In nature, plants are seldom found growing with only their own species; it is usually man’s interference by cultivation which excludes all other species. Most plants will be found growing in the wild as part of a heterogenous community. These communities may be extremely complex or very simple, but in each case they are entirely suited to the immediate conditions. The structure of a plant community is determined by several environmental factors: climate, soil conditions,, availability and also on such conditions as seed- viability and methods of plant reproduction. Not only do these factors act individually, but they also interact, thus ensuring that each community consists only of plants which are adapted to survive the prevailing conditions.
As may be expected of nature, few communities are clear-cut; there is in fact considerable overlapping and gradation. Recognizable plant communities do not occupy sharply defined areas, but merge through zones of competition where typical species of one community intermingle with those of another. These zones of transition are known as ecotones. There is even a school of thought which denies the existence of plant communities, preferring the ‘vegetation continuum’ theory, which measures vegetation as a whole, varying in both time and space. The main disadvantage of this latter theory is that it requires an infinite number of measurements to account for all of the variations before the vegetation can be described. Although this may be the ultimate ideal, it is impossible to carry out. Therefore the plant community is recognized as the acceptable method of vegetation classification.
Classification of plant communities
Plant communities are commonly classified by one of two methods, that is either by the dominant plant form, or by the species present. The first of these two methods is favoured by plant geographers, as it is common for one plant form or growth habit to dominate the vegetation of an area. Perhaps the best and most easily recognizable example of a growth form dominating a geographical region is the forest. Here there is sufficient energy input to the community in the form of moisture, heat, light and nutrients to support the arboreal growth form with its great demands on ecological resources. In direct comparison, desert areas with lowenergy input will only support small slow-growing plants.
The second form of classification is possibly the more obvious of the two: communities with the same floristic composition are the same. Unfortunately, this statement is never true, as no two identical plant populations will ever be found. When it is considered that so many interacting factors are responsible for the make-up of a plant community, it is understandable that the chances of identical conditions existing are negligible. This fact does not render this method useless, however, as it is of great importance to know what species are present; it merely means that floristic composition only enables classification into community type. Well-known community or vegetation types are salt marsh, freshwater marsh, chalk grassland, peat bog, desert, forest, tundra and many others.
Further classification difficulties arise when the ecology of an area is so varied that a patchy vegetation results. Marshland would come into this category, where the whole may be divided into wet dips with a completely different vegetation from the neighbouring dry hummocks. In such cases each unit is called a micro-stand, and together they make up a community complex. Without such limits being imposed, vegetation could be divided into such small units that the word community would become meaningless.
Factors affecting distribution of communities
The- distribution of plant communities is determined by a great number of inter-related factors. Most obvious, perhaps, are those relating to climate, including temperature, precipitation, sunlight and average wind speed, but also of great importance are the physical and chemical composition of the soil and thus the soil parent material. An area with a well-drained soil and a heavy rainfall may support a rich forest, whereas the same rainfall on a less porous soil may result in a peat bog. Thus as the geology and climate vary across the earth’s surface, so the vegetation differs.
The communities vary as the plants within them adapt to the climatic conditions, until ultimately a climax community is reached. Such a community is basically the highest form of plant life which may be supported by each environment. Complete stabilization may be said to exist only when the climax is dominated by trees. This is because trees are the most demanding of their environment of all plants, and it is considered that certain forest communities utilize all of the energy for plant growth available in that environment.
Obviously not all vegetation has stabilized to a climax; most reasonably stable communities are called subclimax. This temporary stabilization is caused by a certain arresting factor, either natural, or as a result of man’s actions, which prevent further changes. A typical example of a subclimax community is heathland, where Calluna andspecies are dominant on what is usually an acid sandy soil. The climax vegetation for such a soil in the temperate regions would be a pine or birch forest, but this is prevented from forming by fire—the arresting factor in this case. In the dry summer conditions on the heath, fires are common, the shrubby heather bushes can withstand burning, and even if almost all growth is destroyed they will soon shoot again at surface level. The young tree , however, which will have just germinated between the heather bushes, will be completely destroyed, thus preventing the establishment of a forest.
Under perfect conditions, vegetation develops from an original pioneer community through what may be a succession of sub-climaxes to a climax community. Such successions are called seres and various types exist. Successions on dry land are called xeroseres. These may begin with the colonization of rocks by microscopic algae and lichens which gradually break down the rock to form a ‘soil’, allowing colonization by slightly larger lichens. This process continues until there is sufficient soil to support higher forms of plant life. The xerosere may then continue through sub-climaxes of perhaps grassland and hazel shrub to a climax of oak forest.
Hydroseres are similar successions, only they start under water. First, small water plants become established near the edge of a, silt and plant remains collect around these, leading to a build-up of soil and a consequent shallowing of the water. These changes enable reeds and bulrushes to colonize; with a further buildup of soil material around their . This is the beginning of the formation of peat, and in an alkaline water area this would probably then become fen, and shrubs and trees which can tolerate a waterlogged soil would eventually take over.
The vegetation of any one area varies both in time and space. The variation in space is easily seen by noting the obvious differences in the vegetation over the earth’s surface. The variation in time is the serai succession as described above. In both, the community figures greatly, and is obviously an important concept which must be considered carefully when studying vegetation.