It may be classified as below:

(i) Initial causes

Primary succession

  1. Erosion: Wind and water erode the soil and deposit it elsewhere and vegetation changes.
  2. Physiography: The configuration of the land surface helps the agents of erosion, i.e. wind, water and gravity to create new soils, eg., land slide may take place on a steep slope, destroying the forest.
  3. Elevation and subsidence: Geological disturbances in the Himalayas result in the formation of new soil for primary succession. 

Secondary succession

  1. Climate: When the vegetation is destroyed by the action of draught, wind, snow or frost.
  2. Physiography: The configuration of the land surface helps the agents of erosion, i.e. wind, water and gravity to create new soils, eg., land slide may take place on a steep slope, destroying the forest.
  3. Biotic factor: A forest is destroyed by the activity of man, his animals, wild animals, insects, etc. causes soil modification, eg., heavy grazing, cutting, burning, etc.

(ii) Continuing causes 

  1. Migration: Mass movement of plants from one place to another is called migration. It begins when germule (spore, seed, fruit or plant) leaves the parent area and reaches the final resting place. This depends upon the degree of mobility of their seeds or their germinating parts, nearness of the parent area, topography, etc.
  2. Ecesis or establishment: The whole process whereby a plant establishes itself in a new area from germination or its equivalent (eg., rooting of some detached portion) to reproduction whether sexual or asexual. Species’ establishment on a new area can only be accomplished when the seeds germinate, which depends on the condition of seeds and sites.
  3. Grouping and aggregation: The colonizers invade new areas  gradually. The immigrants after establishment grow while more migration keeps on taking place. Thus in course of time, the colonizers make a closed canopy. Grouping following establishment of scattered colonizing invaders as a result of propagation.
  4. Competition: Competition starts for food, light and moisture which takes place among species and individuals. Competition is especially more intense among the individuals of the same species of a site and in the process, weakers are left behind in the struggle for existence.
  5. Reaction: The effect of vegetation on site is called reaction, which can be grouped into two classes: (i) Effect on climatic factors (ii) Effect on soil

(i) Effect on climatic factors: Vegetation affects climatic factors by:

(a) Altering the light conditions
(b) Decreasing the air and soil temperature
(c) Reducing the wind velocity
(d) Reducing danger of radiation frost
(e) Increasing relative humidity

(ii) Effect on soil: The vegetation affects the soil by:

(a) Addition of organic matter
(b) Improving the structure of the soil
(c) Improving moisture retentivity of the soil
(d) Improving nutrient status of the soil
(e) Improving the maturity of soil

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