The succession which starts in xeric or dry habitat having minimum amount of moisture (deserts, rocks, etc.) is known as xerach and various stages of succession are called xerosere.
Xerosere begins on barren naked rocks or rocky substratum. Rock surface is devoid of soil without rooted plants cannot grow and colonize. No free minerals and nutrients are available on the surface of rocks. Moisture is also not retained and the environment is xeric (scarcity of water). The process of transformation of a barren rock into a forest site refers to Xerosere (Lithosere).
Stages of xerosere
Various stages of Xerosere are as follows:
- Crustose Lichen Stage - The bare rocks show completely xerophytic conditions like total absence of water, high temperature direct exposure of Sun light etc. On these rocks the development rooted plant is not possible. Under such conditions only crustose lichens can grow on these bare rocks. These crustose lichen gradually spread and secrete some sort of acids with decomposes the superficial layers of rock and make them rough enough for the growth of foliose lichens. (e.g. pioneer, Tropical region)
- Foliose Lichen Stage - After the crustose lichens dies the foliose lichen show their growth. These occupy the depression in the rock and other weathered portions these destroyed quickly. The foliose lichens gradually replace the crustose lichens. Water and humus collects round the foliose lichens and acids move into the rock. Lichens secrete acids and destryoed the Rocks. Gradually a thin layer of soil collects consisting of rock particles, remain of lichens and particles of dust. Formations of layer are due to regeneration of dust particle. (Example- Parmelia, Dermatocarpon)
- Moss Stage - The thin layer of soil formed by the death and decay of crustose and foliose lichen, favours the growth of moss and different types of mosses migrate and develop on the rock. The rhizoids of mosses penetrate into the substratum and form a mass with the foliose lichens. Accumulation of soil takes place rapidly as the erect stems die below but continue to grow upwards. The mosses being taller and able to grow more rapidly make the competition unfavourable for lichens. Example- Polytrichum, Tertula.
- Herbaceous Stage - Mass plants covered the rocks, there is formation of layer of soil and dust particle due to destroyed of masses and flowers. The soil is formed on rock as a result of decay of mosses and lichens and rich in humus etc. favours the development of many herbaceous plants. Many herbs first short lived annual and later biennial and perennial and xerophytic grasses make their appearance. The roots of these herbs further decompose the rock and the amount of both the soil and humus increases, Example - Aristida, poa.
- Shrub Stage - The increased amount of soil and humus favour the growth of shrubs. The falling leaves, the thick net work of roots dense stems produces a good condition for quick accumulation of soil and humus. The roots of the shrub further corrode the rock which is more or less completely covered with soil. Thus soil is further enriched by humus formed from fallen leaves and twigs. The soil is shaded and evaporation from it dry. Wind movement is retarted and humidity increased. All these conditions are favourable for growth of trees, Example- Phytocarpus, Rhus.
- Climax Stages - The trees which appear first are xeric. The trees are smaller and widely spaced. These trees in the due course of time become taller with denser growth. With the increasing shade the underlying shrubs may be disappearing but those shrubs which can grow in shade continue to grow there. Further the under growth of some mesophytic shrub-herbs, trees also takes place. Thus an area covered by bare rock transformed into a forest by the process of xerosere plant succession.
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