1. AN ANALYSIS OF PLANT SUCCESSION
Plant life plays an important role in the weathering of rock, in the entrapment and stabilisation of solid particles that are being moved by wind, water, gravity, and in making organic additions to the rooting medium. Soil accumulation is especially rapid in aquatic habitats where currents carrying silt and clay are reduced in velocity as they move through rooted plants and so the soil which they carry is deposited.
Organic matter and humus
In many environments the advance of succession may depend on the organic matter and nutrients added to the soil surface by leaf litter. Eventually the rate of addition of organic matter to the soil surface is equaled by the rate of decay, so that the soil itself may be said to reach a ‘climax’ state. The time taken to reach such a stage, and the amount of organic matter retained in the soil varies considerably among environments. It is most rapid, and with a lower proportion of organic matter in its stable state, in tropical environments. Soil microbial populations increase with the increase in the humus content of the soil, and the reduced oxygen concentration in the soil brought about by this increased population of microbes may, at least in some grassland areas, prevent the invasion of woody plants requiring better aeration.
The bulk density of the soil declines as succession advances in most stable soils due to the development of soil structure. Plants play a role in this structural development in a number of ways. Roots taking up water dry out the soil so that it shrinks setting up strains in the soil which result in the development of many small cracks and so establishing aggregate boundaries. Also, as the roots thicken they set up pressures within the soil, causing shearing stresses which emphasize the different aggregates. Humus also aids in the building of particles into clusters by acting as a cementing agent which is often more efficient than clay colloids alone.
On loams or heavy soils this development of structure assists in water holding and water penetration, while in sandy soils it reduces the rate of water loss. In both cases, therefore, the tendency is to reduce the drying out of the soil.
A layer of vegetation has other considerable effects, besides those mentioned above, on the water relations of the soil. Prompt infiltration due to improved structure and the slower movement of water across the soil surface result in lower rates of runoff and improved water conditions in the soil. Raindrop interception reduces the force of impact and so the degree of puddling of the soil surface; humus materially increases the water holding capacity of coarse textured soils; the cooling effects of shade reduces evaporation; the interception of fog by foliage can greatly augment soil moisture through fog drip; while increased soil depth. increases the overall water holding capacity of the soil. However, opposite effects may also operate. The canopy of vegetation and the layer of litter on the soil surface may intercept moisture precipitation, so allowing it to evaporate without ever reaching the soil; transpiration remove vast quantities of moisture from all parts of the soil profile.
Commonly the overall effect is a net desiccation of the soil at least during later stages of succession where the soil is relatively deep. However, the soil surface is rendered more moist, so suiting seedling development, while the lower soil layers are rendered drier. This drying out of the lower layers may be critical for the survival of many plant species.
One of the first important changes a .pioneer community makes in a previously unoccupied body of parent material is to bring about a redistribution of many minerals. Solutes taken up throughout the rooting medium are mostly returned to the soil surface in foliage drip or litter. The fertility of the topsoil therefore improves at the expense of the subsoil. Eventually, however, equilibrium is reached where the rate at which plants bring nutrients up from the lower layers is equivalent to the rate at which nutrients are leached to lower layers. However, the removal of soil water by transpiration may considerably reduce the volume of liquid and so reduce the quantity of nutrients which are leached below the root zone. The nutritional status of the topsoil therefore depends on the type of plant community it supports.
Nitrogen, which does not have its origin in weathering rock, is unique among plant nutrients and is usually a limiting factor at the start of primary succession. Plants must generally depend on the nitrogen received from rain water until such time as the litter they cast provides a substrate to nitrogen-fixers in the soil. The soil therefore undergoes a change from extreme nitrogen deficiency in the early stages of succession to, in many cases, an abundance of nitrogen at later stages of succession, although this may not always be in an available form. Sulphur and phosphorus are also deficient in primary bare areas since they are often deficient in parent materials. These nutrients also accumulate as succession advances.
Where high rainfall promotes leaching there is clear evidence that soil fertility may build up to reach a peak before the climax community has developed, and decline as succession advances further. This later decline may reflect leaching losses promoted by the acidification of the soil by the plants and/or to the increased amount of the essential elements being tied up in the litter or humus. The length of the time during which nutrients are tied up in the organic matter may in fact be critical on infertile soils. In tropical rain forests as well as in the early stages of a number of other seres the rate of turnover of nutrients is rapid so, that high sustained production is possible even in relatively infertile soils. In temperate zones, however, a considerable proportion of the soils supply of nutrients may be tied up in the organic matter at any one time.
Many plants produce toxins which may be damaging either to plants of other species or even to seedlings of the same species. The action of toxins within species could well hasten a change in a community, while the action of toxins between species could well affect the botanical composition of some or all successional stages as well as the balance among species at climax.
Plant succession and profile development are closely interrelated. Surface stabilization often prevents soil erosion which would prevent the development of an A horizon; minerals are dissolved by acids secreted by roots; altered temperatures affect the type of weathering of minerals; and more water penetrates the soil although the downward movement of nutrients and clay is prevented deeper in the profile through the removal of water from lower levels. These effects, together with those already mentioned such as an improvement of soil Structure, play an important part in soil development.
The first plants that appear on bare areas are subject to the full intensity of sunlight. As succession advances, the replacement of plants of low stature by others that are taller, and the increasing number of superimposed canopies, results in a gradual reduction in the quantity and a change in the quality of light filtering through to the soil surface. In certain simple communities seedlings and mature plants are intolerant of shade, whereas in more complex communities the dominant plants must be shade tolerant when young, even though they must be able to grow at high light intensities when mature. On the average the intensities of light to which plants are subjected declines as succession advances. Only the dominant plants continue to be subjected to high intensity conditions, with decreased intensity at successively lower levels within the canopy.
The day temperature pattern is greatly altered as plants colonize an area. On bare areas temperature varies considerably, particularly at the soil surface itself so that the conditions under which seedlings must survive are most unfavorable. Even before the canopy closes completely but when it is reasonably dense, temperature variations may be large since air movement will be reduced at the soil surface but the canopy may be ineffective in reducing radiation loss at night. As the canopy closes and so reduces temperature loss through radiation, temperature variations decline considerably. This may favor the germination of the seeds of some species while mitigating against the germination of the seeds of other species. Hence, successional changes are encouraged.
Humidity of air and moisture content of soil at surface
As succession advances in the dryness of the air and the soil at the surface is reduced. This results from reduced air movement, a greater transfer of moisture from the soil to the lower layers of air through transpiration and a higher relative humidity due to the lower soil temperatures.
Although the forces motivating a sere may be purely autogenic (e.g. in bogs) or purely allogenic (e.g. intensively grazed grassland), both processes are generally at work. Their effects may be additive, as when bogs are drained, or they may act in opposite direction, as when the autogenic stabilization of steep slopes is offset by cliffs collapsing or mud slides.
Consequences of changes of environment
Normally, environmental changes brought about by one stage favor invaders of the next, until such time as the climax vegetation occupies an area. An understanding of the changes in vegetation brought about by changes in the environment depends on an understanding of the ecology of the species involved.
Whenever the quantity of useful matter (e.g. nutrients, CO2, water, 02) or energy falls below a certain level needed for the maximal growth of two or more organisms which draw on the same supply, a contest arises. This contest is termed competition. Species differ widely in their genetic capacities to cope with crowded conditions. This factor will be considered in detail in the section dealing with autecology.
Under conditions of crowding, soil fertility has an extremely important impact on the pattern in which vegetation develops. Where soils are so infertile that it is the fertility rather than the genetic potential of even those plants with the lowest potential which sets the limit to plant growth, a uniform stand of stunted plants develops. On the other hand, where genetic differences can be expressed, as under high fertility conditions, the plants with the highest potential grow most rapidly and so gain a competitive advantage over the weaker plants. Natural thinning follows automatically.
Changes in climate
Climate changes slowly through geologic time. Such changes bring about changes in the vegetation through adaptation and migration.
Within the past few centuries man has accidentally or intentionally introduced thousands of plants into new regions where they have sometimes been most successful. The introduction of various insects, parasites, animals, etc. has similarly had an important effect on the vegetation of many regions. The importance of plant immigration can be gauged by the fact that not one important crop plant is indigenous to N. America, Australia or central and northern Europe.
The evolution of new species or ecotypes which are better adapted to a particular set of conditions and can therefore assert a greater degree of dominance, or the evolution of, for example, more virulent parasites, may be responsible for changes in vegetation.
Soils in any region often differ to the extent that they produce different apparently stable communities. Some ecologists believe that any soil which differs from the potential, as determined by the environment, is still in a developmental stage, and so the community which it supports is still developing. Such a community is, therefore, to be regarded as seral. However, the disturbance of such a seral community will initiate another sere which may add to the same relatively stable community within a comparatively short space of time. It seems reasonable, therefore, to regard such a ‘terminal’ community as climax, recognizing that its composition is governed primarily by the nature of the soil. It could therefore be regarded as an edaphic climax.
Edaphic climaxes, because of factors such as low fertility, dryness or solute imbalance, are often characterised by low productivity. However, the moist edaphic climaxes such as those of warm-climate marshes are among the most productive regions of the world.
In regions outside of the tropics, slopes facing the equator and those facing the poles usually have contrasting microclimates; the one receives much more radiant energy than an area of level topography, the other much less. Moisture conditions consequently also vary, so that each of these micro-climates usually support distinctive seres and climaxes. Other topographic effects arise through differences in precipitation, drainage, exposure to dry winds, or through location in thermal belts or violent wind tracts, basins where cold air collects or on steep slopes where snow or stones cascade and so damage the vegetation.
The fire factor
The recurrent burning of vegetation eliminates fire-sensitive species, whose place is taken by fire-tolerant species. Any association that maintains its composition and structure only as a consequence of periodic burning may be referred to as a fire climax
The zootic factor (i.e. the animal factor)
Most, if not all, communities possess animal life of some form. In many places, and notably in grasslands, the structure and composition of the community is in a large measure controlled by the destructive activity of some kind of animal. Communities which become stable under a particular regime of utilization are known as zootic climaxes. The type of zootic climax which develops depends on a number of factors such as the type of animal present, the number of such animals, the season of utilization, etc. The earth’s surface is today, through man’s influence on the animal factor, a mozaic of zootic climaxes or potential zootic climaxes of the region.
2. EXAMPLES OF SUCCESSION
The early stages of the development of vegetation differ quite radically according to the nature of the initial habitat. The two main types of seres are hydroseres and xeroseres. Succession beginning in ponds, lakes, marshes and elsewhere in water i.e. hydric environments, are termed hydrarch, they are initiated by hydrophytes and the different stages of the sere constitute a hydrosere. Succession initiated on bare rock or sand or other situations where there is an extreme deficiency of water i.e. xeric environments, are termed xerarch, they are initiated by xerophytes and the different stages ofdevelopment constitute a xerosere. In both cases development is always towards the mesic or intermediate environment which is occupied by mesophytic plants. A xerosere adjacent to and under the same climatic influences as a hydrosere will tend to end in a similar mesophytic community. Different types of xeroseres are recognised according to the nature of the bare area e.g. lithosere on rock and psammosere on sand. A halosere occurs under extreme saline conditions andmay be classified under either the hydrosere or xerosere, depending on the moisture conditions.
THE HYDROSERE (WET CONDITIONS)
Fresh water is a very uniform medium for plant growth the world over because of its uniform physical properties. For this reason water plants are very widely distributed. What differences there are in water environments are largely in temperature, state of aeration, rate of flow and in the quantity of chemical constituents dissolved in the water.
In general the successive stages in a hydrosere can be outlined as follows:-
The submerged stage
The pioneers of the hydrosere are completely submerged and occur throughout the whole area of open water. They often form quite a dense underwater vegetation of various types of flowering plants, both rooted and unrooted. These plants tend to slow down the rate of flow and so bring about a greater deposition of silt. They also add organic material to the underwater soil. These reactions tend to shallow the depth of water and generally make conditions more suitable for the next stage.
Floating plants generally begin to invade where the depth of the water is about 2 to 3 meters and where soil conditions have been prepared for them by the submerged plants. These plants are rooted in the mud and bear their leaves on the surface of the water. Associated with them and often occurring before them are unrooted floating plants. The amount of light reaching the submerged plants is reduced as these plants become dominant. The soil building process continues and conditions on the edges are made suitable for the following stage.
Reed swamp stage (hydrophytes)
Rooted plants that are partly submerged but which bear their leaves above the surface of the water now invade (e.g. reeds and bulrushes). These plants usually invade where the depth of the water is 30 to 120 cm deep. Like the preceding plants their taller stature and dense growth exert a controlling influence.
Sedge-meadow stage (hygrophilous plants)
As the water becomes shallower still, conditions become less ideal for the rushes and reeds and the moist soils with increasing amount of light make conditions favorable for the sedge meadow stage. Soil conditions may become too dry for these plants and other communities take over. There is, however, a fair amount of variation in the moisture content of the soils under this sedge-meadow stage.
In dry climates this stage is followed by grassland or other xeric climaxes e.g. karoo, but in moist areas it is followed by the woodland stage.
When the low lying vlei areas have been built up to the level where the soil is saturated for only part of the year, certain species of shrubs and trees may appear. Those that can withstand a certain amount of water logging will be the pioneers of this stage.
The accumulation of humus with the better aeration of the dry soil results in the development of forest composed of several distinct layers. Competition for light, nutrients, water, etc. may result in the development of a pure stand of the plant type best adapted to the local conditions or a mixed stand dominated by trees having a similar physiology.
Through the process of succession, therefore; mesophytes have replaced hydrophytes in a succession which is hydroseral. All these stages can generally be seen together in a particular region in zones around an open water area.
THE XEROSERE (DRY CONDITIONS)
The lithosere. (Succession on Rock)
The general stages through which lithosere passes are as follows:
A bare rock surface is very unattractive to plant life. Rock surfaces are smooth, extremely deficient in water except during rains, completely exposed to the sun and subject to extremes of temperature. Crustose lichens alone are likely to grow in such situations. Their small germules are easily dispersed and lodge in cracks on the rock surface. The germules develop and the lichens spread over the adjacent rock surface. They usually have brilliant colors (usually bluish) and adhere very strongly to the patches of rock on which they grow. These lichens flourish during wet weather and remain in a state of desiccation for very long periods during drought. Mineral nutrients are obtained by the secretion of carbon dioxide which forms weak carbolic acid with the absorbed water. This weak acid slowly brings about a disintegration of small sections of the surface of the rock. In this way the rock surface becomes pitted. Soil slowly accumulates in these depressions – this soil either being blown in or washed in, or being formed at the rock surface by the mixing of the particles derived from the disintegrating rock and organic matter derived from decaying lichens. These soil and lichen surfaces then provide sites for the invasion of other types of lichens.
The crustose -lichen stage may persist for hundreds of years on quartzite or basalt in dry climates, but on limestone or sandstone in a moist climate sufficient change of the habitat to permit the invasion of the next stage may take place within a lifetime.
Once a little soil has accumulated these disc shaped lichen, which are attached to the rock surface only at one point, invade the area. They shade the crustose-lichens and become dominant. Soil and organic matter easily accumulate between the discs of the lichens.
A number of other types of lichens may also invade at this stage e.g. dendroid lichens which are slightly taller than the foliose lichens and cause similar reactions.
Once sufficient soil and other material has accumulated, xerophytic mosses appear. These mosses are taller than the lichens and so slowly shade them out. Soil rapidly accumulates among the erect stems as the stems, dying out below but continuing to grow above, build up a substratum and continually increase their area. The soft moss mats which develop are capable of holding water like a sponge and in this way can increase the number of days during which growth is possible. The mats can easily be lifted from the rock face to expose the pitted rock face underneath.
The moss mats are now invaded by xerophytic herbs. Short lived animals and geophytic plants and other perennials which have some or other water storage mechanism are often the first to invade. Grasses become the most important and dominant species at this stage. Within the grassland stage there are a number of successive phases which occur before conditions become suitable for the invasion of the next stage.
The favourable moisture conditions which develop under the grassland sward allow invading shrubs to become established at the expense of the grasses. The improvement of the moisture and temperature conditions which they bring about, coupled with the development of an organic layer on the soil surface, results in the creation of an excellent nursery for tree seedlings and trees may now begin to appear.
The first tree seedlings may be relatively xeric. The pioneers are widely spaced and the hard conditions of life are reflected in their stunted growth. However, as biotic reaction continues and conditions improve trees increase in both number and vigour. With increasing shade the light demanding shrubs are no longer able to survive and other more tolerant and mesophytic shrub species become associated with the forest species forming a characteristic under growth in tropical and sub-tropical forests.
Through this process, then, the xerosere has changed from one of extreme conditions to one of moderate conditions and the vegetation has developed into a mesophytic forest.
The psammosere (i.e. succession on bare sand)
Since the psammosere varies according to the climatic conditions prevailing in an area, succession on sand will be discussed with particular reference to Natal vegetation. Here the psammosere occurs on the narrow strip of land just above the sandy beach.
Pioneers in these areas must be able to withstand extremes of soil moisture, intense insulations from above and below, erosive action of windblown sand, salt spray, and saline conditions. Conditions are particularly severe between the first fixed dune and the high tide line and here only ephermeral plants are able to survive. These plants are not effective as soil binders and this zone is seldom well developed.
At the base of the first dunes, plants occur which have a creeping growth habit with underground stems which can tolerate sand coverage. As their leaves are covered by sand the nutrients are withdrawn for use higher up the stem. The leaves characteristically have thick cuticles to enable them to withstand the effects of windblown sand. Most of the species colonizing such areas are succulents having their stem buds well protected by leaves arising lower down the stem.
In this way the buds are moderately protected against the desiccating effects of high temperatures and salt spray.
One of the main effects of these creeping plants is to stabilize the sand and to act as an obstruction against which windblown sand accumulates. They therefore serve as a nucleus for the development of a new dune. As newly blown sand covers them new stems quickly develop to permeate the layer of loose sand and so stabilize it against the further effects of wind.
In the trough which develops behind the first dune, and so away from the direct effects of salt spray and windblown sand, there develops a community of shrubs with a more upright growth habit. This dune scrub has a very mixed composition and includes a large number of climbers. Further inland and out of the trough, trees which are well adapted to the local conditions become dominant. On the seaward side they form a closed canopy with a clipped hedge appearance. Here the trees are stunted by the salt spray and are very much more branched than trees growing on the landward side. Moving towards the landward side the forest becomes taller and more mixed. Here trees which cannot withstand the effects of direct exposure are able to grow. The canopy is more open than on the seaward side of the dunes and so more undergrowth develops. Climbers are also important in this forest.
Further inland and away from the coastal dunes the typical coastal forest develops. There is, therefore, a gradual change from communities which are resistant to the unfavorable conditions along the seashore to communities which are not resistant to these conditions further inland.
In some coastal areas a series of dunes may develop, all running parallel to the coast. In areas of Zululand, for example, up to 17 dunes have developed. The first dune is relatively unstable, but subsequent dunes have an increasingly complex vegetation which has stabilized these dunes against the erosive action of wind and water.
The halosere (Saline conditions)
The balance is represented by the vegetation of coastal lagoons and estuaries which are subject to flooding. The salt content of the water at the river mouths varies considerably during the year: during high tides the water becomes very salty, while when the rivers flood the salt water is flushed away and replaced by fresh water. Plants growing in lagoon conditions must therefore be able to withstand sudden and considerable changes in osmotic pressure of the lagoon water. They must also be able to withstand anaerobic conditions as the soil is often waterlogged. The plants which are best adapted to these conditions are the mangroves (The association of plants of the muddy swamps at the mouths of rivers and elsewhere in the tropics, over which the tide flows daily leaving the mud bare at low water). Only three mangroves occur as far south as Natal, each having adapted itself to swamp conditions in slightly different ways. The development of aerial prop roots from as high as 5 ft up the stem and of plank buttresses and knee roots has given these plants greater support – a requirement of large plants growing in soil which is permanently moist. Pencil roots, composed mainly of aerated tissue, assist in providing oxygen to the root systems. In. addition, these trees have large seeds which start to germinate before the seeds become attached to the soil. Some of these float readily and so can be widely distributed by water.
Associated with the mangroves are a large number of herbaceous plants which are also resistant to saline conditions. It would be incorrect, however, to give any definite order of invasion since the order seems to depend largely on local conditions. The faster propagating plants will dominate first and here the distance of the bare area from the source of seed and the potential
of the seed for migration will be important. It is possible, therefore, that the mangroves will be the first plants to colonize a bare area.
Further away from the swamp area the vegetation merges into coastal forest. The mangrove swamp generally remains the climax of the halosere since the biotic effects of the communities which develop are nullified by the unchanging effects of the tides.