Prasanna

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Domestication Of Plants

Domestication is the process of bringing a plant species under the control of humans and gradually changing it through careful selection, genetic alteration and handling so that it is more useful to people. The domesticated species are renewable sources that have provided food and other benefis to human.

The possible changes in the plant species due to domestication are listed below;

• Adaptation to a greater diversity of environments and a wider geographical range.
• Simultaneous / uniform flwering and fruiting.
• Lack of shattering or scattering of seeds.
• Increased size of fruits and seeds.
• Change from a perennial to annual habit.
• Change in breeding system.
• Increased yield.
• Increased resistance for disease and pest.
• Developing seedless parthenocarpic fruit.
• Enhancing colour, appearance, palatability and nutritional composition.

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Relationship Between Human and Plants

From the very early times, human beings have co-existed with plants which played a vital role in their survival. Though a long process of trial and error, our ancestors have selected hundreds of wild plants from the various parts of the world for their specific use. The knowledge of the plants and its applications have led to the development of the humans and their civilization in many ways.

People depend on plants for food, clean air, water, fuel, clothing, and shelter. Nearly all food webs begin with plants, the primary producers. During photosynthesis, green plants use sunlight to change carbon dioxide from the air and water into simple sugars made of carbon, hydrogen, and oxygen.

Plants such as trees, on the other hand, can take in this carbon dioxide, which is unusable for humans, and use it to produce their own energy. In a way, they are a cycle – plants help humans breathe by providing us with oxygen, and humans help plants “breathe” by providing them with carbon dioxide.

There are numerous examples of symbiosis in agriculture. Agriculture in a broad sense involves a symbiotic relationship between humans and plants or animals. Humans plant, fertilize, control weeds and pests, and protect crops. Humans also nurture, feed, and protect livestock.

Humans, of course, benefit greatly from their mutualisms with agricultural plants, through the provision of crops of food, fiber, and other products. Similarly, agricultural animals live in a symbiotic mutualism with humans. Even the keeping of animals as pets represents a type of mutualism.

An exploration of the relationship between plants and people from early agriculture to modern-day applications of biotechnology in crop production. Plants and People: Origin and Development of Human-Plant Science Relationships covers the development of agricultural sciences from Roman times through the development of agricultural experiment stations in the United States.

To the rise of agri-business. It underscores the symbiotic relationship and mutuality that define the intertwined histories of plants and people. It does not merely present the latest science but puts the sciences themselves in the context of history.

The book provides the science, chronology, and history that undergird the relationships between humans and plants. It discusses plant anatomy, physiology, and reproduction; evolution of plants and people; early uses of plants; the rise of agriculture in both Old and New Worlds; creation of land grant universities and agricultural experiment stations; the Green Revolution; plant biotechnology; and the future of plant sciences in feeding the growing human population.

The agricultural sciences were not a product of the nineteenth century but of the careful observation and advice of Roman writers who lived some 2000 years ago. This book reveals the malleability of the sciences, the people who practice them, and the plants that are the focus of scientific research.

The author is careful to distinguish between basic and applied science while recognizing that the agricultural sciences pursue both. He also challenges the traditional notion that basic research necessarily yields practical results. The book demonstrates how plants and the agricultural sciences have shaped the everyday world we inhabit.

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Carbon Capture And Storage (CCS)

Carbon capture and storage is a technology of capturing carbondioxide and injects it deep into the underground rocks to a depth of 1 km or more and it is an approach to mitigate global warming by capturing CO₂ from large point sources such as industries and power plants and subsequently storing it instead of releasing it into the atmosphere.

Various safe sites have been selected for permanent storage in various deep geological formations, liquid storage in the Ocean and solid storage by reduction of CO₂ with metal oxide to produce stable carbonates. It is also known as Geological sequestration which involves injecting CO₂ directly into the underground geological formations (such as declining oil fields, gas fields saline aquifers and unmineable coal have been suggested as storage sites).

Carbon Sequestration

Carbon sequestration is the process of capturing and storing CO₂ which reduces the amount of CO₂ in the atmosphere with a goal of reducing global climate change. Carbon sequestration occurs naturally by plants and in ocean. Terrestrial sequestration is typically accomplished through forest and soil conservation
practices that enhance the storage carbon.

As an example microalgae such as species of Chlorella, Scenedesmus, Chroococcus and Chlamydomonas are used globally for CO₂ sequestration. Trees like Eugenia caryophyllata, Tecoma stans, Cinnamomum verum have high capacity and noted to sequester carbon. Macroalgae and marine grasses and mangroves are also have ability to mitigate carbon-di-oxide.

Carbon Foot Print (CFP)

Every human activity leaves a mark just like our footprint. This Carbon foot print is the total amount of green house gases produced by human activities such as agriculture, industries, deforestation, waste disposal, buring fossil fuels directly or indirectly. It can be measured for an individual, family, organisation like industries, state level or national level. It is usually estimated and expressed in equivalent tons of CO₂ per year. The burning of fossil fuels releases CO₂ and other green house gases.

In turn these emissions trap solar energy and thus increase the global temperature resulting in ice melting, submerging of low lying areas and inbalance in nature like cyclones, tsunamis and extreme weather conditions. To reduce the carbon foot print we can follow some practices like

1. Eating indigenous fruits and products
2. Reducing use of electronic devices
3. Reduce travelling
4. Avoid buying fast and preserved, processed, packed foods.
5. Plant a garden
6. Reducing consumption of meat and sea food. Poultry requires little space, nutrients and less pollution compared cattle farming.
7. Reducing use of Laptops (when used for 8 hours, it releases nearly 2 kg. of CO₂ annually)
8. Line drying clothes. (Example: If you buy imported fruit like kiwi, indirectly it increases CFP. How? The fruit has travelled a long distance in shipping or airliner thus emitting tons of CO₂)
   

Biochar

Biochar is another long term method to store carbon. To increase plants ability to store more carbon, plants are partly burnt such as crop waste, waste woods to become carbon rich slow decomposing substances of material called Biochar.

It is a kind of charcoal used as a soil amendment. Biochar is a stable solid, rich in carbon and can endure in soil for thousands of years. Like most charcoal, biochar is made from biomass via pyrolysis.

(Heating biomas in low oxygen environment) which arrests wood from complete burning. Biochar thus has the potential to help mitigate climate change via carbon sequestration. Independently, biochar when added to soil can increase soil fertility of acidic soils, increase agricultural productivity, and provide protection against some foliar and soil borne diseases.

It is a good method of preventing waste woods and logs from getting decayed and instead we can convert them into biochar thus converting them to carbon storage material.

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Environmental Conservation Issues

India due to its topography, geology and climate patterns has diverse life forms. Now this huge diversity is under threat due to many environmental issues for this conservation becomes an important tool by which we can reduce many species getting lost from our native land.

By employing conservation management strategies like germplasm conservation, in situ, ex-situ, in-vitro methods, the endemic as well as threatened species can be protected and also have educational and recreational values for the society.

In-situ conservation:

It means conservation and management of genetic resources in their natural habitats. Here the plant or animal species are protected within the existing habitat. Forest trees, medicinal and aromatic plants under threat are conserved by this method. This is carried out by the community or by the State conservation which include wildlife, National park and Biosphere reserve.

The ecologically unique and biodiversity rich regions are legally protected as wildlife sanctuaries, National parks and Biosphere reserves. Megamalai, Sathyamangalam wildlife, Guindy and Periyar National park, and Western ghats, Nilgiris, Agasthyamalai and Gulf of Mannar are the biosphere reserves of Tamil Nadu.

Sacred groves

These are the patches or grove of cultivated trees which are community protected and are based on strong religious belief systems which usually have a signifiant religious connotation for protecting community. Each grove is an abode of a deity mostly village God Or Goddesses like Aiyanar or Amman.

448 grooves were documented throughout Tamil Nadu, of which 6 groves (Banagudi shola, Thrukurungudi and Udaiyankudikadu, Sittannnavasal, Puthupet and Devadanam) were taken up for detailed floristic and faunistic studies. These groves provide a number of ecosystem services to the neighbourhood like protecting watershed, fodder, medicinal plants and micro climate control.

Ex-situ conservation

It is a method of conservation where species are protected outside their natural environment. This includes establishment of botanical gardens, zoological parks, conservation strategies such as gene, pollen, seed, in-vitro conservation, cryo preservation, seedling, tissue culture and DNA banks. These facilities not only provide housing and care for endangered species, but also have educational and recreational values for the society.

Endemic Centres and Endemic Plants

Endemic species are plants and animals that exist only in one geographic region. Species can be endemic to large or small areas of the earth. Some are endemic to a particular continent, some to a part of a continent and others to a single island. Any species found restricted to a specified geographical area is referred to as ENDEMIC. It may be due to various reasons such as isolation, interspecific interactions, seeds dispersal problems, site specifiity and many other environmental and ecological problems.

There are 3 Megacentres of endemism and 27 microendemic centres in India. Approximately one third of Indian flora have been identified as endemic and found restricted and distributed in three major phytogeographical regions of india, that is Indian Himalayas, Peninsular India and Andaman nicobar islands.

Peninsular India, especially Western Ghats has high concentration of endemic plants. Hardwickia binata and Bentinckia condapanna are good examples for endemic plants. A large percentage of Endemic species are herbs and belong to families such as Poaceae. Apiaceae, Asteraceae and Orchidaceae.

Majority of endemic species are threatened due to their narrow specifi habitat, reduced seed production, low dispersal rate, less viable nature and human intereferences.. Serious efforts need to be undertaken for their conservation, otherwise these species may become globally extinct.

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Alien Invasive Species | Eichhornia Crassipes | Prosopis Juliflra

Invasion of alien or introduced species disrupts ecosystem processes, threaten biodiversity, reduce native herbs, thus reducing the ecosystem services (benefis). During eradication of these species, the chemicals used increases greenhouse gases.

Slowly they alter ecosystem, micro climate and nature of soil and make it unsuitable for native species and create human health problems like allergy, thus resulting in local environmental degradation and loss of important local species. According to World Conservation Union invasive alien species are the second most signifiant threat to bio-diversity after habitat loss.

What is invasive species?

A non-native species to the ecosystem or country under consideration that spreads naturally, interferes with the biology and existence of native species, poses a serious threat to the ecosystem and causes economic loss. It is established that a number of invasive species are accidental introduction through ports via air or sea.

Some research organisations import germplasm of wild varieties through which also it gets introduced. Alien species with edible fruits are usually spread by birds. Invasive species are fast growing and are more adapted. They alter the soil system by changing litter quality thereby affecting the soil community, soil fauna and the ecosystem processes.

It has a negative impact on decomposition in the soils by causing stress to the neighbouring native species. Some of the alien species which cause environmental issues are discussed below.

Eichhornia crassipes:

It is an invasive weed native to South America. It was introduced as aquatic ornamental plant, which grows faster throughout the year. Its widespread growth is a major cause of biodiversity loss worldwide. It affects the growth of phytoplanktons and finally changing the aquatic ecosystem.

It also decreases the oxygen content of the waterbodies which leads to eutrophication. It poses a threat to human health because it creates a breeding habitat for disease causing mosquitoes (particularly Anopheles) and snails with its free flating dense roots and semi submerged leaves. It also blocks sunlight entering deep and the waterways hampering agriculture, fiheries, recreation and hydropower.

Prosopis juliflra

Prosopis juliflra is an invasive species native to Mexico and South America. It was first introduced in Gujarat to counter desertifiation and later on in Andhra Pradesh, Tamil Nadu as a source of firewood. It is an aggressive coloniser and as a consequence the habitats are rapidly covered by this species.

Its invasion reduced the cover of native medicinal herbaceous species. It is used to arrest wind erosion and stabilize sand dunes on coastal and desert areas. It can absorb hazardous chemicals from soil and it is the main source of charcoal.

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An Overview Of Afforestation And Its Acheivements

Afforestation is planting of trees where there was no previous tree coverage and the conversion of non-forested lands into forests by planting suitable trees to retrieve the vegetation. Example: Slopes of dams affrested to reduce water runoff erosion and siltation. It can also provide a range of environmental services including carbon sequestration, water retention.

Afforestation Objectives

• To increase forest cover, planting more trees, increases O₂ production and air quality.
• Rehabilitation of degraded forests to increase carbon fiation and reducing CO₂ from atmosphere.
• Raising bamboo plantations.
• Mixed plantations of minor forest produce and medicinal plants.
• Regeneration of indigenous herbs / shrubs.
• Awareness creation, monitoring and evaluation.
• To increase the level and availability of water table or ground water and also to reduce nitrogen leaching in soil and nitrogen contamination of drinking water, thus making it pure not polluted with nitrogen.
• Nature aided artifiial regeneration.

Achievements

• Degraded forests were restored
• Community assets like overhead tanks bore-wells, hand pumps, community halls, libraries, etc were established
• Environmental and ecological stability was maintained.
• Conserved bio-diversity, wildlife and genetic resources.
• Involvement of community especially women in forest management.

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Environmental Deforestation | Definition, Causes and Effects

Deforestation is one of the major contributors to enhance green house effect and global warming. The conversion of forested area into a non-forested area is known as deforestation. Forests provide us many benefits including goods such as timber, paper, medicine and industrial products. The causes are

• The conversion of forests into agricultural plantation and livestock ranching is a major cause of deforestation.
• Logging for timber
• Developmental activities like road construction, electric tower lines and dams.
• Over population, Industrialisation, urbanisation and increased global needs.

Effects of deforestation

• Burning of forest wood release stored carbon, a negative impact just opposite of carbon sequestration.
• Trees and plants bind the soil particles. The removal of forest cover increases soil erosion and decreases soil fertility. Deforestation in dry areas leads to the formation of deserts.
• The amount of run of water increases soil erosion and also creates flash floding, thus reducing moisture and humidity.
• The alteration of local precipitation patterns leading to drought conditions in many regions. It triggers adverse climatic conditions and alters water cycle in ecosystem.
• It decreases the bio-diversity signifiantly as their habitats are disturbed and disruption of natural cycles.
• Loss of livelihood for forest dwellers and rural people.
• Increased global warming and account for one-third of total CO₂ emission.
• Loss of life support resources, fuel, medicinal herbs and wild edible fruits.

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Environmental Issues On Various Types Of Forestry

Agro forestry

Agroforestry is an integration of trees, crops and livestock on the same plot of land. The main objective is on the interaction among them. Example: intercropping of two or more crops between diffrent species of trees and shrubs, which results in higher yielding and reducing the operation costs. This intentional combination of agriculture and forestry has varied benefits including increased bio-diversity and reduced erosion.

Some of the major species cultivated in commercial Agroforestry include Casuarina, Eucalyptus, Malai Vembu, Teak and Kadambu trees which were among the 20 species identified as commercial timber. They are of great importance to wood-based industries.

Benefits of agroforestry

• It is an answer to the problem of soil and water conservation and also to stabilise the soil (salinity and water table) reduce landslide and water run-of problem.
• Nutrient cycling between species improves and organic matter is maintained.
• Trees provide micro climate for crops and maintain O₂ – CO₂ balanced, atmospheric temperature and relative humidity.
• Suitable for dry land where rainfall is minimum and hence it is a good system for alternate land use pattern.
• Multipurpose tree varieties like Acacia are used for wood pulp, tanning, paper and firewood industries.
• Agro-forestry is recommended for the following purposes. It can be used as Farm Forestry for the extension of forests, mixed forestry, shelter belts and linear strip plantation.

Rehabilitation of degraded forests and recreation forestry

The production of woody plants combined with pasture is referred to silvopasture system. The trees and shrubs may be used primarily to produce fodder for livestock or they may be grown for timber, fuel wood and fruit or to improve the soil. This system is classified into following categories.

(i) Protein Bank:

In this various multipurpose trees are planted in and around farm lands and range lands mainly for fodder production. Example: Acacia nilotica, Albizzia lebbek, Azadirachta indica, Gliricidia sepium, Sesbania grandiflora.

(ii) Livefence of fodder trees and hedges:

Various fodder trees and hedges are planted as live fence to protect the property from stray animals or other biotic inflences. Example: Gliricidia sepium, Sesbania grandiflra, Erythrina spp., Acacia spp.

Social forestry

It refers to the sustainable management of forests by local communities with a goal of climate carbon sequestration, change mitigation, depollution, deforestation, forest restoration and providing indirect employment opportunity for the youth.

Social forestry refers to the management of forests and afforestation on barren lands with the purpose of helping the environmental, social and rural development and benefits. Forestry programme is done for the benefit of people and participation of the people. Trees grown outside forests by government and public organisation reduce the pressure on forests.

In order to encourage tree cultivation outside forests, Tree cultivation in Private Lands was implemented in the state from 2007-08 to 2011-12. It was implemented by carrying out block planting and inter-crop planting with profitable tree species like Teak, Casuarina, Ailanthus, Silver Oak, etc in the farming lands and by a free supply of profiable tree species for planting in the bunds.

The Tank foreshore plantations have been a major source of firewood in Tamil Nadu. The 32 Forestry extension centres provide technical support for tree growing in rural areas in Tamil Nadu. These centres provide quality tree seedlings like thorn / thornless bamboo, casuarinas, teak, neem, Melia dubia, grafted tamarind and nelli, etc in private lands and creating awareness among students by training / camps.

Major activities of forestry extension centres

• Training on tree growing methods
• Publicity and propaganda regarding tree growing
• Formation of demonstration plots
• Raising and supply of seedlings on subsidy
• Awareness creation among school children and youth about the importance of forests through training and camps.

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Green House Effect, Ozone Depletion And Its Effects

Green House Effect is a process by which radiant heat from the sun is captured by gases in the atmosphere that increase the temperature of the earth ultimately. The gases that capture heat are called Green House Gases which include carbon dioxide (CO₂), methane (CH₄), Nitrous Oxide (N₂O) and a variety of manufactured chemicals like chloroflorocarbon (CFC).

Increase in greenhouse gases lead to irreversible changes in major ecosystems and climate patterns. For example, coral ecosystem is affected by increase in temperature, especially coral bleaching observed in Gulf of Mannar, Tamil Nadu.

Human activities lead to produce the green house effect by:-

• Burning fossil fuels, which releases CO₂ and CH₄
• Way of Agriculture and animal husbandry practices
• Electrical gadgets like refrigerator and air conditioners release chloro floro carbons
• The fertilizers used in Agriculture which release N₂O
• The emissions from automobiles.

The increase in mean global temperature (highest in 4000 years) due to increased concentration of green house gases is called global warming. One of the reasons for this is over population which creates growing need for food, fire and fuel and considered to be the major cause of global warming.

Effects of Global Warming

• Rise in global temperature which causes sea levels to rise as polar ice caps and glaciers begin to melt causing submergence of many coastal cities in many parts of the world.
• There will be a drastic change in weather patterns bringing more floods or droughts in some areas.
• Biological diversity may get modified, some species ranges get redefied. Tropics and sub-tropics may face the problem of decreased food production.

Sources of Green House Gases Emission (Natural and Anthropogenic)

CO₂ (Carbon dioxide)

• Coal based power plants, by the burning of fossil fuels for electricity generation.
• Combustion of fuels in the engines of automobiles, commercial vehicles and air planes contribute the most of global warming.
• Agricultural practices like stubble burning result in emission of CO₂.
• Natural from organic matter, volcanoes, warm oceans and sediments.

Methane

Methane is 20 times as effective as CO₂ at trapping heat in the atomosphere. Its sources are attributed paddy cultivation, cattle rearing, bacteria in water bodies, fossil fuel production, ocean, non-wetland soils and forest / wild fies.

N₂O (Nitrous oxide)

It is naturally produced in Oceans from biological sources of soil and water due to microbial actions and rainforests. Man-made sources include nylon and nitric acid production, use of fertilizers in agriculture, manures cars with catalytic converter and burning of organic matter.

Global Warming Effects on Plants

• Low agricultural productivity in tropics
• Frequent heat waves (Weeds, pests, fungi need warmer temperature)
• Increase of vectors and epidemics
• Strong stroms and intense flood damage
• Water crisis and decreased irrigation
• Change in flowering seasons and pollinators
• Change in Species distributional ranges
• Species extinction

Strategies to deal with Global Warming

• Increasing the vegetation cover, grow more trees
• Reducing the use of fossil fuels and green house gases
• Developing alternate renewable sources of energy
• Minimising uses of nitrogeneous fertilizers, and aerosols.

Ozone depletion

Ozone layer is a region of Earth’s stratosphere that absorbs most of the Sun’s ultra violet radiation. The ozone layer is also called as the ozone shield and it acts as a protective shield, cutting the ultraviolet radiation emitted by the sun. Just above the atmosphere there are two layers namely troposphere (the lower layer) and stratosphere (the upper layer).

The ozone layer of the troposphere is called bad ozone and the ozone layer of stratosphere is known as good ozone because this layer acts as a shield for absorbing the UV radiations coming from the sun which is harmful for living organisms causing DNA damage. The thickness of the ozone column of air from the ground to the top of the atmosphere is measured in terms of Dobson Units.

The ozone shield is being damaged by chemicals released on the Earth’s surface notably the chloroflorocarbons widely used in refrigeration, aerosols, chemicals used as cleaners in many industries. The decline in the thickness of the ozone layer over restricted area is called Ozone hole.

Ozone depletion in the stratosphere results in more UV radiations especially UV B radiations (shortwaves). UV B radiation destroys biomolecules (skin ageing) and damages living tissues. UV – C is the most damaging type of UV radiation, but it is completely filtered by the atmosphere (ozone layer). UV – a contribute 95% of UV radiation which causes tanning burning of skin and enhancing skin cancer. Hence the uniform ozone layer is critical for the wellbeing of life on earth.

During 1970’s research fidings indicated that man-made chloroflorocarbons (CFC) reduce and convert ozone molecules in the atmosphere. The threats associated with reduced ozone pushed the issue to the forefront of global climate issues and gained promotion through organisation such as World Meterological Organisation and the United Nations.

The Vienna Convention was agreed upon at the Vienna conference of 1985 but entered into force in 1988 provided the frameworks necessary to create regulative measures in the form of the Montreal protocol.

The International treaty called the Montreal Protocol (1987) was held in Canada on substances that deplete ozone layer and the main goal of it is gradually eliminating the production and consumption of ozone depleting substances and to limit their damage on the Earth’s ozone layer.

Clean Development Mechanism (CDM) is defied in the Kyoto protocol (2007) which provides project based mechanisms with two objectives to prevent dangerous climate change and to reduce green house gas emissions. CDM projects helps the countries to reduce or limit emission and stimulate sustainable development.

An example for CDM project activity, is replacement of conventional electrifiation projects with solar panels or other energy efficient boilers. Such projects can earn Certified Emission Reduction (CER) with credits / scores, each equivalent to one tonne of CO₂, which can be counted towards meeting Kyoto targets.

Effects of Ozone depletion

The main ozone depletion effects are:

• Increases the incidence of cataract, throat and lung irritation and aggravation of asthma or emphysema, skin cancer and diminishing the functioning of immune system in human beings.
• Juvenile mortality of animals.
• Increased incidence of mutations.
• In plants, photosynthetic chemicals will be affected and therefore photosynthesis will be inhibited. Decreased photosynthesis will result in increased atmospheric CO₂ resulting in global warming and also shortage of food leading to food crisis.
• Increase in temperature changes the climate and rainfall pattern which may result in flood / drought, sea water rise, imbalance in ecosystems affecting flora and fauna.

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Ecological Plant Succession – Characteristics, Types and Examples

We very often see that forests and lands in our areas are drastically affcted by natural calamities (Flood, earthquake) and anthropogenic activities (Fire, over grazing, cutting of trees). Due to these reasons all plants of an area are destroyed and the areas become nude. When we observe this area, over a period of a time we can see that it will be gradually covered by plant community again and become fertile.

Such successive replacement of one type of plant community by the other of the same area / place is known as plant succession. The first invaded plants in a barren area are called pioneers. On the other hand, a series of transitional developments of plant communities one after another in a given area are called seral communities. At the end a final stage and a final plant community gets established which are called as climax and climax community respectively.

Characteristics of ecological succession

• It is a systematic process which causes changes in specifi structure of plant community.
• It is resultant of changes of abiotic and biotic factors.
• It transforms unstable community into a stable community.
• Gradual progression in species diversity, total biomass, niche specialisation, and humus content of soil takes place.
• It progresses from simple food chain to complex food web.
• It modifies the lower and simple life form to the higher life forms.
• It creates inter-dependence of plants and animals.

Types of succession

The various types of succession have been classified in different ways on the basis of different aspects. These are as follows:

1. Primary succession:

The development of plant community in a barren area where no community existed before is called primary succession. The plants which colonize first in a barren area is called pioneer species or primary community or primary colonies. Generally, Primary succession takes a very long time for the occurrence in any region.

Example: Microbes, Lichen, Mosses.

2. Secondary succession:

The development of a plant community in an area where an already developed community has been destroyed by some natural disturbance (Fire, flood, human activity) is known as secondary succession. Generally, This succession takes less time than the time taken for primary succession.

Example: The forest destroyed by fire and excessive lumbering may be re-occupied by herbs over a period of times.

3. Allogenic succession

Allogeneic succession occurs as a result of abiotic factors. The replacement of existing community is caused by other external factors (soil erosion, leaching, etc.,) and not by existing organisms. Example: In a forest ecosystem soil erosion and leaching alter the nutrient value of the soil leading to the change of vegetation in that area.

Classification of plant succession

Detailed study of Hydrosere and Lithosere are discussed below:

Hydrosere

The succession in a freshwater ecosystem is also referred to as hydrosere. Succession in a pond, begins with colonization of the pioneers like phytoplankton and finally ends with the formation of climax community like forest stage. It includes the following stages Fig 7.21.

1. Phytoplankton stage:

It is the first stage of succession consisting of the pioneer community like blue green algae, green algae, diatoms, bacteria, etc., The colonization of these organisms enrich the amount of organic matter and nutrients of pond due to their life activities and death. This favors the development of the next seral stages.

2. Submerged plant stage:

As the result of death and decomposition of planktons, silt brought from land by rain water, lead to a loose mud formation at the bottom of the pond. Hence, the rooted submerged hydrophytes begin to appear on the new substratum. Example: Chara, Utricularia, Vallisneria and Hydrilla etc.

The death and decay of these plants will build up the substratum of pond to become shallow. Therefore, this habitat now replaces another group of plants which are of floating type.

3. Submerged free flating stage:

During sthis stage, the depth of the pond will become almost 2-5 feet. Hence, the rooted hydrophytic plants and with floating large leaves start colonising the pond. Example: Rooted flating plants like Nelumbo, Nymphaea and Trapa.

Some free flating species like Azolla, Lemna, Wolff and Pistia are also present in this stage. By death and decomposition of these plants, further the pond becomes more shallow. Due to this reason, floating plant species is gradually replaced by another species which makes new seral stage.

4. Reed-swamp stage:

It is also called an amphibious stage. During this stage, rooted floating plants are replaced by plants which can live successfully in aquatic as well as aerial environment. Example: Typha, Phragmites, Sagittaria and Scirpus etc. At the end of this stage, water level is very much reduced, making it unsuitable for the continuous growth of amphibious plants.

5. Marsh meadow stage:

When the pond becomes swallowed due to decreasing water level, species of Cyperaceae and Poaceae such as Carex, Juncus, Cyperus and Eleocharis colonise the area. They form a mat-like vegetation with the help of their much branched root system. This leads to an absorption and loss of large quantity of water. At the end of this stage, the soil becomes dry and the marshy vegetation disappears gradually and leads to shurb stage.

6. Shrub stage:

As the disappearance of marshy vegetation continues, soil becomes dry. Hence, these areas are now invaded by terrestrial plants like shrubs (Salix and Cornus) and trees (Populus and Alnus). These plants absorb large quantity of water and make the habitat dry. Further, the accumulation of humus with a rich flora of microorganisms produce minerals in the soil, ultimately favouring the arrival of new tree species in the area.

7. Forest stage:

It is the climax community of hydrosere. A variety of trees invade the area and develop any one of the diverse type of vegetation. Example: Temperate mixed forest (Ulmus, Acer and Quercus), Tropical rain forest (Artocarpus and Cinnamomum) and Tropical deciduous forest (Bamboo and Tectona).

In the 7 stages of hydrosere succession, stage1 is occupied by pioneer community, while the stage 7 is occupied by the climax community. The stages 2 to 6 are occupied by seral communities.

Significance of Plant Succession

• Succession is a dynamic process. Hence an ecologist can access and study the seral stages of a plant community found in a particular area.
• The knowledge of ecological succession helps to understand the controlled growth of one or more species in a forest.
• Utilizing the knowledge of succession, even dams can be protected by preventing siltation.
• It gives information about the techniques to be used during reforestation and affrestation.
• It helps in the maintenance of pastures.
• Plant succession helps to maintain species diversity in an ecosystem.
• Patterns of diversity during succession are inflenced by resource availability and disturbance by various factors.
• Primary succession involves the colonization of habitat of an area devoid of life.
• Secondary succession involves the reestablishment of a plant community in disturbed area or habitat.
• Forests and vegetation that we come across all over the world are the result of plant succession.