**4. The role of high magnitude events**

#### **4.1 Extreme events in Africa**

Climatic and hydrologic regimes on the African continent are highly variable in terms of both space and time. Rivers show the highest average extreme flood index of all continents, whilst the runoff ratios are lowest (McMahon et al., 1992). The temporal and spatial variability of rainfalls and rainstorms, as well as the repeated occurrence of periods of extreme droughts in semi-arid tropical and subtropical areas, indicate that extreme events play an important role in the African morphodynamic system. Relatively little is known about the relative work done by rare events of high magnitude when these events are compared with more frequent events with a low magnitude (Gallart, 1995). Although studies indicate that the impact of erosion increases with increasing amounts of rainfall and rainfall intensity, such relationships are not without ambiguity, as events of similar magnitude may have different effects, whilst events with a higher frequency and lowermagnitude are capable of inducing similar effects (Gallart, 1995). While the incidence of drought and rainfall events is determined by the present-day climatic system, human activities may change the magnitude of the impact by changing the vegetation cover, the hydrologic regimes and characteristics of the surface materials and forms. Therefore, an increase in the impact of smaller events with shorter recurrence intervals and lower

Late Quaternary Environmental Changes and Human Interference in Africa 265

frequencies. Although there is a close association between high levels of discharge and warm and wet periods, the most extreme discharge events occurred during a warm interval of the "Little Ice Age", in the period 1500 to 1675 AD (Zawada, 2000). The maximum flood discharge during this brief period exceeded any historically gauged floods by a factor of three. According to Zawada (2000), the high floods cannot be attributed to the increase in rainfall, as during earlier, more humid periods the flood discharge was significantly lower, though these paleoflood discharges exceed all documented floods since the end of the 18th century. Zawada (2000) argues that the sudden onset of warming caused an intense change in the hydrologic regime. Apparently the change affected hillslopes as well as rivers within a time interval that was shorter than the time that is necessary to achieve a full adjustment

Singular events of high magnitude may result in serious damage. Rapp (1976) has documented the effects of a rainstorm in the Mgeta mountains of Tanzania,. The rainfall event achieved an intensity of 100.7mm in less than three hours and triggered more than 1000 shallow landslides in the highly weathered soils. Landsliding affected about 47% of the cultivated land, 46% of the grasslands but less than 1 % of the wooded areas (Rapp, 1976, p.92). The results highlight the link between slope stability, soil properties and changes in the vegetation cover. Trees lower the water table in the soils by transpiration and reduce the amount of rainfall reaching the slope surface as a part of the rainwater is intercepted in the canopy. Both processes counteract soil saturation and delay the development of high pore water pressure. Once deforestation takes place, these positive effects are lost. In combination with the loss of tree roots, this results in a reduced shear strength of the soils, a higher probability of high-pore water pressure and a lower threshold of stability against

The sensitivity to change is a further factor which appears to exert an important influence on the magnitude of events. Most landscapes in Africa have suffered progressive change through time and tend to accumulate the imprints of different environmental conditions. These imprints range from deposits and weathering layers formed during periods with different climatic conditions to hillslope forms and polyphase landscape elements. In the KwaZulu area, Singh et al. (2008) investigated extensive landslide complexes which seem to have been active in the middle and late Holocene. The volume of large individual landslides

apparently reactivated on the larger landslide masses. However, the large landslide complexes appear to be stable. According to Singh et al. (2008), these landslides resulted from a combination of long–term rock-weathering and the location in a seismic active zone. High-intensity rainfall events in 1987 and 1997 in Natal (Southern Africa) indicated a strong association between landsliding and colluvial deposits (Bell and Maud, 2000; Singh et al., 2008). The colluvial deposits in this area are characterised by several non-conformities resulting from differences in the intensity of weathering, the variable thickness, texture and permeability. According to Bell and Maud (2000), landslides on the hillslopes of the Natal group are closely associated with the specific behaviour of the colluvial deposits. The weathered colluvium consists of an upper sandy layer (topsoil) and an illuvial horizon, which lies above a clayey weathered layer. During heavy rainstorms, the silty and clayey layers impede the downward percolation of the water. This promotes the development of high-pore water pressure and of saturated conditions in the upper soil layers. The lateral throughflow in the more permeable layers of the colluvium and weathering layers on the

107 m3. Some smaller, secondary occurrences of slope failure were

of the vegetation cover to the changed conditions.

landsliding (Rapp, 1976).

107 to 2.

ranged from 1.

magnitude is likely. The intensity of the response to these events is a function of the intensity of interference with the ecosystem, coupling of the subsystems and the sensitivity of the subsystems affected. However, changes in environmental conditions frequently bring with them a non-linear behavioural pattern caused by feedbacks (Thomas, 2004). These feedbacks weaken or reinforce the response to changes in different subsystems. In semi-arid areas, a decrease in vegetation cover may reinforce the decline of rainfalls as the degradation of the vegetation decreases the surface roughness and soil moisture. Consequently, the evaporation and transpiration rates decrease, which, in turn, reduces transport of vapour into the atmosphere (Warren, 1999). Further effects are likely to involve changes in the cloudiness, the spatial and temporal distribution and intensity of the rainfalls and the lower inflow of rainwater to the ground water. A decrease of the vegetation cover causes a decrease in the water-retention capacity of the soils, which, in turn, may reduce the threshold of runoff-producing storms (Gallart, 1995). The effects of such changes point towards a higher preparedness of landscape components to react to events of lower magnitude and higher frequency. This appears to bring with it an increase in the impact of events of lower magnitude. The latter is corroborated in studies of sediment yield in Kenya, which indicate that an increase in land use is associated with an increase of the relative work of events of higher magnitude (Dunne, 1979).

With respect to the impact of meteorological events on erosion-processes, the effects of continuous rainfall and short-term high-intensity rainfall events must be distinguished. Long- lasting rainfall events of exceptional magnitude determine the saturation of soils and induce saturation overland flow and liquefaction of the soil layers. High-intensity rainfalls, on the other hand, are capable of inducing Hortonian overland flow causing a rapid increase of runoff. However, the impact of such events depends strongly on the antecedent state of the ground. The role of heavy downpours increases towards the semi-arid and arid tropical areas, where daily rainfall events may exceed the mean annual rainfall by more than 40% (Starkel, 1976). Rainfall intensities ranging from 250mm to more than 400mm have been reported from Mauritania and Tunisia, and daily maximum rainfalls exceeding the annual rainfall by 50mm appear to occur several times within a decade ((Mensching et al., 1970; Starkel, 1976). Such rainstorm events are often accompanied by high discharges and floods (Starkel, 1976). Continuous rainfall events are associated with the adduction of humid air masses, which often occurs in tropical, tropical-monsoonal areas or in areas where air masses are impeded by mountains. Tropical cyclones such as the Mauritius cyclone in the Mozambique channel are also associated with high rainfall events. According to Weischet and Endlicher (2000) about 520 cyclones have been registered in 70 years, and most cyclones deposit large volumes of rainfall along the coast. An extreme event accompanied the cyclone Donoina, which occurred in the year 1984. This cyclone crossed southern Africa, and rainfall intensities achieved about 900mm in a few days. This resulted in severe flooding and intense erosion in Mozambique, Swaziland and South Africa (Goudie, 1999).

#### **4.2 Extreme events and complex response**

The response to extreme events depends not only on the magnitude of the event. Studies on flood frequencies at the Orange River in South Africa indicate that the rate of change and antecedent environmental conditions play an important role. During the last 5500 years, the lower Orange River has experienced marked changes in terms of its hydrologic regime. Zawada (2000) was able to distinguish four periods with different flood magnitudes and

magnitude is likely. The intensity of the response to these events is a function of the intensity of interference with the ecosystem, coupling of the subsystems and the sensitivity of the subsystems affected. However, changes in environmental conditions frequently bring with them a non-linear behavioural pattern caused by feedbacks (Thomas, 2004). These feedbacks weaken or reinforce the response to changes in different subsystems. In semi-arid areas, a decrease in vegetation cover may reinforce the decline of rainfalls as the degradation of the vegetation decreases the surface roughness and soil moisture. Consequently, the evaporation and transpiration rates decrease, which, in turn, reduces transport of vapour into the atmosphere (Warren, 1999). Further effects are likely to involve changes in the cloudiness, the spatial and temporal distribution and intensity of the rainfalls and the lower inflow of rainwater to the ground water. A decrease of the vegetation cover causes a decrease in the water-retention capacity of the soils, which, in turn, may reduce the threshold of runoff-producing storms (Gallart, 1995). The effects of such changes point towards a higher preparedness of landscape components to react to events of lower magnitude and higher frequency. This appears to bring with it an increase in the impact of events of lower magnitude. The latter is corroborated in studies of sediment yield in Kenya, which indicate that an increase in land use is associated with an increase of the relative work

With respect to the impact of meteorological events on erosion-processes, the effects of continuous rainfall and short-term high-intensity rainfall events must be distinguished. Long- lasting rainfall events of exceptional magnitude determine the saturation of soils and induce saturation overland flow and liquefaction of the soil layers. High-intensity rainfalls, on the other hand, are capable of inducing Hortonian overland flow causing a rapid increase of runoff. However, the impact of such events depends strongly on the antecedent state of the ground. The role of heavy downpours increases towards the semi-arid and arid tropical areas, where daily rainfall events may exceed the mean annual rainfall by more than 40% (Starkel, 1976). Rainfall intensities ranging from 250mm to more than 400mm have been reported from Mauritania and Tunisia, and daily maximum rainfalls exceeding the annual rainfall by 50mm appear to occur several times within a decade ((Mensching et al., 1970; Starkel, 1976). Such rainstorm events are often accompanied by high discharges and floods (Starkel, 1976). Continuous rainfall events are associated with the adduction of humid air masses, which often occurs in tropical, tropical-monsoonal areas or in areas where air masses are impeded by mountains. Tropical cyclones such as the Mauritius cyclone in the Mozambique channel are also associated with high rainfall events. According to Weischet and Endlicher (2000) about 520 cyclones have been registered in 70 years, and most cyclones deposit large volumes of rainfall along the coast. An extreme event accompanied the cyclone Donoina, which occurred in the year 1984. This cyclone crossed southern Africa, and rainfall intensities achieved about 900mm in a few days. This resulted in severe flooding and intense

The response to extreme events depends not only on the magnitude of the event. Studies on flood frequencies at the Orange River in South Africa indicate that the rate of change and antecedent environmental conditions play an important role. During the last 5500 years, the lower Orange River has experienced marked changes in terms of its hydrologic regime. Zawada (2000) was able to distinguish four periods with different flood magnitudes and

erosion in Mozambique, Swaziland and South Africa (Goudie, 1999).

**4.2 Extreme events and complex response** 

of events of higher magnitude (Dunne, 1979).

frequencies. Although there is a close association between high levels of discharge and warm and wet periods, the most extreme discharge events occurred during a warm interval of the "Little Ice Age", in the period 1500 to 1675 AD (Zawada, 2000). The maximum flood discharge during this brief period exceeded any historically gauged floods by a factor of three. According to Zawada (2000), the high floods cannot be attributed to the increase in rainfall, as during earlier, more humid periods the flood discharge was significantly lower, though these paleoflood discharges exceed all documented floods since the end of the 18th century. Zawada (2000) argues that the sudden onset of warming caused an intense change in the hydrologic regime. Apparently the change affected hillslopes as well as rivers within a time interval that was shorter than the time that is necessary to achieve a full adjustment of the vegetation cover to the changed conditions.

Singular events of high magnitude may result in serious damage. Rapp (1976) has documented the effects of a rainstorm in the Mgeta mountains of Tanzania,. The rainfall event achieved an intensity of 100.7mm in less than three hours and triggered more than 1000 shallow landslides in the highly weathered soils. Landsliding affected about 47% of the cultivated land, 46% of the grasslands but less than 1 % of the wooded areas (Rapp, 1976, p.92). The results highlight the link between slope stability, soil properties and changes in the vegetation cover. Trees lower the water table in the soils by transpiration and reduce the amount of rainfall reaching the slope surface as a part of the rainwater is intercepted in the canopy. Both processes counteract soil saturation and delay the development of high pore water pressure. Once deforestation takes place, these positive effects are lost. In combination with the loss of tree roots, this results in a reduced shear strength of the soils, a higher probability of high-pore water pressure and a lower threshold of stability against landsliding (Rapp, 1976).

The sensitivity to change is a further factor which appears to exert an important influence on the magnitude of events. Most landscapes in Africa have suffered progressive change through time and tend to accumulate the imprints of different environmental conditions. These imprints range from deposits and weathering layers formed during periods with different climatic conditions to hillslope forms and polyphase landscape elements. In the KwaZulu area, Singh et al. (2008) investigated extensive landslide complexes which seem to have been active in the middle and late Holocene. The volume of large individual landslides ranged from 1. 107 to 2. 107 m3. Some smaller, secondary occurrences of slope failure were apparently reactivated on the larger landslide masses. However, the large landslide complexes appear to be stable. According to Singh et al. (2008), these landslides resulted from a combination of long–term rock-weathering and the location in a seismic active zone.

High-intensity rainfall events in 1987 and 1997 in Natal (Southern Africa) indicated a strong association between landsliding and colluvial deposits (Bell and Maud, 2000; Singh et al., 2008). The colluvial deposits in this area are characterised by several non-conformities resulting from differences in the intensity of weathering, the variable thickness, texture and permeability. According to Bell and Maud (2000), landslides on the hillslopes of the Natal group are closely associated with the specific behaviour of the colluvial deposits. The weathered colluvium consists of an upper sandy layer (topsoil) and an illuvial horizon, which lies above a clayey weathered layer. During heavy rainstorms, the silty and clayey layers impede the downward percolation of the water. This promotes the development of high-pore water pressure and of saturated conditions in the upper soil layers. The lateral throughflow in the more permeable layers of the colluvium and weathering layers on the

Late Quaternary Environmental Changes and Human Interference in Africa 267

The on-site effects of fires range from the immediate impact of the selective burning on the bio-diversity and vegetation structure to changes in the physical, chemical and biological components in soils (Schultz, 2005). However the impact varies as a function of the composition of the plant communities, the size and shape of the woody species, the frequency of fires, the heating temperature during burning, the length of the period of time since the last fire, the onset of the fire during the dry season, and the land-use techniques applied (Schultz, 2005). Some cultivation techniques appear to reinforce the danger of further fires by changing the composition and structure of the ground cover as in the case of "slash and burn agriculture" (Goldammer, 1988). Biomass burning affects the reserves and storage of organic matter in the ground cover and in the soils and hence induces changes in soil-nutrient levels. An immediate effect of burning is an increase in K, Ca, Mg and the pH (Singh, 1994). However, the baring of ground promotes erosion by wind and water, and the transport of ashes contributes to the distribution of nutrients over a larger area. The change in the surface colour results in a higher absorption of the solar radiation and in an increase in evaporation. A further effect involves the enrichment of condensed volatile organic substances in the topsoil. This causes the development of a thin layer, which impedes the infiltration of water (Cass et al., 1984). Accordingly, these changes tend to increase the

The off-site effects of fires are changes in the sediment delivery and in the nutrient level of the rivers' draining areas which are affected by fires. Fires tend to increase the content of dust in the atmosphere as they provide aerosols. Aerosols released by smouldering fires exert control on radiation activity as they increase condensation and cloudiness. However, the surplus of condensation nuclei results in small water droplets that remain suspended in the cloud. Consequently rainfalls are less likely. A further consequence of fires is the emission of oxides of carbon and nitrogen as well as of ozone and halogenides (Helas et al., 1992; Andreae et al., 1996). Particularly methyl chloride and methyl bromide emissions appear to support ozone depletion in the upper atmosphere, though the residence-times of these compounds are shorter than 2 years (Andreae et al., 1996). The estimated amounts of methyl-chloride and methyl-bromide emissions range from 1.8 Tg a-1 to 7 Gg a-1 (Andreae et al., 1996). This indicates that these compounds are capable of contributing significantly to

With respect to the extensive areas which are affected by fires, the question arises whether fires increase the level of greenhouse gases in the atmosphere. Andreae (1991) reports that in each year about 75 % of the African savannas are affected by fires. However, during the savanna fires, parts of the biomass are converted into elementary carbon (e.g. black carbon, charcoal). The estimated amount of charcoal formed during a fire appears to account for 5 to 10 % of the total biomass (Goldammer, 1993; Kuhlbusch et al., 1996). This fraction remains in the soil or sediments or is transported by rivers to the ocean, but cannot reenter the atmospheric carbon cycle (Goldammer, 1993). Consequently, this deficit in carbon has to be compensated for by consumption of atmospheric carbon. According to this concept, savannas may become a carbon sink when the processes are balanced through vegetation regeneration. Studies of the annual gas emissions of fires indicate that in the dry savannas the emissions of carbon dioxide, ammonia and nitric oxide do not exceed the amount dictated in the biomass by processes of nitrification and photosynthesis (Schultz, 2000; 2005). However, in the moist savannas, the changes in the vegetation are more pronounced, particularly if there is no regeneration of woody plants and the vegetation structure is destroyed. Accordingly, this may counteract the compensating effects of regeneration.

likelihood of soil erosion by the first rainfall events.

ozone depletion in the upper atmosphere.

upper hillslope-segments increases the flow of ground water to the middle and lower hillslope-segments. This causes the development of excess pore water pressure and artesian conditions on the lower hillslope segments, which, in turn, is accompanied by viscous flow movements and liquefaction of the soils (Bell and Maud, 2000). During the 1987 event most landslides were triggered by an extreme rainstorm episode with an intensity of 576 mm in 72h (Bell and Maud, 2000, p. 1034).

However, antecedent moisture conditions seem to play an important role, as prior to 1987 no records of larger landslide events are documented, while it is likely that rainfall events of similar magnitude have occurred several times in the past. The importance of antecedent moisture conditions and of the properties of the colluvial layers is indicated in the critical precipitation coefficients for slope failure that were calculated by Bell and Maud (2000). According to their investigations, major landslides and landslide episodes will occur when rainfall intensities exceed the mean annual precipitation by 20%. However, most landslides were triggered in the latter months of the rainy season when the colluvium was almost saturated with water. Accordingly, occurrences of slope failure in this area depend on the rainfall intensity and on the antecedent moisture conditions (Bell and Maud, 2000). On the other hand, the investigations emphasise the important role of permeability nonconformities in the colluvium and at the weathered-unweathered rock boundary. This indicates that the occurrence of landslides depends strongly on local conditions and that several factors must be kept in mind in the analysis of landslides. These factors include the association between slope parameters, mechanical parameters of the soils, rocks or sediments, and the presence of palaeolandslides.

#### **5. The role of fires**

Fires play an important role in African environments, and few areas in the African savannas appear to have ever escaped fires. In the savanna areas, fires appear to determine the volume of biomass above the ground and the turnover of herbivores and saprophytes.

Wildland fires can be induced by lightning, volcanism and rockfalls. Most fires in savanna environments are ignited in the dry season by lightning. In west Namibia, lightning ignites about 60% of the savanna fires (Held, 2006). However, in mountainous terrains, rockfalls may be also an important factor. Reports from the Cedar Hills in South Africa indicate that rockfalls contribute to the development of about 25% of the fires (Goldammer, 1993). Since the appearance of humans, the impact of fires on vegetation patterns has progressively increased. The modification of the vegetation in the savannas began in an early epoch, when hunters and gatherers used fire to make hunting easier. Evidence of the early use of fire ranges from sedimentary layers in the Swartkrans Cave in South Africa, with an age of about 1.5 Ma BP (Gowlett et al., 1981) to changes in the vegetation pattern on the Nyika Plateau in Malawi, which seems to indicate the repeated burning of the savanna vegetation at the end of the Pleistocene (Goldammer, 1993).

In more recent times, increasing demand for arable land has resulted in a regular burning of larger savanna areas and in the development of extensive grasslands. Within the moistsavanna-zone, this has caused the development of "derived savannas", which consist of grasslands and are a secondary vegetation formed by fires (Goldammer, 1993; Schultz, 2005). The effects of fires decrease from the moist savannas to the dry savannas, largely as a function of the available biomass.

upper hillslope-segments increases the flow of ground water to the middle and lower hillslope-segments. This causes the development of excess pore water pressure and artesian conditions on the lower hillslope segments, which, in turn, is accompanied by viscous flow movements and liquefaction of the soils (Bell and Maud, 2000). During the 1987 event most landslides were triggered by an extreme rainstorm episode with an intensity of 576 mm in

However, antecedent moisture conditions seem to play an important role, as prior to 1987 no records of larger landslide events are documented, while it is likely that rainfall events of similar magnitude have occurred several times in the past. The importance of antecedent moisture conditions and of the properties of the colluvial layers is indicated in the critical precipitation coefficients for slope failure that were calculated by Bell and Maud (2000). According to their investigations, major landslides and landslide episodes will occur when rainfall intensities exceed the mean annual precipitation by 20%. However, most landslides were triggered in the latter months of the rainy season when the colluvium was almost saturated with water. Accordingly, occurrences of slope failure in this area depend on the rainfall intensity and on the antecedent moisture conditions (Bell and Maud, 2000). On the other hand, the investigations emphasise the important role of permeability nonconformities in the colluvium and at the weathered-unweathered rock boundary. This indicates that the occurrence of landslides depends strongly on local conditions and that several factors must be kept in mind in the analysis of landslides. These factors include the association between slope parameters, mechanical parameters of the soils, rocks or

Fires play an important role in African environments, and few areas in the African savannas appear to have ever escaped fires. In the savanna areas, fires appear to determine the volume of biomass above the ground and the turnover of herbivores and saprophytes. Wildland fires can be induced by lightning, volcanism and rockfalls. Most fires in savanna environments are ignited in the dry season by lightning. In west Namibia, lightning ignites about 60% of the savanna fires (Held, 2006). However, in mountainous terrains, rockfalls may be also an important factor. Reports from the Cedar Hills in South Africa indicate that rockfalls contribute to the development of about 25% of the fires (Goldammer, 1993). Since the appearance of humans, the impact of fires on vegetation patterns has progressively increased. The modification of the vegetation in the savannas began in an early epoch, when hunters and gatherers used fire to make hunting easier. Evidence of the early use of fire ranges from sedimentary layers in the Swartkrans Cave in South Africa, with an age of about 1.5 Ma BP (Gowlett et al., 1981) to changes in the vegetation pattern on the Nyika Plateau in Malawi, which seems to indicate the repeated burning of the savanna vegetation

In more recent times, increasing demand for arable land has resulted in a regular burning of larger savanna areas and in the development of extensive grasslands. Within the moistsavanna-zone, this has caused the development of "derived savannas", which consist of grasslands and are a secondary vegetation formed by fires (Goldammer, 1993; Schultz, 2005). The effects of fires decrease from the moist savannas to the dry savannas, largely as a

72h (Bell and Maud, 2000, p. 1034).

sediments, and the presence of palaeolandslides.

at the end of the Pleistocene (Goldammer, 1993).

function of the available biomass.

**5. The role of fires** 

The on-site effects of fires range from the immediate impact of the selective burning on the bio-diversity and vegetation structure to changes in the physical, chemical and biological components in soils (Schultz, 2005). However the impact varies as a function of the composition of the plant communities, the size and shape of the woody species, the frequency of fires, the heating temperature during burning, the length of the period of time since the last fire, the onset of the fire during the dry season, and the land-use techniques applied (Schultz, 2005). Some cultivation techniques appear to reinforce the danger of further fires by changing the composition and structure of the ground cover as in the case of "slash and burn agriculture" (Goldammer, 1988). Biomass burning affects the reserves and storage of organic matter in the ground cover and in the soils and hence induces changes in soil-nutrient levels. An immediate effect of burning is an increase in K, Ca, Mg and the pH (Singh, 1994). However, the baring of ground promotes erosion by wind and water, and the transport of ashes contributes to the distribution of nutrients over a larger area. The change in the surface colour results in a higher absorption of the solar radiation and in an increase in evaporation. A further effect involves the enrichment of condensed volatile organic substances in the topsoil. This causes the development of a thin layer, which impedes the infiltration of water (Cass et al., 1984). Accordingly, these changes tend to increase the likelihood of soil erosion by the first rainfall events.

The off-site effects of fires are changes in the sediment delivery and in the nutrient level of the rivers' draining areas which are affected by fires. Fires tend to increase the content of dust in the atmosphere as they provide aerosols. Aerosols released by smouldering fires exert control on radiation activity as they increase condensation and cloudiness. However, the surplus of condensation nuclei results in small water droplets that remain suspended in the cloud. Consequently rainfalls are less likely. A further consequence of fires is the emission of oxides of carbon and nitrogen as well as of ozone and halogenides (Helas et al., 1992; Andreae et al., 1996). Particularly methyl chloride and methyl bromide emissions appear to support ozone depletion in the upper atmosphere, though the residence-times of these compounds are shorter than 2 years (Andreae et al., 1996). The estimated amounts of methyl-chloride and methyl-bromide emissions range from 1.8 Tg a-1 to 7 Gg a-1 (Andreae et al., 1996). This indicates that these compounds are capable of contributing significantly to ozone depletion in the upper atmosphere.

With respect to the extensive areas which are affected by fires, the question arises whether fires increase the level of greenhouse gases in the atmosphere. Andreae (1991) reports that in each year about 75 % of the African savannas are affected by fires. However, during the savanna fires, parts of the biomass are converted into elementary carbon (e.g. black carbon, charcoal). The estimated amount of charcoal formed during a fire appears to account for 5 to 10 % of the total biomass (Goldammer, 1993; Kuhlbusch et al., 1996). This fraction remains in the soil or sediments or is transported by rivers to the ocean, but cannot reenter the atmospheric carbon cycle (Goldammer, 1993). Consequently, this deficit in carbon has to be compensated for by consumption of atmospheric carbon. According to this concept, savannas may become a carbon sink when the processes are balanced through vegetation regeneration. Studies of the annual gas emissions of fires indicate that in the dry savannas the emissions of carbon dioxide, ammonia and nitric oxide do not exceed the amount dictated in the biomass by processes of nitrification and photosynthesis (Schultz, 2000; 2005). However, in the moist savannas, the changes in the vegetation are more pronounced, particularly if there is no regeneration of woody plants and the vegetation structure is destroyed. Accordingly, this may counteract the compensating effects of regeneration.

Late Quaternary Environmental Changes and Human Interference in Africa 269

The increase in population in Africa is expected to result in an extension of the area cultivated land, even in steeply sloping mountainous regions. The impact of change in the climate and the intensified land use are likely to cause a reinforcement of degradation processes in the landscapes and may result in a lowering of the carrying capacity of land. However, predictions on future rates of change also depend on socioeconomic processes and political decisions. The devastating impact of desertification in the Sahel was not only a result of drought but was also associated with one of the highest population growth rates in

Andreae, M.O., 1991. Biomass burning. Its History, Use and Distribution and its impact on

Andreae, M.O.; Goldammer, J.G.;Schebeske, G.; 1996. Methyl chloride and methyl bromide

Beckedahl, H. R., 2002. Bodenerosion in Afrika: ein Überblick. Petermanns Geographische

Bell, F.G.; Maud, R.R., 2000. Landslides associated with the colluvial soils overlying the

Bingelli, P., 1989. The Ecology of Maesopsis Invasion and Dynamics of Evergreen Forest of

Biot, Y., 1990. THEPROM: an erosion-productivity model. In. Boardman, J.; Foster, I.;

Bork, H.-R., 2004. Soil erosion during the 20th century. Examples form South Africa, the

Botha, G.A.; Partridge, T.C., 2000. Colluvial Deposits. In. Partridge, T.C.; Maud, R.R. (Eds.).

Braun, J.-J., Ngoupayou, J.R.N., Viers, J., Dupre, B., Bedimo, J.-P.B., Boeglin, J.-L., Robain, H.,

Bryan, R.B.; Jones, J.A.A., 1997. The significance of soil piping processes: inventory and

Calles, B.; Kulander, L., 1996. Likelihood of erosive rains in Lesotho. Z. Geomorph. N.F.,

Netherlands, 6-10 May 1996. Annales Geophysicae 14, Supp. II, C 595. Areola, O., 1999. Soils. In. Adams, W.M.; Goudie, A.S.; Orme, A.R. (Eds.). The Physical

Geography of Africa. Oxford Univ. Press, Oxford, pp. 134-147.

East Usambara Mountains. Tanzania. Gland, 269-300.

Bork, H.-R., 2006. Landschaften der Erde. WGB-Darmstadt, Darmstadt.

Geochimica et Cosmochimica Acta 69 (2), 357-387.

prospect. Geomorphology 20, 209-218.

Suppl. Bd. 106, 149-168.

Environmental Quality and Global Climate. In. Levine, J.S. (Ed.). Global Biomass

emission from vegetation fires. EGS XXI General Assembly, The Hague, The

Natal Group in the greater Durban region of Natal, South Africa. Environmental

East Usambara and their Implications for Forest Conservation and Forestry Practices. In. Hamilton A.C.; Bensted-Schmith, R. (Eds.). Forest Conservation in the

Dearing, J. (Eds.). Soil Erosion on Agricultural Land. Wiley, Chichester, pp. 465-479

Americas, China and Europe. In: Li, Y.; Poesen, J.; Valentin, C. (Eds.). Gully Erosion under Global Change. Sichuan Science and Technology Press, Chengdu, China, pp.

The Cenozoic of Southern Africa. Oxford Monographs on Geology and Geophysics

Nyeck, B., Freydier, R., Nkamdjou, L.S., Rouiller, J., Muller, J.-P., 2003. Present weathering rates in a humid tropical watershed: Nsimi, South Cameroon.

the world.

**7. References** 

Burning. Cambridge, Mass., p.3-21.

Mitteilungen (PGM) 146, 18-23.

Geology 39, 1029-1038.

3-10.

40, pp.88-99.

However, we have a poor understanding of the turnover of carbon in quantitative terms in the savannas due to the complex interaction of weathering, soil formation, vegetation and litter production and different reaction-times. Finally, a full assessment of the climatic impact of biomass-burning depends also on the reliability of the data and on the quality of case studies.
