Climate
Climatic factors affect vegetation growth and plant succession
The climate is one of the most important factors determining how far plant succession will proceed the distribution of plant species and the type of vegetation which will be established. The species which are involved in the process of plant succession, the rate of succession and the final stage of succession are largely determined by climatic and soil conditions.
Should the climatic conditions improve, the climax vegetation would change as follows: Mixed grass – shrub veld – short grass veld – tall grass veld – tall grass with reeds and shrubs – forest. In the higher rainfall areas, the vegetation is more complex because of a greater diversity of plant communities that can be found than in the drier western areas. In the central and eastern parts of the Free State region, under relatively high rainfall conditions, the plant succession develops as far as dense grassveld. A definite pioneer, sub-climax and climax stage can be recognised in these areas.
In the drier western area, simple climax vegetation with a sparse cover develops (an open savanna) while only two distinct stages namely, a pioneer and a sub-climax stage can be recognised. Owing to the nature of the plant succession in the higher rainfall areas, the vegetation is more resistant to deterioration than that of the lower rainfall areas.
A plant which may be regarded as a pioneer in one area can belong to the climax in another as a result of soil and climatic differences. An example of this is Themeda triandra which is a climax grass in the central Free State but is a sub-climax plant in the Drakensberg under conditions of high rainfall, lower temperatures, and volcanic soils.
Other examples are the main pioneer genera which occur in the eastern grassveld areas namely, Aristida, Eragrostis, Cynodon and Sporobolus. These self-same genera are dominant in the climax stages of the primitive, semi-open grassveld of the drier parts of the west which extend into the Karoo and semi-desert areas.
Extreme climatic conditions such as prolonged droughts can impede succession and cause the vegetation to deteriorate with subsequent retrogression. This is usually of a temporary nature and in good seasons the veld recovers. It should be stressed, however, that the detrimental effects of drought are far less drastic in veld which is in good condition than in veld which is in a poor state. The productive ability of veld during dry periods is markedly lower and if the dry periods coincide with overgrazing, retrogression may increase tremendously.
The effect of temperature on vegetation is partly due to the influence it has on water relations in the plant as well as more directly as a result of extreme and sudden changes. The incidence and duration of frost is a limiting factor in the distribution of vegetation. Succulent plants are dominant in dry areas because they are closely related to the occurrence of frost. Frost, particularly in the spring, can promote the recovery of the veld very considerably because the young shoots of encroaching plants such as sickle bush, blackthorn and Mopani are killed by frost. The leaf surface and thus the expenditure of moisture is also considerably reduced, and the perennial grasses have no opportunity for establishment.
Hot desiccating winds contribute a great deal to evaporation and transpiration, which leads to a loss of moisture. Anything which raises the evaporation potential must necessarily have a big effect on plant growth.
Soil
Different soils affect the vegetation which can be found in them
As a result of differences in physical, chemical, and biological properties under different soil conditions, characteristic vegetation types occur. Even under uniform climatic conditions, the pattern of plant succession can differ on different veld types and thus different veld types can be distinguished in an area as a result of differences in soil.
Physical and chemical properties such as soil depth, structure, mineral content, and pH have an important influence on the pattern of succession. The depth, texture and structure markedly affect moisture conditions in the soil and will largely determine how the succession can develop to the climatic climax. The mineral content and the pH of the soil have an important bearing on the type of plant which will grow there. For example, if soils have a high pH or are rich in lime, only the climax plants which are specially adapted will grow there.
Mostert (1958) states that grassland is the most extensive type of veld in the Bloemfontein – Brandfort area. He distinguishes three subdivisions of this grassveld namely, Valley- Marsh grassveld, Themeda grassveld, and limestone veld. The distribution of these subdivisions of the grassveld is primarily due to differences in soil conditions and topography. The Valley-Marsh grassveld occurs on clay soils, at the confluence of streams, while Themeda veld is found on the higher-lying plains and ridges with sandy loam soils. The limestone communities occur on limestone ridges.
Veld deterioration may indicate undesirable changes in soil conditions. When the veld deteriorates the soil surface is denuded, the surface soil is lost, and extremely unfavourable soil conditions occur. When the cause of retrogression is removed, succession will again take place, but if the soil has suffered much damage, succession will be delayed, and it may be many years before the climatic climax can be restored.
When grassveld has cover in the climax stage, the soil is usually of such a nature that it does not easily erode. When the grass cover is destroyed, Karoo bushes usually become dominant, and this type of vegetation cannot provide effective protection to the soil compared with a grass cover. The result is a large-scale loss of soil by erosion.
The sandier the soil, the less likelythe soil to cake when the veld deteriorates. Thus, the infiltration capacity is not adversely affected with the result that the percentage run-off does not change much. The opposite is also true, however, the more loamy or silty the soil particle aggregation is, the worse the soil structure becomes when the veld deteriorates. The disadvantage is that the more clayey soil is, the more difficult is germination, establishment, and growth of plants (particularly grasses) and the slower the recovery of the veld.
Typography
The influence of climatic factors is also affected to a marked degree by topographical factors. The slope and aspect of the slope affect the temperature, rainfall, humidity, evaporation, and the insolation of the particular surface vegetation by the rays of the sun. It is particularly noticeable on mountain tops and the northern and southern slopes of mountains and in deep ravines. The result is different vegetation types are found.
The influence of aspect on micro-climate and vegetation (according to Hoberts 1975). a) Visual representation of the difference between vegetation on the North and South side; b) Northern aspects of Xerocline; c) Southern aspects of Mesocline
The effect of aspect on the micro-climate and vegetation of Thaba ‘Nchu in the south-eastern Free State is summed up above and shows clearly why the two main aspects differ so much in botanical composition. It applies to most parts of South Africa although more so in the eastern regions than in the dry western parts.
As a general rule veld recovery is slower, the steeper and longer the slopes of hills, ridges and mountains are, because the run-off is greater, and the surface soil can be removed to expose the sterile subsoil. One advantage is that ridges and mountains are usually covered with stones which reduce the compacting effect of raindrops, the speed of run-off water and evaporation and thus increase the effectiveness of the rainfall so that, in some cases, recovery is not delayed too long.
Steep mountain side with exposed soil and some rocks
Veld Management Practices
Without proper management and planning, many veld management practices can be severely detrimental to the environment
Through the centuries there has been a balance between plants and physical factors of the environment on one hand and the animal factor on the other. This balance must be maintained and unfortunately, in some cases restore first because of the disturbing influence of animals and their management. Judicious veld management is the most important aid in the process of plant succession. Plant succession is encouraged by effective veld management and veld improvement is accomplished. Not only can veld improvement be affected but the veld can be so utilised that it has the least possible harmful effect on the plant cover and, in addition, the greatest possible quantity of good quality grazing can be provided.
One of the aims of veld management is to maintain, develop, and stabilise that stage in the process of plant succession that will provide the most suitable grazing and ensure effective soil and water conservation.
Overgrazing, selective grazing, continuous grazing, unadapted types of animals, injudicious veld burning, and other malpractices have resulted in the deterioration of the veld (retrogression). For this reason, the grassveld in large parts of the Free State region has completely disappeared and has been replaced by karroid pioneer vegetation. The nature and composition of a veld type can thus be completely changed by mismanagement. Because the process of change is slow, however, it is not always noticed. The changed conditions are often accepted as the natural normal condition. A well-known example of such changes in the veld is that in the southern Free State where the grassveld has been replaced by karroid vegetation.
By the application of controlled grazing and the provision of suitable rest periods, it is sometimes possible to bring about a fairly rapid recovery of the veld. In many cases, however, veld restoration through judicious veld management is a slow process. Where veld deterioration has already reached serious proportions and soil conditions are very unfavourable it is necessary to assist veld restoration through reseeding because the source of seed has dwindled over the years. The closer the veld has deteriorated towards the pioneer phase the longer it will take to recover. It is important to remember that it is much easier to initiate retrogression than it is to check it or restart the succession.
Factors Influencing Plant Production
The productivity of natural veld in any region is determined by the environmental and climatic conditions of the veld.
(1) Environmental factors:
Environmental factors determine the natural vegetation of any area and thus the quantity and quality of the pasture that is produced.
The natural production potential in any area is largely influenced by climate, soil, and topographic conditions. The effect of environmental factors on the vegetation is as follows:
(2) Climactic factors:
Climatic factors affect the natural vegetation
Vegetation is often an indication of the type of climate. Many different factors contribute to climate:
- Rainfall: Water is essential for plant growth and rain is the only natural way by which water can reach the soil. The following characteristics of rain are of great importance:
- Amount.
- Duration.
- Intensity.
- Distribution through the year.
- Reliability.
- Type.
- Temperature as in the case of water, influences just about all the functions in the plant. It is not only necessary for life processes but provides most of the energy for these processes.
- Frost may have important effects on vegetation.
- Wind affects humidity, temperature and therefore evaporation and transpiration, but also has a mechanical effect on the vegetation. Wind also transports pollen and seed.
- Sunlight: The sun supplies light which is necessary for photosynthesis and supplies heat (energy) to the atmosphere and surroundings so that chemical processes can take place. While temperature and moisture are often limiting factors, which can retard the growth of plants, light is very seldom a limiting factor on the veld.
- Humidity and evaporation are mainly determined by other factors like temperature, wind, and rainfall. Humidity has a very important effect on transpiration.
- Except for the rainfall, man has succeeded to modify the climate only on a very small scale, e.g., glass houses, windbreaks, and so on. In extensive, arid areas the climate must be treated as part of the ecosystem and the production system. Knowledge of the climate, and especially the rainfall characteristics and the influence thereof on the vegetation, is therefore of help with proper veld management.
(3) Soil factors (edaphic factors):
The different characteristics of the soil which affect the vegetation
Soil, together with climate, are the two primary factors which control the type and production of the veld. The following are the most important soil characteristics that influence the vegetation:
- The texture is the ratio between the amounts of clay, silt and sand in the soil. It plays an important role in the water retaining capacity, aeration, fertility, absorption capacity of the soil and the movement of water and salts in the soil.
- The vegetation of sandy soils usually differs a lot from that of clay soils.
- The structure is the arrangement of the individual soil particles and aggregates of soil particles. Structure determines the porosity of the soil which in turn influences the absorption, run-off of water, erodibility, aeration, and the movement of water in the soil.
- Humus and micro-organisms: Humus is well-rotted organic matter descending from plant material by the working of micro-organisms. It normally gives a good structure to the soil. Its colloidal nature plays an important role in the retention of mineral plant nutrients i.e., adsorption of cations and fertility. The amount of humus in the soil depends on the vegetation and climate.
- Soil reaction or pH value is the acidity or alkalinity of the soil solution. It affects the solubility and availability of plant nutrients in the soil. Some plants are very specific concerning the pH values of the soil.
- Other soil factors include factors like soil depth, the number of stones, the presence of compact soil layers, and so on, which can also have important influences on the vegetation.
(4) Physiographic factors:
This includes altitude, steepness of slope and aspect of slope. An increase in altitude causes a decrease in temperature as well as bigger changes in temperature and general weather conditions. The steepness of a slope affects the runoff of rainwater and the stability of the soil which will in turn influence the vegetation.
In the southern hemisphere, northern slopes are generally warmer and drier than southern slopes. This is the result of the more direct sun rays a north slope receives because the sun follows a northern orbit. The vegetation on a northern slope is therefore usually more xerophytic and sparser.
Physiographic factors, therefore, act indirectly, as they act by changing the climate, especially the micro-climate.
(5) Biotic factors:
The biotic factors are those having to do with the influence of living organisms on the ecosystem. This includes, among others, the following:
- The effect of plants on other plants (in other words competition).
- The effect of the vegetation on the soil and micro-climate.
- The effect of the vegetation on the animals.
- The effect of the animals on the vegetation.
The latter factor revolves around the defoliation of plants and is also the most important aspect of veld management.
(6) Abiotic factors:
Temperature and moisture are the two most important abiotic elements impacting plant primary productivity in an environment.