2.2 Tropical and non-tropical southern Africa

The state of knowledge of fire regimes in African forest and non-forest ecosystems is best developed in tropical and non-tropical southern Africa. Thus, special emphasis is given to this region. The editor is greatly indebted to Brian van Wilgen, Robert Scholes and the University of Witwatersrand Press, Johannesburg, South Africa, for permission to partially reproduce the book chapter cited (Van Wilgen and Scholes 1997). Country reports of Mozambique, Namibia and South Africa are provided in full.

Determinants of fire in Southern Hemisphere Africa

The occurrence of fire in the different vegetation types of Southern Africa is dependent on several factors. The vegetation providing the fuel available for fires is a product of both the soil and the prevailing climate. Climatic conditions conducive to fire must also be present, as must a source of ignition. Herbivory competes with fires for the available grass fuels, and may prevent fires in some areas as fuels are eaten before they can burn. Finally, human intervention through active management can materially affect the occurrence of fires.

Climatic conditions as a determinant of fire

The climates of southern hemisphere Africa range from continuously very wet in the equatorial regions of the western side of the continent and at high altitudes on some of the mountains of the eastern side, to very arid in the southwest and northeast. Associated with this aridity gradient is increasing seasonality of rainfall. The rainfall over most of the range is concentrated into one or two rainy seasons (the latter in the monsoonal climates of East Africa). The rainy season is generally in the summer. In the extreme southwest there is a winter rainfall region dominated by evergreen, sclerophyllous shrublands (fynbos). Although the hot, dry summers of this area provide ideal fire conditions, the slower rates of fuel accumulation and the coarser nature of the fuels results in much longer intervals between fires (between ten to twenty years).

The largest portion of southern hemisphere Africa therefore experiences a climate in which there is a prolonged dry period every year during winter. The atmosphere is warm and dry due to the high radiation, and fine fuels readily dry out sufficiently to ignite. These climates coincide with the distribution of savannahs (in the broad sense), shrublands and grasslands. We will argue that this coincidence is not accidental: the existence of this fire climate is one of the main factors determining the distribution of these vegetation types.

Climate also has an indirect influence on fire frequency and intensity through its control on the rate of fuel accumulation. The most important factor controlling aboveground primary production is the water balance of the site, followed by the fertility of the soil. At the regional scale, plant water availability is positively related to mean annual rainfall, whereas site fertility is negatively related to rainfall. The arid areas of southern hemisphere Africa burn infrequently because there is rarely enough fuel present to carry a fire across the landscape (a minimum of about 0.5-1 t/ha is needed). Several years of fuel accumulation or an exceptionally wet growing season, especially in the presence of herbivory, are required to generate this minimum fuel load in arid areas.

Most of the rainfall in southern Africa occurs as convective storms associated with lightning. In areas where there are few human ignition sources, lightning may be the dominant form of ignition, but currently it accounts for only 1-10 percent of the observed ignitions. Lightning is rare during the dry months. It has generally been inferred that the pre-human fire season was therefore concentrated in the early part of the wet season, when lightning occurs but the fuel is still dry enough to ignite.

The meteorological conditions at the time of the fire have a profound influence on fire behaviour. Fire managers make skilful use of the knowledge that high humidity, low air temperature and low wind speed reduce the rate of spread of fires. High fire risk is associated with dry, hot and windy conditions, the coincidence of which is related to the regional macroclimate and very intense fires can result. For example, when air masses spill off the interior plateau they create hot, dry winds (known in South Africa as bergwinds). These bergwind-driven fires are responsible for the confinement of forest patches in the southern Cape to fire-protected locations such as the leeward side of steep slopes and rivers.

Soils and fuel accumulation

The second major factor controlling the rate of fuel accumulation in southern hemisphere Africa is the soil type. In arid environments, an equal amount of rain falling on a clayey soil is less available to plants than if it had fallen on a sandy soil. The reverse can be true in wet environments. More grass grows per unit plant-available water on fertile than infertile soils, but the grass is more nutritious and thus more likely to be eaten by herbivores. Infertile soils are less productive, but the fuel accumulates because it is unpalatable.

Ground fires are rare in southern Africa, with the exception of some organic (peat) soils associated with montane wetlands and low-nutrient tropical wetlands such as the Okavango swamps. When the dynamics of channel formation in the swamps causes the peat to dry out, these soils can ignite and smoulder, sometimes for months on end. In the process, a thick layer of peat is consumed, lowering the land surface and preparing the way for future reflooding.

Herbivory and fuel depletion

Fires and herbivores can be thought of as competitors for grass, since only the grass that is not eaten is available as fuel. To ignore herbivory could therefore lead to serious overestimates of the rate of grass fuel accumulation. Grasses growing in moist, infertile soils are fibrous and have low nutrient content except when they are young (for instance, after a fire). There is insufficient protein in the dry grass to sustain the digestive symbionts of large mammalian herbivores, and little of this grass is eaten. Decomposition rates of the dry, standing grass are low, so fuel accumulates and is burned. If it remains unburned it reduces the primary productivity of the grass sward in subsequent years by shading the new growth, and reduces the secondary productivity by restricting access by herbivores to the more nutritious new leaves. Pastoralists burn off the old growth for these reasons.

On fertile soils (and infertile soils in arid regions, since the nutrient characteristics of a grass depends on its growth rate) the grass is digestible even when dead, and so it is usually eaten rather than burned. Fires only occur in years when abnormally high rainfall causes more grass to grow than the herbivores can consume, or where herbivory is abnormally low.

Effects of management on fire regimes

Traditional practice

Historically, both African pastoralists and hunter-gatherers set fires, especially during the dry season, to create patches of green grass, to supplement the nutrition of domestic stock in the case of pastoralists or to attract game in the case of hunters. Burning begins early in the dry season and continues until the spring rains. The vegetation pattern in landscapes ignited this way is distinctive: it consists of a patchwork of irregularly-shaped overlapping scars of different ages, some large but most quite small.

The first European settlers in southern Africa copied the fire management practices of the Khoi and Bantu people. Despite early attempts to regulate the use of fire, effective legal control on burning practices only began in the twentieth century. The use of fire was officially discouraged in all of the southern African colonies, so the mean fire frequency may have decreased during the colonial period in some areas. Since the 1950s the regulated, periodic use of fire has been promoted as a tool for grazing management. The fires set for grassland management on commercial ranches in South Africa and Zimbabwe are clustered in the late dry season and first few weeks of the wet season, and the aerial impression is of regularly spaced polygons, each a few hundred hectares in extent.

In the miombo woodlands, especially in Zambia, a traditional ash-fertilisation agriculture called chitimene is practised. In one form, the trees over a radius of about 200 m are pollarded and the branches dragged into a central area of about 50 m radius, where they are dried and burned. Crops are planted in the ash bed. After a few years, the patch is abandoned for another.

Policy and laws

Most of the countries of the region have laws regulating vegetation burning. The vigour with which these laws are applied varies greatly among countries and between districts within countries. In general the laws are anti-burning. Examples of fire policies and laws in southern Africa are given in the country reports of South Africa and Namibia.

Ecological considerations

There is much evidence that fire has an important and usually beneficial role in maintaining the diversity, structure and function of southern hemisphere African ecosystems. In nutrient-poor ecosystems the primary production in the seasons following a fire is slightly enhanced relative to unburned areas due to the release of nutrients from the ash. The enhanced nutrients in the new growth are attractive to grazers, which in some environments would not be able to survive without the patchwork of burns. Fire is also one of the key factors in maintaining the competitive balance between trees and grasses in savannahs. Many plant species have growth and reproduction attributes directly linked to the fire cycle, for instance germination, flowering or seed dispersal only after burning. Similar examples can be found for birds and animals, all of which suggest that fire has a long evolutionary history in Africa.

In general, ecologists would like to see the maintenance of a historical fire regime, including its natural variability in time and space. The details of the historical fire regime are unknown. In most areas it probably peaked in the late dry season, but was unlikely to have been as limited to that period as it is now. The widespread increase in woody biomass in savannahs under commercial management suggests that the current fire frequency and intensity are lower than the historical levels, favouring trees over grass. In intensively managed landscapes, such as parts of South Africa and Zimbabwe, the size of the areas burned is typically a few hundred hectares (the size of a grazing paddock or the area which can be conveniently burned in a morning). Historically the patch sizes would have been much more variable and not repeatedly confined to the same areas.

Major vegetation types and their fuel properties

The vegetation types of southern hemisphere Africa have traditionally been mapped and described on the basis of the species that they contain (a floristic classification) rather than on the basis of their physical structure. The floristic approach results in a large number of classes with no clear association to their fire-related properties. The variation in fire regime within each floristic class can be largely due to structural variations. National-scale maps usually contain more structural information than the continental-scale maps. We therefore used the best-available national vegetation maps for southern Africa to define functional vegetation type units on the basis of their fire ecology (Figure 2-2). The fuel characteristics of the classes are summarised in Table 2-1 and some additional notes are provided below.

Ecologists usually distinguish between total fuel (the maximum amount of vegetation that could burn under the most extreme conditions) and available fuel (the amount of vegetation that actually burns under a given set of conditions). The aboveground biomass of vegetation represents total fuel in some cases, but usually certain categories (for example, the live trunks of large trees) are not included, as they do not burn even under extreme conditions. In order to assess the amount of aboveground biomass consumed in fires, we provide estimates of consumption from the literature where these are available, or from local experience. The most important fuel properties of these vegetation types are summarised in the sections below.

Figure 2-2 Map of southern hemisphere Africa showing the distribution of the most important vegetation types. See Table 2-1 for a description of the vegetation types

Evergreen forest

Most forests (stands of trees ranging in height from 10 to 50 m or more with a continuous multi-layered canopy) in Africa are evergreen or semi-evergreen. These forests have a complex structure, with the many co-dominant species reaching different sizes when mature. The most widespread type is the Guineo-Congolian rainforests of Zaire and neighbouring countries. Swamp and riparian forests are widespread in the Zambezian region but of limited areal extent, while in the eastern half of Africa, afro-montane and coastal forests have a localised and fragmented distribution.

The fuel properties of evergreen forests are poorly studied. In afro-montane forests in the Western Cape province of South Africa, dead fuels were restricted to densely packed litter layers and the canopies of trees were sparse and consisted of live material. Fires in these vegetation types are rare and restricted to occasional extreme fire weather conditions.

Dry/deciduous forests

These forests have a canopy that is near-continuous and multi-layered, dominated by deciduous trees. They occur in areas where there is a two to three month dry period in the year. Areas where the dry season is three to six months in duration support woodlands and savannahs, so the dry forests can be considered as transitional between evergreen forests and woodlands. They also occupy the ridge tops in some areas where the valleys are filled with evergreen forests due to the ridgetop soils being shallower and better drained and therefore having a lower water storage capacity. Dry forests would be more prone to fire than evergreen forests and would therefore burn periodically because of the accumulation of fuel in the form of dry leaf and twig litter.

Plantations

Plantations of non-indigenous trees, predominantly species of Pinus and Eucalyptus, supply most of the timber needs of countries outside the tropical forest region. The plantations are actively defended against fire, with varying degrees of success. In South Africa, for instance, an average of 6 430 ha (0.5 percent of the planted area of 1.3 million hectares) per annum was burned during 1986-1993. In some plantations, slash resulting from pruning or felling operations is intentionally burned (see initial remarks and country report of South Africa).

Forest/savannah mosaics

Forest/savannah mosaics can be edaphic (determined by soil conditions) or anthropogenic (created by human land use). In the former case, the forests usually occupy either the moist places in the landscape (for example, the valley floors and the southern slopes of steep ridges) or the places that are topographically protected from fire, such as the leeward side of rocky outcrops. Anthropogenic mosaics are created in areas that were formerly forested, particularly through the process of slash-and-burn agriculture. Once the forest cover has been fragmented it is prevented from regeneration by frequent fires or by continuous harvesting of woody regrowth.

Moist/infertile savannahs

Savannahs are vegetation types in which the biomass is shared by trees and grass. Savannahs are the dominant vegetation of the seasonally wet tropics, and are the location of the greatest area of vegetation burned in the world. There are conflicting views regarding the necessity of fire in the creation and maintenance of savannahs. Many studies have shown that the exclusion of fire from savannahs leads to an increase in woody biomass, and this has been interpreted as evidence that fire is essential for the persistence of savannahs. However, in dry environments fire exclusion does not eventually lead to a forest or even woodland, but rather to a denser form of savannah. Clearly competition for water, nutrients and light is also important in determining savannah structure.

The savannahs of Southern Africa can be classified into two broad groups, with a minority of outliers and intermediate cases. The two groups are associated with the broad soil fertility classes described above as well as with the rainfall gradient. The moist/infertile savannahs are identifiable by the predominance of broad-leafed, thornless trees in the families Ceasalpinaceae and Combretaceae. The most extensive example of this vegetation is the miombo woodlands, dominated by Brachystegia and Julbernardia species, that occupy a broad belt from Angola to Tanzania. A very large fraction of all the biomass burnt in southern Africa occurs in this type. Many miombo ecosystems burn every one to three years, although the area-weighted average is closer to three years. As in all savannahs, the grass fuel load varies inversely with the tree cover. Grass growth is suppressed except where the tree canopy has been opened up, for instance, by ash-fertilisation agriculture. Fallen leaves and twigs can make up a large part of the fuel, especially where the tree cover is high.

Arid/fertile savannahs

Thorny, fine-leafed trees of the families Mimosaceae (predominantly Acacia species) and Burseraceae (Commiphora spp.) dominate the arid/fertile savannahs. These savannahs are more widespread in the countries south of the Zambezi. The frequency with which they burn varies greatly, depending on rainfall, herbivory and local management practice, but it is seldom more often than once every three years, and probably averages around once every eight to ten years. The fuel is predominantly fine dry grass. Dead tree trunks and dung are a minor part of the available fuel load, but their contribution to emissions is disproportionate to their mass.

The most widespread non-conforming savannah types belong to the Colophospermum mopane woodlands. Mopane, a broad-leafed tree belonging to the Ceasalpinaceae, almost completely dominates the tree layer in a broad belt from Northern Namibia to Mozambique. It occurs on base-rich, arid sites, often where the soil profile includes a compacted layer within half a metre of the surface. Mopane woodlands should be treated as arid/fertile savannahs, despite their broad leaves and absence of thorns. The resinous live and dead mopane leaves provide a large part of the fuel, especially in the low-growing scrub form of this vegetation.

Other ecosystems

Fire characteristics of ecosystems other than forest, other wooded lands and arid and moist savannahs include:

• Arid shrublands (e.g. karoo in South Africa);

• Succulent arid shrublands;

• Deserts;

• Fynbos (sclerophyllous shrubland vegetation of the western and eastern Cape Provinces in South Africa);

• Infertile grasslands;

• Fertile grasslands;

• Wetlands (e.g., the Okavango swamps in Botswana and Lake Bangweulu in Northern Zambia);

• Crop agriculture.

Conclusions

The climate and soils of southern hemisphere Africa are conducive to vegetation types that support fires of moderate intensity. The bulk of these fires occur in savannahs between the latitudes of 5° and 20° S, with other vegetation types, such as grasslands, fynbos and wetlands supporting the balance of fires in the subcontinent. Moist evergreen forests and arid areas that account for 18 percent of the land surface of southern hemisphere Africa are less important in terms of fire.

There can be little doubt that fires are a permanent, inevitable and necessary process that drives the ecology of the savannah, grassland and fynbos areas in southern hemisphere Africa. Other areas (such as fertile areas) may escape fires for longer periods as fuel loads are removed through herbivory, but even these areas burn from time to time. All plants in fire-prone areas have evolved adaptations to cope with fire, and many are even dependent on fire to the degree that they can become locally extinct in areas protected from fire.

The colonisation of Africa by European peoples, many of whom brought an abhorrence of fire with them, was marked by attempts to prevent fires. Such attempts were made for various reasons – to protect forest resources, to prevent the putative destruction of grazing, to protect infrastructure or crops, or in often misguided attempts at the conservation of indigenous plants. As an understanding of the role of fire in the ecology of African vegetation developed, these strategies were replaced by others that recognised and utilised fire as an integral process in the ecology and management of vegetation.

The importance and inevitability of fires in southern hemisphere Africa needs to be recognised in the development of global policies for the management of the atmosphere. While fires in Africa are no doubt an important contributing factor to global emissions, a factor that needs to be understood and quantified, they are not necessarily in the same category as the felling and burning of non-fire-prone tropical forests, or fossil fuels. Calls for the elimination of fire are neither feasible nor even desirable.

References

Scholes, R. J., Kendall, J. & Justice, C.O. 1996. The quantity of biomass burned in southern

Africa. J. of Geophysical Research 101 (D19), 23 667-23 676.

Van Wilgen, B. & Scholes, R.J. 1997. The vegetation and fire regimes of Southern-

Hemisphere Africa. In Van Wilgen, B., Andreae, M.O., Goldammer, J.G. & Lindesay, J. eds., Fire in Southern African savannas. Ecological and atmospheric perspectives p. 27-46. Johannesburg, South Africa, The University of Witwatersrand Press, 256 pp.

Table 2-1 Fuel properties of major vegetation types in southern hemisphere Africa.

Table 2-1. Cont.

Table 2-1. Cont.

Table 2-2 Dominant fire regimes of the major vegetation types in southern hemisphere Africa.

Table 2-2. Cont.

Table 2-2. Cont.

Table 2-3 Estimates of the quantity of biomass burned in vegetation fires in southern Africa.¹

¹ The values are for an average year (nominally 1989) and exclude biomass burned during land clearing (for instance, tropical deforestation) and biomass fuels burned for domestic or industrial energy. The fuel loads are simulated using an empirical plant production, herbivory and decomposition model, driven by rainfall and calibrated for each vegetation type. The fraction of the area burned is determined using a calibrated fire detection algorithm applied to daily NOAA satellite data. The biomass burned column is not simply the product of the preceding columns, since it is an annualised sum, while the fuel load is only reported for a single month. The combustion completeness, not reported here, also varies month by month. Tg = teragrams; 1 teragram = 10² grams. Source: Scholes et al. (1996).