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The following points highlight the the seven main stages of lithosere in plants. The stages are:- 1. Crustose Lichen Stage 2. Foliose and Fruticose Lichen Stage 3. Moss Stage 4. Herb Stage 5. Shrub Stage 6. Trees Stage 7. Climax Stage.
1. Crustose Lichen Stage:
Lichen species like Graphis Rhizocarpon, Rinodina and Lacanora forms are the pioneer community in a lithosere, as they can tolerate desert or extreme hot condition. Organic acids produced by lichens leach the rock surface and release minerals for proper growth of lichens. A barren rock consists of solid surface or very large boulders and there is no place for rooting plants to colonize.
Crustose lichens can adhere to the surface of rock and absorb moisture from atmosphere; therefore, these colonize the barren surfaces of rocks first. The propagules of these lichens are brought by air from the surrounding areas. When these lichens die their thalli are decomposed and add humus. This promotes soil formation and the environment becomes suitable for growth of foliose and fruticose type of lichens.
2. Foliose and Fruticose Lichen Stage:
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Foliose lichen retain more water and accumulate more soil particles, helping in the development of a fine layer of soil on the rock surface. As the soil formation proceeds it leads to the growth of foliose lichens like Parmelia, Physcia etc. Foliose lichens have leaf-like thalli, while the fruticose lichens are like small bushes. They are attached to the substratum at one point only, therefore, do not cover the soil completely.
They can absorb and retain more water and are able to accumulate more dust particles. Their dead remains are decomposed to humus which mixes with soil particles and help building substratum and improving soil moisture contents further. The shallow depressions in the rocks and crevices become filled with sold and topsoil layer increases further.
3. Moss Stage:
Accumulation of soil and humus leads to the growth of mosses such as Polytrichum and Grimmia. After some times as a lot of soil and organic matter accumulate, that favour the growth of moisture loving mosses like Hypnum, Bryum etc. The spores of xerophytes mosses, such as Polytrichum, Tortula and Grimmia, are brought to the rock where they succeed lichens.
Their rhizoids penetrate soil on the rocks. The bodies of mosses are rich in organic and inorganic compounds. When these die they add these compounds to the soil, increasing the fertility of the soil. As mosses develop in patches they catch soil particles from the air and help increase the amount of substratum. The changing environment leads to alter of lichens and helps invasion of herbaceous vegetation that can out- compete mosses.
4. Herb Stage:
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Death and decay of mosses produce a mat of organic moss rich in organic soil, that help the germination to seeds of hardy grasses like Eleusin, Aristicla, Poa, etc. Further decomposition of these annual grasses promotes the growth of perennial grasses like Cymbopogon, Heteropogon etc.
Their roots penetrate deep down, secrete acids and enhance the process of weathering. Leaf litter and death of herbs add humus to the soil. Shading of soil results in decrease in evaporation and maintains soil moisture and slightly increases in temperature.
As a result the xeric conditions begin to change and biennial and perennial herbs and xeric grasses. These climatic conditions favor growth of bacterial and fungal populations, resulting in increase in decomposition activity.
5. Shrub Stage:
The herb and grass vegetation is replaced by shrub species, such as Rhus and Phytocarpus. This results in dense organic soil and making conditions unfavorable for the growth of herbs, which then begin to migrate.
The soil formation continues and its moisture content increases. Further weathering or rocks and death of herbs make the habitat more suitable for the growth of shrubs like Rhus, Caparis and Zizyphus etc. Since the shrubs are larger in size and their roots penetrate more deeply in the rocky substratum causes more weathering and soil formation.
6. Trees Stage:
Change in environment favors colonization of tree species. The trees begin to grow among the shrubs and establish themselves. The kind of tree species inhabiting the area depends upon the nature of the soil. In poorly drained soils oaks establish themselves. The trees form canopy and shade the area.
Shade-loving scrubs continue to grow as secondary vegetation. Leaf litter and decaying roots weather the soil further and add humus to it making the habitat more favorable for growth to trees. Mosses and ferns make their appearance and fungi population grows abundantly.
7. Climax Stage:
The succession culminates in a climax community, the forest. Many intermediate tree stages develop prior to establishment of a climax community. The forest type depends upon climatic conditions. Vegetation finally becomes mesophytic. A steady state is reached between the environment and the biotic community. Type of climax community depends upon the climate.
The climax forest may be:
(i) Oak-Hickory Climax Forest:
In dry habitat oaks and hickories are climax vegetation. There is only one tree stage and forests are characterized by presence of scrubs, herbs, ferns, and mosses.
(ii) Beech-Hemlock Climax Forest:
These climax forests develop in mesic climates. The dominant vegetation is Beech and Hemlock. There are many intermediate tree stages. The other vegetation types include herbs, ferns, and mosses.
