (IVI) of the species. The different important species of a community are arranged always in the order of their decreasing IVI values. The IVI gives the complete phytosociological picture of a species in the community but does not give the dimension of relative density, relative dominance and relative frequency.
A phyto-graph can help to remove this problem. In Lutz’s phyto-graph (1928, 1930) a circle is made and then it is divided into four equal parts by two radial lines lying at right angles to each other (Fig. 1.9). Divide the three radii from centre to circumference into 100 equal parts and the fourth radius into 300 parts.
Mark first three radii with R.D., R. Dm. and R.F. and the fourth as IVI. Place the values of R.D., R. Dm., R.F. and IVI of the species concerned on their respective arms, connect these four points and that will give a quadrangular phyto-graph for the species. Such a phyto-graph gives an impression of ‘mass effect’ of several factors, and, at the same time, any particular factor is readily isolated for study.
II. Synthetic Characters:
Data collected from analytic characteristics result in tentative grouping of stands into associations on the basis of the following synthetic characters:
1. Fidelity:
The faithfulness of a species to its community is referred to as fidelity. While plants of low fidelity grow in several types of communities, a high fidelity plant occurs in only one kind in community. So, the characteristic species of a community has high fidelity value but a low ecological amplitude. In other words, the species is tolerant to a narrow range of environmental conditions.
The degrees of fidelity is usually expressed in the following manner:
Species belonging to F3, F4 and F5 classes are called characteristic or key species of a community.
2. Presence and Constancy:
The uniformity of a species over a number of sample plots or stands of the same type of community is expressed in terms of presence or constancy. The term Constancy is used if the sample areas are of equal size and Presence is used when the sample areas are of variable sizes.
3. Physiognomy:
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 forms of life, as for example forest grassland, desert community, etc..