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One of the main themes of plant geography is that vegetation at a particular place evolves with time, usually starting with very simple plant communities, then leading gradually to more complex communities and ultimately to the establishment of the relatively stable plant community-the climax. Starting with a newly formed ground surface, or one that has been denuded to vegetation, the process of succession takes place, in which one plant community invades the area and is followed in turn by other plant communities in an orderly sequence, or sere, culminating in the vegetation climax. A new site for the development of vegetation may have one of several origins: a sand dune, a sand beach, the surface of a new lava flow or of a freshly fallen layer of volcanic ash, or the deposits of silt on the inside of a river bend which is gradually shifting. Such a site will not have a true soil with horizons; rather it may be a lithosol-perhaps little more than a deposit of coarse mineral fragments. In other cases, such as that of the floodplain silt deposits, the surface layer may represent redeposited soil endowed with substantial proportions of soil colloids and bases. Ground surface from which vegetation has been destroyed by fire will have the soil profile largely intact.

Stages of succession

The first stage of a succession is a pioneer stage, consisting of a few plant species unusually well adapted to adverse conditions of rapid water drainage and drying of soil, and to excessive exposure to sunlight, wind and extreme ground and lower air temperatures. As these plants grow, their roots penetrate the soil; their subsequent decay adds humus to the soil. Fallen leaves and stems add an organic layer to the ground surface. Bacteria and animals begin to live in the soil in large numbers. Soon conditions are favorable for other plant species, which invade the area and displace the pioneers. The new arrivals may be larger plant forms providing more extensive cover of foliage over the ground. In this case the climate near the ground, or microclimate, is considerable altered towards one of less extreme air and soil temperatures, higher humidities, and less intense insolation. Now still other species can invade and thrive in the modified environment.

When the succession has finally run its course there will exist a stable community consisting of certain definite proportions of the various species, each of which contributes to the total structure of the vegetation. This climax stage represents an ideal model for the so-called natural vegetation of a region. Doubt exists as to whether a climax, unchanging with time can truly be maintained. Some plant geographers consider that the climax must be followed by disturbance leading to a regression, or return to one of the earlier stages of the succession, so that a type of self-repeating cycle (is the rule. There is also the possibility that climatic change prevents the climax from being maintained in one place.

One type of succession is that in which the normal geomorphic processes build new ground continuously, as on a floodplain, delta, or sandpit. The succession associated with such continuous accretion of ground is described as allogenic. The twelve zones shift gradually to the left as silting by the river raises the level of the ground surface and shifts the water line to the left. Here we see the hygrophytes, living largely submerged, giving way to plants which thrive under conditions of intense sunlight on ground that is alternately exposed and inundated. Higher upon the bank are successive forest zones: willow, elm, and finally maple, representing the climax of mesophytic trees.

Another form of succession, described as autogenic, results from the alteration of the environment by the plants themselves, and not by outside agencies (such as the silt accumulation from flood stages of a river). The gradual covering of a sand dune by small plants, then by forest, would illustrate autogenic succession. Another illustration is provided by the evolution of a shallow pond or lake of glacial origin in the regions of the cold continental climate such as prevail in Canada and northern Europe. This bog succession is illustrated by stages beginning with the lake as it was left following disappearance of glacial ice, perhaps 10,000 to 15,000 years ago. A remarkable feature of the bog succession is that the organic matter produced by the growth and partial decay of the plants accumulates to such thickness that the open water is actually replaced by an organic mass of a substance known as peat. At the water's edge is a zone of sedges, followed by rushes. These construct a floating layer that encroaches upon the open water. There follows a zone of sphagnum (peat moss), which the eventually completely fills the lake. Now the peat deposit supports hygrophytic trees (largely spruce), which produce a woody peat. This community may in turn be replaced by mesophytic tress, marking the climax stage. In the shallower upland ponds, shown on either side of the profile, the mesophytic growth ii achieved much earlier than over the larger water body.

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