Plant succession

The gradual replacement of one community by another in the development of vegetation towards a climax is the culmination stage in plant succession for a given environment.

1. Introduction The gradual replacement of one community by another in the development of vegetation towards a climax which is the culmination stage in plant succession for a given environment. The plant communities involved in the succession before the climax is reached are called ' seres ’. Sere is defined as the series of plant communities resulting from processes of succession or any recognizable stage in plant succession' .

2. EVOLUTION OF THE CONCEPT OF PLANT SUCCESSION Richard Peters, who in 1806 described the departure of southern pine timber as a proof of the tendency in the nature to a change in products on the same soil. Dawson (1847) and Thoreau (1863) described the changes in localized places. Thoreau who, first, used the term ’forest succession’.

3. EVOLUTION OF THE CONCEPT OF PLANT SUCCESSION Cawles (1899) described the change in vegetation on sand dunes. Clements (1916) who developed the idea and elaborated the theory of plant succession.

4. Concept of plant Succession There is a continual change in the vegetation as a result of interaction of plant community and the habitat factors. The succession is inherently and inevitably progressive and the end product is the climax. The succession is the progressive development of vegetation on the same site in course of time.

5. Kinds of Plant succession Succession can be classified mainly in two ways: On the basis of moisture conditions of the place On the basis of presence or absence of vegetation in the place.

6. On the basis of moisture conditions of the place On the basis of the moisture conditions, succession is classified into: Xerarch succession which is defined as the succession initiated in extremely dry situations such as bare rock, wind blown sand, rocky talus slopes, etc. Hydrarch succession which is defined as the "succession beginning in water, or very wet land as in ponds, lakes, marshes, etc.' The various stages of this succession are called Hydrosere .

7. On the basis of the presence or absence of vegetation in the place On the basis of the presence or absence of vegetation in the place, succession is classified into: Primary succession which is defined as the succession which takes place on ‘sites which have previously not borne vegetation' . Primary succession is, sometimes, termed as Autogenic succession because it takes place as a result of autogenic factors which are defined as the 'dominating factors of change which are due only to the individuals in a plant community .

8. On the basis of the presence or absence of vegetation in the place Secondary succession which is defined as the succession which takes place on site 'after the destruction of the whole or part of the original vegetation' . This succession is, sometimes, termed as Allogenic succession as it takes place due to allogenic factors which are defined as the 'factors which operate, independently of the plant themselves, to alter the habitat gradually and thus cause changes in vegetation.

9. Different types of habitats in which succession takes place Hydrosere : succession occurs where water is plenty, e.g. pond. Derosere : succession occurs on a dry soil or rock. Xerosere: succession occurs on dry habitat like dry desert or bare rock Psammosre : succession which occurs on sand dunes. Halosere : succession which occurs on saline soil Lithosere : Succession on newly exposed rock surface Oxylosere : Succession on acidic soil

10. Causes of succession The causes of succession may be classified as under: Initial causes Continuing causes

11. Initial causes of primary succession Erosion- Wind and water erode the soil and deposit it elsewhere. Thus, new soils are created in the form of alluvial deposits, coastal sands, estuarian deposits, sand dunes, landslips and scree. Physiography- The configuration of the land surface is an initial cause to the extent it helps the agents of erosion i.e., wind, water and gravity, to create new soils. Elevation and subsidence- Seismic disturbances result in elevation and subsidence of the soil resulting in the formation of new soils. Due to these disturbances, river beds are silted up or the rivers change their courses leaving their original beds for starting primary succession.

12. Initial causes of secondary succession Climate- Climate is the initial cause when the vegetation is destroyed by the action of drought, wind, snow or frost. For instance, a fair portion of a forest may be killed by drought. If left to it self, secondary succession will start on this bare area. Wind may lay bare an area by uprooting the original crop. Snow may, similarly, destroy forest by sliding. Physiography - Physiography is the initial cause when configuration of the land surface is responsible for the destruction of vegetation in combination with some other factors. For instance, land slide may take place on a steep slope, destroying the forest. Biotic factor- Biotic factor is the initial cause where a forest is destroyed as result of the activity of man, his animals, or even wild animals. For instance, a forest may be destroyed by reckless cutting, clearing, burning, indiscriminate heavy grazing, etc.

13. Continuing causes Continuing causes- Continuing causes are those causes of succession which help the development of plant communities and their replacement by other plant communities. Thus, while the initial causes create suitable conditions for starting succession, the continuing causes help in the formation of plant communities and their gradual replacement by other communities leading to a climax. They consist of the following: Migration; Ecesis or establishment; Grouping and aggregation; Competition; and Reaction.

14. Hydrosere or Hydrarch It is succession occurring in the aquatic environment. Such a type of succession does not necessarily lead the aquatic communities toward the development of land communities. If the body of water is large and very deep or very strong wave action and other powerful physical forces are at work, the succession results in a stable aquatic community in which any considerable further change is hardly recognizable. Succession is recognizable only if the colonization of plant communities takes place in artificial small and shallow ponds, lakes, etc. where wave action speeds up the process by allowing the erosion of soil towards edge regions. In this way, the filling process also speeds up quickly and consequently the body of water disappears within few years time.

15. The different types of vegetation at different depths in a pond

16. Xerosere or Xerarch The original substratum is deficient in water and lacks any organic matter, having only minerals in disintegrated unweather state. The pioneers to colonize this primitive substratum are crustose type of lichens, and through a series of successive seral stages the succession finally terminates into a forest which constitutes the climax community. The various stages in xerosere can be enumerated as follows— Lichen stage Moss stage Herbaceous stage Shrub stage Climax forest

17. Various stages in xerosere

18. Climax BCFT(1953) defines climax as the culmination stage in plant/community succession for a given environment A climax which owes its distinctive characters to climatic factors in conjunction with only such biotic influences as plants and animals naturally occurring in the area, bring about This is also, sometimes, referred to as formation which is defined as ‘the major unit of vegetation comprising the climax communities of an area uniform in its major physiognomic features ’ . Pro-climax, which is defined as a term applied to all communities that suggest something of a permanence or extent of a climax but are not typical of the existing climate'. It includes sub-climax, sere-climax, pre-climax, post-climax and dis-climax.

19. Theories of Climax Monoclimax theory Polyclimax theory Mosaic theory Vegetational gradient and climax pattern theory

20. Aubreville's mosaic theory Aubreville proposed Mosaic theory of climax. In English language, mosaic means a form or work of art in which designs are made by fitting together differently coloured bits of stones. Even if the pattern of fitting stone pieces is changed, the design is still called mosaic. Aubreville believed that the forest is composed of a number of irregular small units with one or few dominants, which develop on more or less predictable but different lines. These small units may be compared *to the small bits of stones of the engineering mosaic. T he pattern of mosaic of the climax community keeps on changing continually and is never constant.

21. Whittaker's theory of vegetational gradient and climax patterns Whittaker was of the view that there is no absolute climatic climax for any area and the climax is a function of the sum total of all the factors of a mature ecosystem, i.e., climate, soil, other site factors, biotic influences, availability of species, their characteristics, dispersal, etc. It is thus a partially stabilized steady state adapted to the whole pattern of environmental factors in which it exists, exhibiting similar or convergent patterns in adaptation to similar environments but showing continuous change along the various continuous environmental gradients.

22. Classification of climax Even though the concept of climax is still developing it is convenient for practical use to classify the climax as follows: Climatic climax- It is the climax which owes its distinctive characters to climatic factors in conjunction with only such biotic influences as plants and animals naturally occurring in the area bring about. Edaphic climax- Edaphic climax is defined as a community which differs from the climatic climax of the area owing to the influence of special soil factors'. In other words, within the general climatic climax, there may be characteristic vegetation locally due to the influence of soil peculiarities Pre-climax- Pre-climax is the plant community immediately preceding in seral development the climatic climax of the region and found under conditions drier than are usual in the climate of the region

23. Classification of climax Post climax- Post climax is 'a plant community more exacting than the climatic climax of a given region and found under exceptionally favorable site conditions within that region’ Biotic climax or subclimax- Biotic climax is 'a climax which differs from the climatic climax of the area owing to the action of biotic factors'. Sub-climax is a vegetation whose development towards climatic climax has been arrested at some stage short of the normal climax by the action of factors, natural or artificial, other than the climate and which is more or less stable under the prevailing conditions.

24. Importance of the study of plant succession The study of plant succession is very important from the point of view of silviculture because it helps in : Classification of forests into forest types-- It is one of the bases for classification of forests into forest types. Choice of species for artificial regeneration- The species to be raised in a plantation have to be selected keeping in view the stage of development which the soil has reached. If an attempt is made to raise a climax species on soils which are still very immature, it would result in failure. Determination of successional stage of economically most valuable crop and the method of obtaining it- Knowledge of succession shows how the composition of the crop is changing and will change in future. It also indicates the factors by which the succession can be altered to the best advantage of the people

25. Uses of succession Classification of forests Species choice for afforestation Evaluating the sites Delineating economic species

26. Thank You