Some of the most important process involved in evolution of plant community are as follows:
(1) Nudation (2) Migration including initial colonization (3) Ecesis (4) Aggregation of germules (5) Evolution of community relationships (6) Invasion (7) Reaction (8) Stabilization (9) Climax.
Evolution of plant community on a bare area is quite a prolonged process. This involves a number of stages. Each stage is characterised by particular assemblage of plant populations and dominants. It is difficult to recognize; these stages because of the fact that the process of community evolution is continuous. However, these stages can be defined on the basis of their characteristic vegetation.
Evolution of plant community involves the following important steps and processes:
The development of bare areas is initial prerequisite. The naked areas develop either by emersion, submergence, glacial recession, erosion, deposits and climatic change or by biotic agencies.
(2) Migration including initial colonization:
When the area becomes bare some plants from the nearby localities move into it in the form of germules, propagules or migrules (structures or off-springs reaching from different places). This process is known as migration. Migration starts when germules leave their parent areas and terminates when they reach the final resting place.
The movement between these two places may complete in one or two steps. It is only by migration, plants from an area are brought into new areas. Several agencies help in the migration of plants to new areas. They are wind, water, animals, man, glacier, etc. Migrules may jump into the new area from all the surrounding localities or from one side only.
It is a process of establishment of immigrants. When the migrants enter a new area, they germinate, grow and reproduce there. It is not necessary that all the migrules reaching the new area must stabilize. The stabilization depends greatly on the conditions prevailing in that area. The first plants growing in the new area are known as pioneer colonisers. The germination may be affected by a number of external and internal factors. Dormancy may be a barrier in the germination. Viviparous germination is helpful in the establishment of halophytes in the saline marshy places as the saline habitat has marked inhibiting effect on germination.
(4) Aggregation of germules:
In the beginning, pioneer plants may be present in very small number and they grow far from one another. These plants produce reproductive structures which will be dispersed in the open areas around them and after germination they form their family groups. In the course of evolution, more new migrules reach the open areas and become stabilized there. This grouping together of colonizing individuals in bare area after migration is termed as aggregation.
Aggregation may be of two types:
(i) Simple aggregation:
In this, the germules are aggregated in a group around the parent. This is independent of immigration. It increases the number of individuals of only one species, e.g., Gloeocapsa, Tetraspora. Falling down of fruits and seeds of the plant just below it is also aggregation.
(ii) Mixed aggregation:
When the individuals from the family groups migrate away and some more new immigrants are brought by some means into the area under colonisation, it is called mixed aggregation.
(5) Evolution of community relationships:
When the bare areas become occupied by the individuals of colonizing species, they become related with one another.
The relationship may be of the following three types:
In this, one species lives at the expense of another.
In this, one or both species benefit from the relationship but none suffers.
In this, the species live together in some measures of actual or potential competition for same necessities, such as light, moisture, space and nutrients. Competition starts among the constituents of vegetation when the supply is in adequate to meet full requirements of all Competition may be interspecific or inter generic.
It increases with the increase in the number of individuals in the populations. If the populations over the entire community range are habitually at levels producing interspecific competition, coexistence will be possible only for species best adapted to some recurring variants of the ecological pattern of the community.
Competition is, perhaps, most acute in the early stages of growth when mortality is highest. Between individuals of the same species an equilibrium may be established so that over a wide range of population density the total productivity is very similar. The reactions vary greatly between the individuals of different species. In perennials, the mode of growth and rate of potential spread play important role in competition. Competition continues until the vegetation is fully established.
In the process of colonization, germules of aggressive and more adapted plants reach the adjacent area from time to time. There they grow and become established. This process is termed as invasion and the new aggressive and more adapted organisms are called invaders. Invasion may be intermittent (periodic) or continuous. The invaders establish themselves in the new area either temporarily (partial invasion) or permanently (permanent invasion). New invaders may come either from the areas adjacent to the locality under colonization or they come from other places in the same locality. There are several barriers to check the invasions.
Some of them are as follows:
(i) Topographic barriers mountains, valleys, slopes, etc.
(ii) Physical barriers oceans, lakes, rivers, deserts, etc.
(iii) Biotic barriers man, animals, insects, etc.
This essentially involves the changes that are brought in the habitat conditions by the plants themselves. This is the effect of interactions between vegetation and habitat.
Plants modify the environment particularly in two ways:
(i) By changing the nature and reaction of soil, and
(ii) By modifying the climate.
Acute competition among developing plant communities causes the disappearance of many individuals. The dead remains of colonisers are added to the soil in the form of humus. Humus changes the soil structure to a considerable extent. It also increases the water holding capacity, aeration and mineral contents of the soil. Shadow of the plants checks the rise of temperature and increases the humidity of the air.
Reaction is continuous process. This leads to the development of such conditions as are less favourable for the growth of previous colonizing individuals and more favourable to the new invaders. In this way reaction plays important role in the replacement of pre-existing plants by the new invaders.
Continuous competition and reaction bring about several marked changes in the environment and consequently introduce gradual change in the structure of vegetation. After a long time, some such individuals come and dominate in the area as are least affected with the new changes in the habitat. Climate at this stage plays principal role in determining the nature of community. This process is called stabilization.
The final stage of vegetation development after the stabilization is called climax stage. The dominant species of climax community are nearly in complete harmony with its habitat and environment. Climax community is nearly stable and will not change so long as the climate and physiography remain the same. However, complete climax is impossible because the community and environment both are changing, i.e., they are in dynamic state.