Plant Communities; Characteristics: a Plant Community Is a Recognizable and Complex Assemblage of Plant Species Which Interact W

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Plant communities; characteristics: A plant community is a recognizable and complex assemblage of plant species which interact with each other as well as with the elements of their environment and is distinct from adjacent assemblages. A plant community is not a static entity: rather it may vary in appearance and species composition from location to location and also over time. What makes each of these communities distinguishable to us is its general physiognomy or physical structure. This overall appearance is created by the particular species present, as well as their size, abundance, and distribution relative to one another. Dominant species, those whose presence most influences the community environment and composition, are often the largest or the most abundant and may be a single species or several codominant species. Dominance may also be sociologic, expressed in the form of allelopathogens, chemical compounds manufactured by some plants that inhibit the growth and development of other species and/or seedlings of the same species within a certain distance. Plant communities may occur as relatively obvious ribbons across a landscape, such as the lush green path of a river through the desert. More often, however, adjacent communities interdigitate, their boundaries less distinguishable. Ecotones are areas where adjacent plant communities overlap with, transition, and grade into one another and have a unique set of characteristics which are defined spatially, temporally, and by diverse interactions among the adjacent communities. Ecotones vary in size and species composition, containing elements of each of the bordering communities. They may be predictable and recognizable entities whose physiognomy and even species composition may be similar from one geographical area to another. In other instances, the boundary between communities may be a band of barren soil caused by animal browsing or by phytotoxins (plant-produced allelopathogens), for example, between the grassland and the black sage (Salvia mellifera)-dominated coastal scrub on several hillsides of Poly Canyon. Where chaparral and coastal scrub adjoin in the Canyon, the ecotones vary in proportion, probably most directly as a result of varying soils and topography. Communities vary over time. Fires, floods, grazing, and plowing are some disturbances which quickly change a community. As the vegetation returns to a disturbed area, it may represent the same species that were there before, or other species may invade the area. This change in communities is called succession. Natural ecological succession tends to proceed at a relatively gradual pace, sometimes taking hundreds of years. During this time, communities evolve from one to another, through a series of seral (temporary, non-climax) stages. Eventually, in the absence of further disturbance, a climax community develops, one which is

CORE COURSE BOTANY-PAPER II- Plant Ecology and Taxonomy (B. Sc. II Semester CBCS 2016) at equilibrium with the environment. Just as individual plants and animals are named and sorted into groups of similar types, plant communities are also classified. There are many ways of looking at communities. One can focus on the characteristics of the habitat, such as alpine meadow, where the habitat remains the same although the species composition and physiognomy or community structure may vary from site to site. Another focus is the physiognomy of a community, for example chaparral: dense vegetation of woody shrubs with evergreen hardened leaves and relatively little understory or leaf litter. The species composition varies from locality to locality, but the overall appearance of the community remains the same. A third focus of community classification is the species composition. For example, the sole dominant species of a coastal Characteristics

Structure of plant communities can be determined by analytical characters and synthetic characters.

1. Analytical characters: These are of two types (a) quantitative, which are expressed in quantitative terms, and (b) qualitative, which are expressed only in qualitative way. (a) Quantitative characters: These include such characters as frequency, density, cover, basal area and abundance etc. (i) Frequency: Frequency is the number of sampling units (as %) in which a particular species occurs. Thus, frequency of each species is calculated as follows: Frequency (%) = Number of sampling units in which the species occurred / Total no. of sampling units studied x 100 After determining the percentage frequency of each species, various species are distributed among Raunkiaer’s (1934) five frequency classes depending upon their frequency values as follows:

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Frequency% Frequency class 0-20 A 21-40 B 41-60 C 61-80 D 81-100 E

(ii) Density: Density represents the numerical strength of a species in the community. The number of individuals of the species in any unit area is its density. Density gives an idea of degree of competition. It is calculated as follows. Density = Total no of individuals of the species in all the sampling units / Total no. of sampling units identified (iii) Cover and Basal area: The above ground parts (such as leaves, stems and inflorescence) cover a certain area—if this area is demarcated by vertical projections, the area of the ground covered by the plant canopy is called foliage cover or herbage cover or canopy cover. It is a good measure of herbage availability and is estimated by chart quadrat or point quadrat method. 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 2.5 cm above the ground or actually on the ground level by callipers, line interception or point-centered quadrat method. (iv) Abundance: This is the number of individuals of any species per sampling unit of occurrence. It is calculated as follows— Abundance = Total no. of individuals of the species in all the sampling units. / No. of sampling units in which the species occurred. But, abundance thus obtained in quantitative terms gives little idea of the distribution of the species. (b) Qualitative characters: These include physiognomy, phenology, stratification, abundance, sociability, vitality and vigour, life form (growth form), etc. (i) Physiognomy: This is the general appearance of vegetation as determined by the growth form of dominant species. Such a characteristic appearance can be expressed by single term. For example, on the basis of appearance of a community having trees and some shrubs as the dominants, it can be concluded that it is a forest.

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(ii) Phenology: It is the scientific study of seasonal change i.e., the periodic phenomenon of organisms in relation to their climate. Different species have different periods of seed germination, vegetative growth, flowering and fruiting, leaf fall, seed and fruit dispersal, etc. Such data for individual species are recorded. A study of the date and time of these events is phenology. In other words phenology is the calender of events in the life history of the plant. Environmental factors tend to influence the phenological behaviour of a species population. (iii) Stratification: Stratification of communities is the way in which plants of different species are arranged in different vertical layers in order to make full use of the available physical and physiological requirements. (iv) Abundance: Plants are not found uniformly distributed in an area. They are found in smaller patches or groups, differing in number at each place. Abundance is divided in five arbitrary groups depending upon the number of plants. The groups are very rare, rare, common, frequent and very much frequent. (v) Sociability: Sociability or gregariousness expresses the degree of association between species. It denotes the proximity of plants to one another. Braun-Blanquet (1932) classified plants into the following five sociability groups:

S1 – Plants found quite separately from each other, growing singly

S2 – Plants growing in small groups (4 to 6 plants)

S3 – Plants growing in small scattered patches.

S4 – Several bigger groups of many plants at one place

S5 – A large group occupying larger area. (vi) Vitality: It is the capacity of normal growth and reproduction which are important for successful survival of species. In plants, stem height, root length, leaf area, leaf number, number and weight of flowers, fruits, seeds, etc., determine the vitality. (vii) Life form (growth form): Ecologists generally use Christen Raunkiaer’s classification (1934) of plant life forms. A life form is ―the sum of the adaptation of the plant to climate‖. On the basis of the position of perennating buds on plants and the degree of their protection during adverse conditions, Raunkiaer classified plants into five broad life-form categories which are as follows: (a) Phanerophytes: Their buds are naked or covered with scale, and are situated high upon the plant. These life forms include trees, shrubs and climbers generally common in tropical climates.

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(b) Chamaephytes: In these plants, the buds are situated close to the ground surface which gets protection from fallen leaves and snow cover. Chamaephytes commonly occur in high altitudes and latitudes, e.g., Trifolium repens. (c) Hemicryptophytes: These are mostly found in cold temperate zone. Their buds are hidden under soil surface protected by the soil itself. Their shoots generally die each year. Examples-most of the biennial and perennial herbs. (d) Cryptophytes or Geophytes: In these plants, the buds are completely hidden in the soil as bulbs and rhizomes. Cryptophytes include the hydrophytes (buds remaining under water), halophytes (marshy plants with rhizomes under the soil) and geophytes (terrestrial plants with underground rhizomes or tubers). (e) Therophytes: These are seasonal plants, completing their life cycle in a single favourable season, and remain dormant throughout the rest unfavourable period of year in the form of seeds. They are commonly found in dry, hot or cold environment (deserts). 2. Synthetic characters: These are determined after computing the data on the quantitative and qualitative characters of the community. For comparing the vegetation of different areas, community comparison needs the calculation of their synthetic characters. Synthetic characters are determined in terms of the following parameters: (i) Presence and Constance: It expresses the extent of occurrence of the individuals of a particular species in the community, i.e., how uniformly a species occurs in a number of stands of the same type of community. The species on the basis of its percentage frequency may belong to any of following five presence classes that were first proposed by Braun- Blanquet. (a) Rare—present in 1 to 20% of the sampling units. (b) Seldom present—present in 21-40% of the sampling units. (c) Often present—present in 41-60% of the sampling units. (d) Mostly present—present in 61-80% of the sampling units. (e) Constantly present—present in 81-100% of the sampling units. (ii) Fidelity: Fidelity or ―Faithfulness‖ is the degree with which a species is restricted in distribution to one kind of community. Such species are sometimes known as indicators. The species have been grouped into five fidelity classes which were first formulated by Braun Blanquet: (a) Fidelity 1: Plants appearing accidentally (Strangers) (b) Fidelity 2: Indifferent plants may occur in any community (Indifferents).

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(c) Fidelity 3: Species which occur in several kinds of communities but are predominant in one (Preferentials). (d) Fidelity 4: Specially present in one community but may occasionally occur in other communities as well (Selectives). (e) Fidelity 5: Occur only in one particular community and not in others (Exclusives). (iii) Dominance: It is used as a synthetic as well as analytical characters (Daubenmire, 1959). The number of organisms sometimes may not give correct idea of the species. If one bases his conclusion on number, a single or few trees in a grassland, or few grasses in a forest should be of little value. But if he considers the species on the basis of area occupied or biomass, the situation may be different. Thus, cover is included as an important character in dominance. Relative dominance (cover; RDO) is calculated as follows: Relative Dominance (lover) = Dominance (cover) of the species / Total dominance (cover) of all the species x 100 (iv) Importance Value Index (IVI): This index is used to determine the overall importance of each species in the community structure. In calculating this index, the percentage values of the relative frequency, relative density and relative dominance are summed up together and this value is designated as IV or importance value index of the species. It provides the idea of the sociological structure of a species in its totality in the community, but does not indicate its position separately with regard to other aspects. For IV, values of relative density, relative frequency and relative dominance are obtained as follows: Relative Density = Density of the species / Total density of all the species x 100 Relative Frequency = Frequency of the species / Total Frequency of all the species x 100 Relative Dominance = Dominance (cover) of the species / Total dominance (cover) of all the species x 100 (v) Other synthetic characters: Besides the above mentioned ones, there have also been proposed some other characters which have been quite useful in comparative studies on communities. Such characters include, interspecific association and association index, index of similarity, dominance index, diversity index etc.

Ecotone: An ecotone is a transition area between two biomes. It is where two communities meet and integrate, for e.g. the mangrove forests represent an ecotone between marine and terrestrial ecosystem. Other examples are grassland (between forest and desert), estuary (between fresh

CORE COURSE BOTANY-PAPER II- Plant Ecology and Taxonomy (B. Sc. II Semester CBCS 2016) water and salt water) and river bank or marsh land (between dry and wet). It may be narrow (between grassland and forest) or wide (between forest and desert). As it is a zone of transition, it has conditions intermediate to the adjacent ecosystems. Hence it is a zone of tension. Usually, the number and the population density of the species of an outgoing community decreases as we move away from community or ecosystem. A well-developed ecotone contains some organisms which are entirely different from that of the adjoining communities. Edge Effect: The edge effect is an ecological concept that describes how there is a greater diversity of life in the region where the edges two adjacent ecosystems overlap, such as land/water, or forest/grassland. At the edge of two overlapping ecosystems, you can find species from both of these ecosystems, as well as unique species that aren’t found in either ecosystem but are specially adapted to the conditions of the transition zone between the two edges. The organisms which occur primarily or most abundantly in this zone are known as edge species. In the terrestrial ecosystems edge effect is especially applicable to birds. For example the density of birds is greater in the mixed habitat of the ecotone between the forest and the desert.

Ecological or biotic succession : changes in a community succession: The occurrence of relatively definite sequence of communities over a long time in the same area resulting in establishment of stable or climax community, is known as ecological or biotic succession. It is so because the biotic communities are never stable but dynamic, changing more or less regularly over period and space. These are never found permanently in complete balance with their component species or with the physical environment. Ecological succession is an orderly and evolutionary process of community development in a habitat. The first community to inhabit an area are called pioneer community while the last and stable community in an area is called climax community. The intermediate communities between the pioneer and climax communities are called transitional or seral communities. The entire series of ,communities is called sere. (i) Characteristics. An ecological succession is characterized by: (a) A continuous change in the kinds of plants and animals towards a state of stability. (b) Tendency towards increase in the species-diversity. (c) An increase in the organic matter and biomass supported by available energy flow in autotrophic succession.

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(d) Decrease in net community production or annual yield. (e) In an area, the plant and animal communities undergo succession side by side. (f) Biotic succession is according to specific laws and towards particular direction so future seral communities can be predicted. (g) Biotic succession on bare ground progresses towards increasing wetness, while biotic succession in open water progresses towards increasing dryness. General concept of ecological succession Ecological succession is an orderly and evolutionary process of community development in a habitat. The first communities invading the habitat are beginners of this developmental process and are called pioneers. With the passage of time and changes in environmental conditions, the pioneer Stage is successively followed by other stages till a mature and stable community, called as climax community, develops. The whole series of changes in community characteristics from pioneer stage to climax stage constitute a 'sere'. The intermediate stages are seral stages, comprising of respective seral communities. The intermediate communities are also called as transition communities. The initial phases of succession are represented by lower life forms which establish themselves to the new area. With the passage of time, their number increase and they act upon the abiotic components, specially the soil and microclimate of the habitat. The constant action and reaction of populations and habitat upon each other changes the environmental conditions. This changed environment now becomes unsuitable for the growth and development of existing populations and may be favorable for the establishment of other species of higher life forms. The latter repeat the cycle and process goes in till the climax stage is reached. According to Odum (1971), species replacement in a sere occurs because populations tend to modify the physical environment, making conditions favorable for other populations until an equilibrium between biotic and abiotic components is reached. (ii) Causes of Ecological Succession. These can be divided into two categories: (a) Biotic factors. The interactions among the organisms in a community are collectively called biotic factors. These influence the structure, composition and function of a community. In ecological succession, each community drives itself out of its habitat because it makes the area less favourable for itself and more favorable for the next seral community and so on. (b) Physiographic factors. These include the physical and chemical factors of the environment which determine the nature and composition of a community. These include landslides, erosion, catastrophic factors, hails and storms, frost, fire etc.

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Since succession involves a series of complex processes so is controlled by a number of causeswhich are of three types: initiating causes (produce bare areas); continuing causes or ecosis (cause successive waves of population) and stabilizing causes (stabilize the community). (iii) Basic types of succession. These are of following types: 1. Primary succession (perisere). In any of the basic environments (terrestrial, fresh water; marine), one type of succession is primary succession which starts from the primitive substratum where there was no previously any sort of living matter C-sterile area) e.g. land formed by volcanic lava or a newly formed estuarine mud bank or sand dunes or glaciated surface. Conditions at extreme so unfit for the growth of most plants and animals. The first group of plants establishing there are known as the pioneers, primary community or primary colonisers. It takes longer period e.g. development of forest climax on a sand dune or barren land may take about 1,000 years. 2. Secondary succession (subsere). Another general type of succession is secondary succession which starts from previously built up substrate with already existing living matter. The action of any external force, as a sudden change in climatic factors, biotic intervention, fire etc. causes the existing community to disappear. Thus, area becomes devoid of living matter but its substratum. instead of primitive, is built up. It has organic matter so is biologically fertile. Such successions are comparatively more rapid. Biotic community undergoes several changes and ultimately gives rise to a climax community. Time taken is about 50-100 years in case of a grassland about 100-200 years for a forest. 3. Autogenic succession. After the succession has begun, in most of the cases, it is the vegetation itself which as a result of its reactions with the environment, modifies its own environment and thus causing its own replacement by new communities. This course of succession is known 35 autogenic succession. 4. Allogenic succession. In some cases, however, the replacement of the existing community is caused largely by any other external condition and not by the existing vegetation itself. Such a course is referred to as allogenic succession. On the basis of successive changes in nutritional and energy contents, successions art sometimes classified as: 5. Autotrophic succession. It is characterized by early and continued dominance of autotrophic organisms like green plants. It begins in a predominantly inorganic environment and the energy flow is maintained indefinitely. There is gradual increase in the organic matter content supported by energy flow.

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6. Heterotrophic succession. It is characterized by early dominance of heterotrophs, such‖ bacteria, actinomycetes, fungi and animals. It begins in a predominantly organic environment there is a progressive decline in the energy content. Depending mainly upon the nature of the environment (primarily based upon moisture relations) where the process has begun, and thus it may be a hydrosere or hydrarch-starting in regions where water is in plenty e.g. ponds, lakes, streams, swamp, beg soil etc. a mesarch-where adequate moisture conditions are present; and a xerosere or xerach-where moisture is - present in minimal amounts such as dry deserts, rocks etc. Sometimes, these are further distinguished: the lithosere initiating on rocks, psammosere-on sand and halosere-in saline water or soil. iv) General process of ecological succession. The whole process of a primary autotrophic succession is actually completed through a number of sequential steps like : (a) Nudation. It involves the development of a bare area without any form of life. The cause of nudation may be topographic (e.g. soil erosion, landslide, volcanic eruption etc.) or climatic (cg. glaciers, hails, storms, fire etc.) or biotic (epidemics, human activities etc.) (b) Invasion. It involves successful establishment of a species in a bare area. It involves three steps: (1) Migration (dispersal). It involves teaching of seed, spores. etc. in a bare area through the agencies like air, water etc . (2) Ecosis (establishment). It involves the successful adjustment of a species with the prevailing conditions of that area. (3) Aggregation. It involves the increase in 'number of organisms through the process of reproduction. (c) Competition and Co-action. It involves the development of intraspecific as well as interspecific competition among the members due to their large number but limited food and space; and development of positive and negative interspecific as well as intraspecific interactions between them. (d) Reaction. It involves the modification of the environment through the influence of living organisms. The modified area becomes less favorable for the existing community so it is sooner or later replaced by another community called seral community and the process is repeated. The whole sequence of communities that replaces one another in the given area is called a sere and various communities constituting the sere are called seral communities. (e) Stabilization (Climax). In this, the final terminal community becomes more or less stabilized for a longer period of time which can maintain itself in equilibrium with the climate of the area, This final community is called climax community and the final stage is called

CORE COURSE BOTANY-PAPER II- Plant Ecology and Taxonomy (B. Sc. II Semester CBCS 2016) climax stage. The climax community is the most complex and stable, providing food and variety of niches for animals. The climax stage is reached When the existing climax community can tolerate the conditions created by itself and there are no more successful species to replace them. But sometimes there is retrogressive succession in which the organisms exert some destructive effects and the process of succession become retrogressive instead of progressive e.g. replacement of a forest community by a shrubby or grassland community. (v) Significance of Biotic Succession (a) Its knowledge helps in maintaining a specific biotic seral stage by interfering the process of biotic succession eg. maintenance of teak forest. (b) Dams are protected by preventing siltation and biotic succession. (c) it gives information about the techniques to be used during reforestation and afforestation.

Hydrosere: Hydrosere, also called hydrarch, involves the ecological succession in the newly formed pond or lake ; (i) Stages in a Hydrosere (a) Plankton stage. It is the pioneer community of the hydrosere and is formed by the germination of encysted spores in the newly formed water body. These spores reach the water body. These spores reach the water body through wind or animals. The planktonic stage mainly includes the minute autotrophic organism, called phytoplanktons e.g. diatoms, phytoflagellates, cyanobacterial and green algal cells. The population of the phytoplanktons is regulated by zooplanktons. Death and decomposition or planktons produce organic matter which mixes with the silt and form a soft mud at the bottom of pond which favors the growth of next seral stage. (b) Rooted-submerged stage. It is formed of rooted submerged hydrophytes like Hydrilla Potamogeton, Vallisneria, Utricularia, Myriophyllum, Elodea etc. Due to death and decay of these plants and sand and silt deposited around the plants, the bottom level of the pond rises slowly. The older plants and buried parts of other plants form humus which favor the growth of next seral stage (c) Rooted-floating stage. It is formed of rooted hydrophytes like Nymphaea, Limnanthemum Aponogeton, Trapu, Monochoria etc. Some free-floating species like Azolla, Lemna, Woljia, Pistia Salvinia etc. are also found. The organic matter formed by the death and decomposition of these plants further increases the level of the substratum so the pond becomes more shallower. Finally, the floating species disappear.

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(d) Reed-Swamp stage. It is also called amphibious stage as most parts of the rooted plants remain exposed to air. It includes the plant species like Sagittaria, Phragmites, Typha, Scirpus etc are also found. These have well developed rhizomes and form a dense vegetation. The organic matter added by these plants further raises the substratum so the pond becomes unsuitable for the growth of these amphibious plants. (e) Sedge-meadow stage. It is mainly formed of Plant species like Carex (sedge), Juncus, Cyperus Eleocharis, Diochanthium and herbs like Caltha, Polygonum etc. They form a mat- like vegetation towards the centre of the pond with the help of their much branched rhizomatous systems. This develops the mesic conditions in the area and marshy vegetation disappears gradually. (f) Wood land stage. In this stage, first the peripheral part of the area is invaded by some shrubby plants which can tolerate bright sunlight as well as water logged conditions e. g. Camus (Bog Wood), Cephalanthus (Button brush) etc. Next to invade are trees capable of tolerating bright sunlight e.g. Populus (cotton wood), Almus (Alder) etc. These further lower the water table by their transpiration and build up more soil. Mineralisation and soil favors the arrival of plants of next stage. (g) Forest stage. It is climax community. It depends upon the climatic conditions eg. tropical deciduous or monsoon forests in regions of moderate rainfall, tropical rain forests In areas with heavy rainfall and tropical climate; and mixed forests of Almus (Elm) and Quercus(Oak) in the temperate regions.

Llthosere: It is a type of xerosere and involves the ecological succession on bare rock surfaces. (i) Stages in a Lithosere. The rocky surface is characterized by (a) Deficiency of water. (b) Absence of organic matter. (c) Surface temperatures are very high. The various stages in the ecological succession in a lithosere are : (a) Crustose lichens stage. It forms the pioneer community in a lithosere and is represented by lichen species like Graphis Rhizocarpon; Rinodina and Lacanora. The lichens can tolerate desiccation. These produce organic acids which cause weathering of rocks so that minerals essential for proper growth of lichens are released. The lichens hold the fine particles of rock and sand to initiate soil formation. This invites the foliose type lichens.

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(b) Foliose lichens stage. It includes the lichens like Pamellia, Dermatocarpon etc. Which are large sized lichens with leafy thalli. These foliose lichens retain more water and are able to accumulate dust particles which help in further formation of the substratum. The weathering of rocks and its mixing with humus leads to the development of a fine soil layer on the rock surface which favors the upcoming of hardy mosses. (c) Moss stage. It is characterized by extensive growth of xerophytic mosses like Polytrichum, Torula and Grimmia on thin soil layer on rock surface. Their death and decomposition add more soil and organic matter so the thickness of soil increases. Weathering of rocks also continues. The soil now remained moist for longer periods which favor the growth of moisture-loving mosses like Hypnum, Bryum etc. (d) Herbs stage. The mat formed by mosses on the partially fragmented rock becomes suitable for the germination of seeds of annual hardy grasses like Eleusin, Aristida etc. These grasses have more sandbinding properties. Their death and decomposition accumulate more soil so the annual grasses are replaced by perennial grasses like Cymbopogon, Heteropogon etc. (e) Shrub stage. Due to further weathering of rocks and death of the herbs, more soil is accumulated. So the habitat becomes suitable for the growth of shrubs like Rhus, Phytocarpus, Zizyphus, Caparis etc. The shrubs are large in size and their roots penetrate more deeply in the rocky substratum causing more weathering and soil formation. This favors the invasion of the area by next seral stage. (f) Forest stage. It is a climax community in lithosere and is formed of several hardy trees. Further weathering of rocks and increasing humus content of the soil favors the growth of more trees. The vegetation finally becomes mesophytic. Type of climax community depends upon the climate e.g a rainforest in a moist tropical area. Animal communities in Lithosere Due to high temperature, the fauna is represented by a few mites, ants and spiders associated With lichens. Their species increase in moss stage and become diverse quantitatively as well as qualitatively during herb stage. More species of ants, spiders and mites along with nematodes, larval insects appear as the environment reaches to climax stage. The climax forest is occupied by a rich fauna constituted by invertebrates and vertebrates. Functional aspects of succession As succession progresses, there is a change in structure and composition of plant and animal communities along with changes in soil and the environmental conditions. Major trends observed during succession are summarized below : 1. The conditions change from hydrism or xerism to mesism.

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2. Soil development takes place gradually with the increase in non-living organic matter. 3. Total organic matter, species diversity, bio-chemical diversity and size of organisms increase from initial stages to mature stage. 4. Stratification changes from its poor organization to well organized towards climax. 5. The life cycles changes from simple and short to complex and long towards maturity of succession. 6. Mineral cycles are open in beginning and closed in later stages. Exchange of nutrients between organism and environment is rapid initially but slows down gradually. Detritus organisms play an important role in mature stage. 7. Gross production and community respiration (P/R) ratio is greater or less than one but approaches to unity towards climax. 8. Gross production and standing crop biomass (P/ B) and net community production decrease With the progress of succession. 9. Nutrient conservation, stability and iternal symbiosis are poor in lower stages and good in mature stage. 10. Entropy changes from high to low as succession proceeds.

The concept of climax The final successional stage consisting of stabilized community in harmony with the prevailing environmental conditions was termed as climax community (Clements 1916). The climax community is determined by the climate. There are two important theories about the concept of climax : 1. Monoclimax theory. This theory has been derived from the work of E.E. Clements (1916) which shows the importance of climate on the stabilization of communities. This theory has been supported by many ecologists. According to this theory, the climax community is determined by climate of that area. Thus, stable climatic conditions will consists of only one climax community. It is named as climatic climax. 2. Polyclimax theory. There are several criticisms to the monoclimax theory. Supporters of polyclimax theory do not consider me climax community, only under the control of climate. There are several edaphic and biotic factors in addition to climatic factors which can influence the development of stable community. Monoclimax theory explains the development of only one climax, irrespective of the kind of succession (lithosere, psammosere, hydrosere). However, climatic climax may not develop at all places in a climatic region. The succession may be arrested at a stage preceding the climax due to some natural or artificial factors like

CORE COURSE BOTANY-PAPER II- Plant Ecology and Taxonomy (B. Sc. II Semester CBCS 2016) grazing, burning, cutting, floods etc. This imperfect stage of succession is termed as subclimax. When the succession is arrested and held indefinitely at some other earlier stage, is termed as a serelimax. The subclimax is also termed as disclimax. Sometimes, within a climax, a localized strip may have stable community of lower life forms due to some adverse climatic, topographic or edaphic factors. This is known as pro-climax. If this community is made up of higher life forms than the main climax, it is termed as post-climax. Another different stable community may develop at the border area of two climaxes and is called as proclimax. Pie-climax, sub-climax, disclimax, post-climax or proclimax communities are considered to be more or less stable communities but not fully developed (Monoclimax theory). The poly-climax theory, however, consider them as fully developed communities controlled by edaphic or physiographic factors, in addition to climate, within a climatic region. It is a general consideration that the climax, once reached will remain stable indefinitely. But as an individual matures and then deteriorate with ageing, a climax community and its environment undergo deterioration leading to alteration over a long period of time. Supporters of polyclimax theory believe that during the course of succession, a community with higher life forms may be degraded due to adverse conditions. This type of succession is termed as retrogressive succession. This phenomenon is against the viewpoint of monoclimax theory which consider that succession is always a progressive phenomenon.