agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 10.1 1999
MANAGEMENT OF WETLANDS
C R Hurt
ARC - Range and Forage Institute, Cedara
D Kotze
Department of Range and Forage Resources, University of Natal
W B Russell
KwaZulu-Natal Department of Agriculture
The Value of
Wetlands
Wetland Legislation
Use and Management of Wetlands
INTRODUCTION
Land is considered to be wetland or vlei when waterlogging, permanent or periodic, in part of the soil is a major determinant of soil properties as well as the plant and animal communities inhabiting the site. There is a tremendous variation in the extent to which waterlogging occurs in different wetland systems. At one extreme are those areas where the water table is always above the soil surface. At the other extreme are areas where the surface water is present for only very brief periods during the wet season, and the water table generally lies below the soil surface. In the latter case, however, the water table is still high enough to influence the plant and animal communities living at the surface.
One generally finds that, within any given wetland, zones with different waterlogging regimes occur, and each zone is characterized by different plant communities (Figure 1). The aquatic zone com-munities consist of free-floating aquatic plants which are adapted to survive on the water surface (e.g. duck weed), and species adapted to survive underwater (e.g. water grasses, Potamogeton spp.). The reedswamp zone is characterized by tall emergent species such as reeds (Phragmites australis) and bullrushes (Typha capensis). Plant communities of the sedge meadow zone are dominated by sedges (grass-like plants) and a few grasses (e.g. red swamp grass, Hemarthria altissima). The hygrophilous grassland zone contains grass species which occur in wetland as well as non-wetland areas, but also contains species which are restricted to wetland areas. Individual wetlands vary considerably depending on the relative proportion of each zone within the system. For example, a wetland may consist almost entirely of hygrophilous grassland with only a small area of sedge meadow in the wettest zone, while another may be dominated by reed swamp with a central aquatic zone and only a small area of fringing sedge meadow and hygrophilous grassland.
Wetlands may be found in different landscape positions, and, broadly speaking, these may be divided into three major groups.
- Plains wetlands are usually the largest of the wetlands and generally occur on mature river floodplains. They have very gentle gradients, and contain all four vegetation zones.
- Valley wetlands occur in steeper, younger valleys, and are very narrow and long, usually following the course of a stream for some distance. The reedswamp zone in this type of wetland is usually less developed than in plains wetlands.
- Headwater wetlands are small depressions, generally on steep hill slopes in the headwater region of the catchment. The reedswamp and sedge meadow zones are often not present.
Wetlands are found in all the Bioresource Groups of KwaZulu-Natal, but are most common in the high rainfall upland regions (e.g. the Mgeni and Franklin vleis) and along the coast plain of northeastern KwaZulu-Natal (e.g. the wetlands which form part of the greater St Lucia system).
Wetlands are probably one of the most mismanaged habitats. Most people regard wetlands as wastelands, because superficially they do not appear to have much value in real terms. Of major concern is the extent to which wetlands in KwaZulu-Natal have been lost through development and poor land-use practices. Once one starts to examine these fragile systems, however, it soon becomes apparent how important they are to many other habitats, including most veld-based agricultural systems. In order to manage wetlands properly, it is essential to understand how they function and how they interact with other ecosystems. The objective of this Production Guideline is to outline concepts of wetland ecology, and to use these principles as a basis for recommendations for wetland management.
Wetlands in their natural state perform many functions which are of value to society, and these are discussed in this section.
Wetlands contribute substantially to the improvement of water quality by removing suspended particles (such as silt, clay and other solid pollutants), excess nutrients (most importantly nitrates and phosphates) and toxicants (such as pesticides, herbicides and heavy metals). Certain characteristics of wetlands contribute to the improvement of water quality.Water quality enhancement
- The gradual slope of wetlands and the resistance provided by the specialized vegetation causes the rate of water flow to decrease as runoff water enters a wetland system. When water moves slower, suspended particles are able to settle out to the bottom of the system.
- The shallow nature of wetlands allows a relatively high degree of contact between the water passing through a wetland and the wetland sediments. This contact leads to significant exchanges occurring between the water and the sediments, and excess nutrients and toxic chemicals are thereby more readily removed from the water.
- Nutrients and toxicants can enter the system when a rain storm follows fertilizer, herbicide or pesticide application to croplands, and the chemicals accumulate in runoff water. The chemicals are taken up by the plants, and ‘held’ in the system. By the time the above-ground parts of the plant die, most of these chemicals have been transferred to the roots of the plants where they are ‘buried’ in the bottom sediments.
- Wetland soils generally have a high organic matter content (which has a high ion exchange capacity) as a result of the prevailing waterlogged conditions. This facilitates the trapping of pollutants such as heavy metals.
Flood attenuation and water storage
Wetlands usually have a number of features (such as relatively flat gradients, dense vegetation and outflow constrictions) which slow down the rate of water flow, and dissipate flood waters locally. By delaying the passage of water through the catchment, wetlands have value in two respects:
- they provide a more sustained supply of water during periods of low flow (i.e. they have a ‘sponge’ effect); and
- they dampen flood peaks and hence decrease the damage which is caused by flood waters downstream from the wetland.
Wetland vegetation has evolved under conditions of frequent flooding and is able to recover rapidly from physical damage following a flood.
Bank protection
Wetland vegetation also has a physical function. The emergent vegetation in the reed swamp and sedge meadow zones forms a buffer between open water and the banks of streams, lakes and estuaries. The plants dissipate the energy of the waves and prevent bank erosion by wave wash.Species diversity
Wetland plants often have to survive unfavourable periods during their life cycle (e.g. flood damage, excessive soil deposition, saline conditions), and consequently have hard leaves which make them unpalatable to most herbivores. As a result, most dead plant material decays and is broken down by a multitude of invertebrates and other organisms (e.g. bacteria) present in the system. These animals are a source of food for higher animals, and thus the vegetation forms the basis of a complex series of food chains. In addition to the plants providing food for other organisms, they also provide refuge for frogs, birds and mammals, and thus are also responsible for maintaining animal diversity. Certain rare species (such as wattled crane, whitewinged flufftail and long-toed tree frog) are entirely dependent on wetlands in their natural state. Thus, if the loss of wetlands through development or mismanagement continues at its present rate, these species may be threatened with extinction. In addition, many other species depend on wetlands to varying degrees. These range from serval which obtain most of their prey (particularly vlei rats) from wetlands, to common reedbuck which rely on wetlands for cover.The high natural plant and animal diversity is important in conservation areas. In agricultural systems, it is nevertheless important to keep the wetland functioning as naturally as possible, because a break in the link of any part of the food web can cause changes to the system which may decrease its value.
Resource utilization
Certain wetlands, if managed properly, are agriculturally highly productive, primarily because of the ready availability of soil water. The vegetation can provide medicinal plants, or reeds and rushes for basketry. Wetland systems can be used as a source of game birds and mammals for trophy and recreational hunting. Thus wetlands in their natural state can provide a direct source of revenue for the landowner. Another important use of certain wetlands is the provision of grazing for domestic stock.
At present, the legislation which provides for control of these systems are the regulations promulgated under the Conservation of Agricultural Resources Act (Act 43 of 1983) - Regulation 7 of Government Notice R.1048. In terms of the Act, the land user is obliged to abide by certain restrictions which are set out below.
Utilisation and protection of vleis, marshes, water sponges and water courses
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Regulation 7(1). Subject to the provisions of the Water Act, 1956 (Act 54 of 1956), and subregulation (2) of this regulation, no land user shall utilise the vegetation in a vlei, marsh or water sponge or within the flood area of a water course or within 10 metres horizontally outside such flood area in a manner that causes or may cause the deterioration of or damage to the natural agricultural resources.
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Regulation 7(2). Every land user shall remove the vegetation in a water course on his farm unit to such an extent that it will not constitute an obstruction during a flood that could cause excessive soil loss as a result of erosion through the action of water.
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Regulation 7(3). Except on authority of a written permission by the executive officer, no land user shall:
(a) drain or cultivate any vlei, marsh or water sponge or a portion thereof on his farm unit;
(b) or cultivate any land on his farm unit within the flood area of a water course or within 10 metres horizontally outside the flood area of a water course.
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Regulation 7(4). The prohibition contained in subregulation (3) shall not apply in respect of:
(a) a vlei, marsh or water sponge or a portion thereof that has already been drained or is under cultivation on the date of commencement of these regulations, provide it is not done at the expense of the conservation of the natural agricultural resources; and
(b) land within the flood area of a water course or within 10 metres horizontally outside the flood area of a water course that is under cultivation on the date of commencement of these regulations, provided it is already protected effectively in terms of regulation 4 against excessive soil loss due to erosion through the action of water.
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Regulation 7(5). The provisions of regulation 2 (2), (3) and (4) shall apply mutatis mutandis with regard to an application for a permission referred to in subregulation (3).
Wetland development
Tree planting permits which are issued by the Forestry
Branch, Department of Water Affairs and Forestry, restrict the planting of trees
in the vicinity of wetlands. These norms, while being site specific, are never
less than those specified under Act 43/1983.
Any land owner wishing to develop a wetland in any manner (e.g. drain, plough, plant trees) is required to apply to the local agricultural extension office for permission to do so. A soil conservation officer will conduct an inventory of the site which will include such information as soil type, degree of saturation, frequency of flooding, size of catchment and crop to be planted. This inventory, together with the land owner's application, is submitted to the chairman of the Wetland Advisory Committee (WAC) at Cedara. The WAC comprises representatives from the Departments of Water Affairs and Forestry, Agriculture, the KwaZulu-Natal Provincial Administration, and the Natal Parks Board. Their respective recommendations on the advisability of developing the wetland are collated and sent to the Executive Officer appointed in terms of the Conservation of Agricultural Resources Act (Act 43/1983). The Executive Officer will accede to the request, adding if necessary any provisions, or he may refuse the request.
USE AND MANAGEMENT OF WETLANDS
Unfortunately, many farmers consider wetlands in their natural state as wastelands, and they have therefore often not been managed properly in the past. In many cases, ignorance of the importance of these systems has led to their misuse, and subsequent degradation. Wetlands, including many large and important wetlands, have been dammed, drained, ploughed or planted to pasture species. In order to ensure that wetlands function properly, it is essential that the system as well as its surrounds are managed properly. Because wetlands comprise systems where a delicate balance exists between the various complex physical and biological processes, any abnormal influence on any component of the system may have a major effect on the entire system. Wetlands must therefore be managed holistically, rather than to gain some kind of benefit from one, or a few, components of the system. For example, converting a wetland to an improved pasture would be detrimental to other processes in the system (such as a decrease in water quality enhancement, nutrient retention ability or species diversity, or, in certain cases, a decrease in flood attenuation). The legal restrictions on the modification of wetlands (as stated above) must also be considered. Wetlands which have already been disturbed, and which are not performing agriculturally, should preferably be rehabilitated as far as is possible. A principle to which all management options must adhere is that soil erosion must not be increased by management.
Wetland management as a whole applies to the catchment, the wetland habitat and the system below the wetland, as management of each of these individual components has an influence on the functioning of the wetland.
Wetland management begins in the catchment because any form of catchment degradation will have an effect on the water regime of the catchment as well as that of the wetland. In this respect, a number of aspects of veld management are important.Catchment management
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Erosion. Natural erosion is a normal process in a catchment (refer to Production Guideline 7.1 in this series). The degree of erosion is largely dependent on vegetation cover which effectively intercepts rainfall, forms a barrier to overland flow of runoff water and binds the soil. Any reduction in cover will result in increased runoff and erosion. As has been seen above, wetland vegetation is adapted to trap sediments in runoff water, and infilling by sediments and the subsequent colonisation by vegetation are natural processes of the system. If excessive erosion is allowed to occur in the catchment, however, the wetland vegetation will be unable to cope with the abnormally high sediment loads, and the system as a whole could be smothered.
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Stream flow rates. Canalization of streams and rivers above wetlands causes an increase in the rate of flow of the water. This is disadvantageous to the wetland as increased stream flow rates are usually associated with scouring of the stream bed, and thus increased erosion, or drainage of the wetland.
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Change in water regime. Damming the stream, or extracting water for irrigation above the wetland will reduce the water input to the wetland during periods of low flow. Depending on the extent to which this occurs and the wetland's minimum water requirements, this may have a severe impact on the system.
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Afforestation. Replacing the natural veld of the catchment with exotic evergreen, plantations can affect wetlands in a number of ways. Afforestation will affect the hydrology of the area by reducing streamflow. Aerial application of herbicides to prepare areas of veld prior to planting timber species is fairly widely practised. Unfortunately, the spraying programme often does not exclude wetlands (in spite of the fact that it is illegal to kill wetland vegetation), and the wetland vegetation is adversely affected. Legislation prevents the planting of trees within 20 m of either side of the stream. A wider belt should, however, be maintained. In this regard the KwaZulu-Natal Nature Conservation Services have prepared guidelines for the afforestation of wetland catchments, and recommend that trees are not planted within a 100 m belt around the wetland, and that at least one slope adjacent to the wetland is not planted if wattled crane are present in the wetland.
In summary therefore, catchment management should be aimed at providing a sustained supply of water with low sediment levels. Veld in the catchments should be stocked according to recommendations (never over-stocked), and should be burnt only when necessary. Afforestation of catchments should be avoided as far as possible.
Management within the wetland
The management which is applied to the wetland itself
is of extreme importance to the functioning of the system. A number of factors
are important in this respect.
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Drainage. Due to their steep terrain and importance in erosion protection, wetlands in valleys and in the headwaters of rivers should be maintained in their natural state, and under no circumstances be drained or disturbed in any manner whatsoever. In most cases, however, the production of crops or improved pastures on drained wetlands which are located on the plains are sustainable practices, provided the soils are not of a high erodibility, the correct soil conservation measures are applied, and they occur in the appropriate wetland type. For example, wetlands with an organic topsoil which remains saturated for most of the year are totally inappropriate for drainage. It is the responsibility of the Wetlands Advisory Committee (refer to the section on "Wetland Legislation" above) to decide whether a wetland is appropriate for development. Even though crop and improved pasture production might appear to be sustainable, the fact that they require the water table to be lowered (through drainage) greatly decreases the capacity of the wetland to perform its various functions. For example there could be a reduction in the ability of the wetland to purify water since drainage channels decrease the extent to which water is held within the system, and this, together with the replacement of the specialized vegetation, greatly reduces the purification process. Additional values such as water storage and the provision of habitat for wetland-dependent fauna and flora are also lost. Wetlands which have been disturbed through the construction of drains, or by ridge-and-furrow ploughing, can be rehabilitated to a certain extent by blocking the drains and allowing the natural hygrophilous vegetation to re-establish. If the wetland is stable and providing sustained yields, however, rehabilitation should not normally be necessary.
Another form of wetland drainage is associated with gulley erosion. Gulley erosion leads to a significant drop in the water table which has similar consequences as intentional drainage. Where gulley erosion has occurred in a wetland, every effort should be made by the land owner to rehabilitate the wetland through such measures as the exclusion of stock and the construction of mechanical structures. The Department of Agriculture provides a free conservation engineering service to farmers which is allied to a financial aid scheme. Land owners with this type of problem are urged to contact their local extension office for details.
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Damming. The construction of dam walls at wetland discharge sites leads to flooding of wetlands, and is a relatively common practice. Although dams are able to perform certain of the functions previously carried out by wetlands (e.g. sediment trapping and water storage), not all functions are as efficient as in an undisturbed state. Dams may appear to be beneficial to the wildlife of the area, as they attract wildfowl such as ducks and geese, but these are usually generalist species whose habitat is not threatened. On the contrary, dams result in the loss of habitat for specialized and often rare species which are dependent on wetlands.
The benefit of the stored water must therefore be carefully weighed against the loss of specialized habitat and the other values of wetlands described previously before embarking on the expensive measure of dam construction. However, in a country as poorly endowed with water as South Africa, there is a strong case for conserving as much water as possible to ensure continued growth and development in the country.
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Burning. Wetlands are often burnt to improve grazing and to decrease the fire hazard in afforested areas. If the vegetation in a wetland is burnt too frequently, it results in an increase in the extent of soil exposure and the removal of organic matter which would otherwise be incorporated into the soil. This in turn decreases the water retention capacity of the wetland, as well as detracting from other wetland values associated with high levels of organic matter. Less frequent burning of the vegetation is, however, considered a natural part of the system, as, historically, veld fires would normally spread into dry wetland vegetation from time to time.
Controlled burns in wetlands should be implemented every two years and, if possible, half the area should be burnt each time. If a number of wetlands are present on a farm, these can be burnt individually in successive years. Both these practices would create a mosaic of wetland vegetation in different stages of post-burn recovery, and thus valuable cover would be provided for wetland-dependent animals. Fires should be ‘cool’ (i.e. burnt under conditions where the fuel is relatively moist, such as at night or early in the morning, and under windless conditions). Burning during autumn or early winter will also result in a ‘cool’ fire or a patchy burn which provides refuge sites for animals. Care should be taken that burns do not affect winter-breeding birds (e.g. wattled and crowned cranes). Fire is one of the most important causes of chick mortality in wattled cranes, as the chicks are usually unable to fly when the wetland is burnt in winter or early spring. These mortalities may be avoided by burning the wetland in blocks, changing the timing of the burn to suit the birds, or to temporarily remove the chicks while the wetland is burnt.
If a wetland has soils with high proportions of organic matter (e.g. Champagne form), it should not be burnt when the water table is relatively low (i.e. at the end of the dry season, and particularly during very dry years). This is because the risk of igniting the soil is high, and will have obvious detrimental effects on the system.
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Grazing. Certain wetlands which consist pre-dominantly of sedge meadows and hygrophilous grassland can be grazed on a sustainable basis. Excessive disturbance of wetlands by domestic stock can affect the system through the removal of the vegetation and physical disturbance of the soil through trampling. This type of mismanagement can often lead to gulley erosion, particularly in the drier Bioresource Groups. Where the grazing of natural wetland vegetation is possible, stocking rates should be conservative, and adequate rest periods should follow grazing to allow the vegetation to recover and maintain good vigour.
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Alien vegetation. Introduced plants are usually able to establish themselves easily and outcompete indigenous vegetation. Plants which are a threat to the aquatic zones of wetlands include Myriophyllum spp. (parrot's feather) and water hyacinth, while species such as Sesbania punicea (sesbania, rattlepod) and Rubus cuneifolius (American bramble) tend to invade the sedge meadow and hygrophilous grassland zones. The ability of alien plants to invade the latter habitats is substantially higher in cases where the water table has been lowered through some form of mismanagement. This is because the competitive ability of the natural wetland vegetation is lowered following a change in the water regime. Afforestation of wetlands with species which are able to tolerate wet conditions (e.g. poplars) will also affect the water regime in the system, and this practice is therefore not recommended.
Alien plants should be removed from wetlands before they predominate in the system. Continual checks should be made following a clearing operation to detect reinvasion, and control measures should be applied timeously.
Management below the wetland
The major threat to wetlands below their discharge
points is caused by erosion creeping back up the river, and eventually draining
the wetland. This is particularly common in the lower rainfall areas (less than
700 mm) where highly erodible expansive clays and duplex soils commonly
occur. The most important aspect involved in the management of this part of the
system is the conservation of river and stream banks. Stream banks should be
disturbed as little as possible, and the natural vegetation occurring there
should be maintained.
LITERATURE CONSULTED
BATCHELOR, G., 1984. Current land-use and conservation problems (of the Steenkampsberg). Fauna & Flora 41 : 28 - 32.
BEGG, G. 1987. The wetlands of Natal (Part 1) An overview of their extent, role and present status. Pietermaritzburg : Natal Town & Regional Planning Commission.
GAIGHER, C., undated. Wetlands. Cape Conservation Series 6, Cape Department of Nature and Environmental Conservation.
JOHNSON, D.N., 1988. Vlei management (2nd edition). Wildlife Management. Technical guides for farmers No. 7, Natal Parks Board.
SCOTNEY, D.M. & WILBY, A.F., 1983. Wetlands and agriculture. Journal of the Limnological Society of Southern Africa 9(2): 134 -140.