Xerarch denotes the plant succession formed in regions acutely deficient in water. As in the case of hydrosere, there are various kinds of plants successively occupying the same regions here as well and at last a climax forest is formed. Xerosere succession stages are similar in all types of succession in various arid areas. Lithosere succession is a type of xerosere succession that happens on bare rock surfaces.
Xerosere Succession Stages
The various xerosere succession stages are as follows.
Crustose Lichen Stage
In regions of water deficiency such as the surface of rocks, initial vegetation of lichens occurs in the crustose forms. These are symbiotic associations of fungus and algae. They can live in extreme climatic conditions on the surface of rocks.
Besides water deficiency, minerals, and soil are scarce as well. Plants are exposed to high intensity of light. During the rainy season, they can retain water due to their spongy nature. In periods of drought, their plant body undergoes a resting period. But they can be restored to normal activity by the presence of a small amount of water.
They can liberate carbonic acid which dissolves the rocks gradually. This enables the rhizoids of crustose lichens to penetrate rock. Lichens obtain nitrogen from rainwater and also from dust particles.
They convert rocks into soil over the period. These plants are propagated by propagules like soridia as in the case of Lesidia, Rhizocarbon, etc.
Foliose Lichen Stage
After soil formation by the activities of crustose lichens, foliose lichen makes its appearance. They are attached to the soil by a single part. The plant body is much elaborated and expanded. Most often such plants develop leaf-like extensions of thalli. Hence they do not get proper light and are thus defeated in their struggle for existence.
During this stage, the temperature is reduced and thus evaporation of water is kept in check. These plants retain more water and lead a successful life until the next invader comes in.
Moss Stage
Vegetation of moss commences only after the formation of some more soil. Their rhizoids can penetrate into rocky crevices and absorb water and minerals in a much better way.
Peculiar leaf arrangement and close association of these plants help them retain more water. These plants spread over rocky regions like velvety structures. Eg. Black moss and Polytrichum.
Herbaceous Stage
The growth of lichens and mosses makes the soil better inhabitable for highly specialized forms of plants like angiosperms. Seeds of xerophytic herbs reach these areas due to dispersal and they gradually grow by penetrating the root systems to more deeper regions of soil.
As a result of the growth of these plants, humus content is increased and the soil becomes gradually fertile. Temperature and evaporation are greatly reduced. Examples of plants in this stage include Solidago, Potentilla, etc.
Shrub Stage
The growth of xerophytic herbs helps shrubs grow because of a greater amount of soil. Thus shrubs are more elaborate and thus shape of herbs and herbaceous plants gradually vary.
The root system of shrubs is greatly elongated and passes deeper into the soil. Moreover, they produce more humus in the soil. This allows the seeds of trees to germinate and develop a climax forest.
Climax Forest
In the climax forest, the first invaders will have xerophytic adaptations or characters. Gradually as the soil and climatic conditions are better, all types of plants are developed here to form the climax forest.
Thus it becomes clear that in plant succession, the last stage is reported by the same climax forest, whether it is a xerosere or hydrosere. They vary only in their intermediary stages with different types of vegetation.
Depending on the prevailing climatic conditions, the climax forest could have different types of trees.
- Oak-Hickory in dry habitat.
- Beech-Hemlock in mesic climates.
- Spruce-Alpine Fir at high altitude rocky mountains.
Xerarch Succession Types
Zerosere succession is of two types- lithosere and psammosere. Lithosere succession occurs on bare rocks and psommosere succession occurs in sand dunes.
Lithosere Succession
Lithosere succession occurs on bare rock surfaces where there is no soil.
Psammosere Succession
Psammosere succession begins with the pioneer colonization of grasses and lichens that can tolerate harsh sandy conditions. Over the period, soil accumulates and stabilizes where shrubs and trees can establish, altering the dune ecosystem.
Psammosere succession stabilizes and diversifies sand dune habitats to provide for other plant and animal species.
References
- Shukla, R.S. and Chandel, P.S. (2001) Plant Ecology. S. Chand and Company Ltd., New Delhi.
- Verma, P.S., Agarwal, V.K. (1999). Cell Biology Genetics Molecular Biology Evolution and Ecology. New Delhi: S.Chand Co.(Pvt) Ltd.
- G. (2024, April 23). Xerarch Succession. GeeksforGeeks. https://www.geeksforgeeks.org/xerarch-succession/
