**3. What are we talking about?**

A vegetation map of northern and western Africa was established in order to explain clearly the large vegetation types and their repartition. It is based on direct observation during several expeditions (see the small included map) and on published vegetation maps [27]. It deals with the physiognomic units such as forest, shrub-land or grass-land and gives the main floristic components. There is no differentiation between natural formation, near to nature formation or cultural units; however, their dynamics are shortly discussed. Here, we will concentrate on the South-Mediterranean steppe, on the Sahara and on the Sahel to give a background to the discussion about limits, their dynamics and their regeneration potential. Block diagrams and designs will support it. Thus, we will try to avoid the various confusions on terms such as 'steppe or prairie'. The map is to document visible units and their limits and to work as a modern model when reconstructing the past. The question of limits might be regarded as an academic one. However, it is an assessment of resource areas-mainly of pasture.

### **3.1 The southernmost formation of the Mediterranean realm is the 'steppe'**

The term 'steppe' is freely used in literature – comparable to the term 'savanna'. For both the statement of Cole [28] is still valid: 'Most discussed and least

*Plant Communities and Their Environment*

demography.

potential

**1. Introduction**

were initiated [1–13].

Wall' too [17, 18].

rebellion of the 1990s, put a sudden end on it. The general insecurity of the last decades caused by civil war and/or various terrrorist groups led to a re-evaluation of a great number of regeneration initiatives including the pharaonic 'Great Green Wall', a continent wide forest belt. However, smaller projects on the village level may better develop as they are under the responsability of local population, which can reactivate their long experience. The 'regreening' might be restricted to the region of the southern Sahara and the northern Sahel as well as to the traditional park systems. Anyhow, even if a long-time amelioration of production systems will happen, the former must be regarded on the background of a rapidly increasing

**Keywords:** Sahara, Sahel, vegetation, landscape types, present situation, historical development, stability of limits, cultural, landscapes, degradation, regeneration

In the last years, a 'Greening' or 'Regreening' of the Sahel was a most disputed topic. It mutated to a general discussion of regeneration potential of the ecosystems and the possibilities to find production modes for the necessary food production. Moreover, conservation and nature protection were discussed and great projects

On the other side, the general political insecurity of the last 15 years supressed fieldwork and made an end to several initiatives. Many of the conservation projects are now classified as 'in suspense.' This stands especially for the big National Natural Parks in the Sahara of Niger and Chad [14–16] and more or less for the 'Great Green

Thus, the reasoning on degradation or regeneration is often based on pure remote sensing without the necessary ground check or field work. In addition, for the case of Sahara-Sahel-complex, there is still a deep confusion on the nature and dynamics of ecosystems and landscapes as well as on their definitions. Limits and

On this background, we will characterise the main ecosystems – landscapes of Sahara-Sahel by a general vegetation map in order to avoid further confusions. This should also work as a base to interpret palaeorecords. Furtheron, we will try to reconstruct the landscape evolution during the last 200 years. Finally, we will

It is fascinating to see that the old concept of an extension of one large ecosystem on the cost of another – here the advancing/encroaching desert into the savannas is still taken as valid. The alarm of Stebbing [19] of an advancing desert in the Niger-Nigeria border region was rapidly disproved by a common French-English – Forester expedition [20]. More than half a century later, Tucker et al. [21] presented the model of an expanding and retracting Sahara, which he considered as desert for the whole in the scale of years. Their conclusions were based on interpreted vegetation changes with help of satellite images; however, without any differentiation between permanent and short-time plant cover. Another less meaningful approach was presented by Thomas and Nigaru [22], who claimed a 10% expansion of the Sahara/desert since 1920 both to the North and to the South. The authors based

boundaries seem to be free floating – sometimes on an annual scale.

discuss the chances of measures of regeneration and conservation.

**2. The 'bandoneon desert'. Concepts and nature of the Sahara**

**64**

understood' (see **Figure 1** nr. 10 and 2). Thus, 'steppe' is rarely referred to the original definition as a tussock-grassland of the genus *Stipa* under continental winter cold conditions [29, 30]. In this area, it is mainly characterised by *Stipa tenacissima* and *Lygeum spartium* on fine-grained substrates such as loess. It stretches over the plateaus of the Atlas Mts., and it is severely exploited for pasture, agriculture or paper production.


### **Figure 1.**

*Schematic presents vegetation map of northern and western Africa. Also shown are the national parksnational reserves in the Sahara of Niger and Chad and the location of the planned 'Great Green Wall'. From [27], modified and enlarged.*

**67**

**Figure 2.**

**Figure 3.**

*Tunisia. Drawing Schulz.*

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example…*

*Aspects of the steppe (cf.* **Figure 1***, 10). (A) Block diagram of Djebel Chaambi in Central Tunisia. In the upper part, the southernmost stand of* Quercus*-forests, in the lower part the* Juniperus-Rosmarinus*-shrubs and on the plain the* Stipa-Lygeum Artemisia*-steppe. (B) Djebel Dahar, Southeast Tunisia. The southernmost outpost of steppe on the loess plateaus. (C) Aspects of the* Stipa-Lygeum*-steppe near Kasserine, Central* 

To the North, the steppe interfingers with Mediterranean *Rosmarinus-Juniperus* formations and a clear limit is hardly visible. However, at the southern part of the Sahara-Atlas, it ends with the loess cover. But there is an outpost of steppe on the

*Aspects of the semidesert. (A)* Acacia-Rhantherium*-stands in the Bou Hedma, southern Tunisia. (B)*  Rhantherium-*semidesert South of Remada, southern Tunisia. (C) The southern limit of semi desert with* 

This follows the basic 'law of relative constancy' [30]. It means that plants or animals change the type of their habitat in the border region of their main area in order to guarantee the basic needs of the respective organism. Finally, it fits well to the original definition as a grassland under continental and wintercold conditions (**Figures 2, 3**).

Descriptions and characterisations of the Sahara are manyfold, see [31, 32]. Mostly it is taken as the greatest desert on earth with an extension of about 2000 × 5000 km. The area is structured by a system of wide basins and ridges often topped by mountains of more than 4000 m. Climatically, it is characterised by the

loess plateaus of the Dahar Mts. in southeast Tunisia.

Calligonum-Ephedra *at 30°N, South of El Golea, Algeria. Drawing Schulz.*

**4. The landscape system of the Sahara**

*DOI: http://dx.doi.org/10.5772/intechopen.87030*

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example… DOI: http://dx.doi.org/10.5772/intechopen.87030*

### **Figure 2.**

*Plant Communities and Their Environment*

paper production.

understood' (see **Figure 1** nr. 10 and 2). Thus, 'steppe' is rarely referred to the original definition as a tussock-grassland of the genus *Stipa* under continental winter cold conditions [29, 30]. In this area, it is mainly characterised by *Stipa tenacissima* and *Lygeum spartium* on fine-grained substrates such as loess. It stretches over the plateaus of the Atlas Mts., and it is severely exploited for pasture, agriculture or

*Schematic presents vegetation map of northern and western Africa. Also shown are the national parksnational reserves in the Sahara of Niger and Chad and the location of the planned 'Great Green Wall'. From* 

**66**

**Figure 1.**

*[27], modified and enlarged.*

*Aspects of the steppe (cf.* **Figure 1***, 10). (A) Block diagram of Djebel Chaambi in Central Tunisia. In the upper part, the southernmost stand of* Quercus*-forests, in the lower part the* Juniperus-Rosmarinus*-shrubs and on the plain the* Stipa-Lygeum Artemisia*-steppe. (B) Djebel Dahar, Southeast Tunisia. The southernmost outpost of steppe on the loess plateaus. (C) Aspects of the* Stipa-Lygeum*-steppe near Kasserine, Central Tunisia. Drawing Schulz.*

**Figure 3.**

*Aspects of the semidesert. (A)* Acacia-Rhantherium*-stands in the Bou Hedma, southern Tunisia. (B)*  Rhantherium-*semidesert South of Remada, southern Tunisia. (C) The southern limit of semi desert with*  Calligonum-Ephedra *at 30°N, South of El Golea, Algeria. Drawing Schulz.*

To the North, the steppe interfingers with Mediterranean *Rosmarinus-Juniperus* formations and a clear limit is hardly visible. However, at the southern part of the Sahara-Atlas, it ends with the loess cover. But there is an outpost of steppe on the loess plateaus of the Dahar Mts. in southeast Tunisia.

This follows the basic 'law of relative constancy' [30]. It means that plants or animals change the type of their habitat in the border region of their main area in order to guarantee the basic needs of the respective organism. Finally, it fits well to the original definition as a grassland under continental and wintercold conditions (**Figures 2, 3**).

## **4. The landscape system of the Sahara**

Descriptions and characterisations of the Sahara are manyfold, see [31, 32]. Mostly it is taken as the greatest desert on earth with an extension of about 2000 × 5000 km. The area is structured by a system of wide basins and ridges often topped by mountains of more than 4000 m. Climatically, it is characterised by the interaction of the Westafrican monsoon and the tradewinds – see below. However, the most important feature is the general lack of water – a fact, which all living organisms have to cope with.

For a useful partition of the Saharan area, we need criteria, which are applicable to the whole area. Moreover, they must summarise the ecological effects of the respective region and in principle it must be visible and recognisable even in a reduced form, and it is not useful to choose volatile elements. In that way, the vegetation is the most appropriate way to characterise the whole region and to divide it in several parts. Thus, it has an indicator function. On a second level, the plant cover shall be described by its floristic content. In addition, the plant cover can be understood from detailed descriptions – even by non-specialists. And we should not forget that vegetation is the most important resource for various organisms. In that way, we will describe and divide the Sahara in units, which are easy to recognise – also from ancient descriptions.

*Sahara est. omnis divisa in partes tres.......*

### **4.1 The semidesert**

South of Atlas Mts. there is a double change in landscape. It is from grassland (steppe) to shrub land and from the Mediterranean realm to the Saharan one (see **Figure 1**, 11–15). Vegetation is still diffuse, but rarely exceeds 30% of soil cover, and the greater part of biomass is below the surface. Saharan floristic elements like *Fagonia arabica, Rhantherium suaveolens, Gymnocarpus decander* or *Stipagrostis pungen*s on dunes dominate in the small- or dwarf shrub lands. It is the northern part of the Saharan landscapes – the semidesert.

The authors [33–37] claimed that the double stress by frost and drought impedes a tree development. However, the double strategy of life in the Sahara is already visible. Only a restricted number of organisms are equipped against drought and frost. On the other hand, there is the strategy of mass and accident. Aleatoric rainfall may activate the seed bank of herbs and grasses. These therophytes must fulfil their lifecycle in the short time of limited rainfall.

These accidental floras are an important resource for nomadic animal keeping.

Anyway, we must not forget that *Acacia raddiana*-stands still exist in southern Tunisia (Dj. Bou Hedma) or in southwestern Morocco. Perhaps, future records will convince us to rethink the dynamic of the northern Sahara [38–42].

The southern limit of semidesert is easy to recognise. Around 30°N (31° N in the East or along the Atlantic coast of Morocco), it changes from diffuse stands of *Calligonum* or *Ephedra* to another mode, (conracted or linear) of the *Acacia-Panicum*-type. This characterises the change from semidesert to desert.

### **4.2 The desert**

The desert is extremely difficult to define because of the emotional component of the term (see **Figure 1**, 16-19, **Figures 4, 5**). Here, we follow the definition of Monod [44]. He stressed the difference of diffuse modes of semidesert or savanna to the contracted one – the desert. The desert is the region where permanent life is only possible in favourable places such as wadis (dry valleys) or depressions where groundwater and run off are available. Thus, permanent vegetation is contracted or linear.

It follows the oasis system, as few places, where the basic needs are guaranteed. There are several modes to cope with the scarce water resources such as the Acaciastrategy. Aleatoric rainfalls may induce germination of the seeds – perhaps already prepared by the intestines of animals. After germination, all resources are mobilised to develop a tap root to reach ground water. In that case, the plant gets independent

**69**

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example…*

*Aspects of the desert. (A) Contracted vegetation in the Wadi Achelouma, northeastern Niger. (B) Achab in the Ténéré, northern Niger. (C) Wild cereal fields in the southwestern foreland of the Air Mts., N-Niger. From [27]* 

from climate. However, there is the other strategy of life – that of achabs, already discussed in regard to the semidesert. The seed bank rapidly reacts on aleatoric rainfalls with a short time-flora. It may be the case once in 3 years or several times a year. There are also wild cereals as part of the achabs – an important resource for

*The modes of altitudinal change in the Sahara. (A) From desert to semidesert (Ahaggar/Algeria and Tibesti/*

*Chad) (B) From desert to savannah (Air Mts./Niger). From [43], modified.*

The contracted vegetation, mainly of the *Acacia-Panicum*- (tree-tussock grass) type, is typical for the wadis of mountain areas and their forelands (see **Figure 1**, 16, 17). Large wadis in the forelands – especially in the Southwest of Adrar des Iforas and Air Mts. might touch for a short distance but they separate afterwards. In that way the impression of a diffuse plant cover may exist. It is perfectly demonstrated by Voss et al. [45, 46] for the western forelands of the Adrar des Iforas in northern Mali.

As in other regions, the plant cover changes with altitude in the Sahara (see **Figure 1**, 12, 22, **Figures 5, 6**). There is an altitudinal change of vegetation in the High Mts. of the Sahara (**Figure 5**). In the Ahaggar Mts/South Algeria, the characteristic *Acacia-Panicum* vegetation of the desert wadis changes from about 2000 m into a diffuse *Artemisia*-shrub vegetation-a semidesert of a Mediterranean affiliation. In small gorges, some tree groups of *Olea lapperinii* or *Pistacia atlantica* exist. The Tibesti Mts. show similar features, however, on the highest peaks, some stands of *Erica arborea* survived (**Figure 7**). This is the Mediterranean type of

The Air Mountains are different. Above 1800 m the contracted *Acacia-Panicum*plant cover changes to a diffuse *Acacia-Commiphora-Rhus*-savanna (savanna seen as a tree grass – vegetation under a tropical climate). Thus, it is a Sahelian type of altitudinal change. **Figure 6** gives a general overview of the Air Mts. (A) with the locations of the change to High-Mts.-savannas and the upper catchment of the wadi

*DOI: http://dx.doi.org/10.5772/intechopen.87030*

**Figure 4.**

*modified.*

**Figure 5.**

human food (see **Figure 4C**).

**4.3 Altitudinal change**

altitudinal change [48].

Anou Mekkerene (**Figure 8** see below, see also [49]).

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example… DOI: http://dx.doi.org/10.5772/intechopen.87030*

### **Figure 4.**

*Plant Communities and Their Environment*

organisms have to cope with.

**4.1 The semidesert**

part of the Saharan landscapes – the semidesert.

lifecycle in the short time of limited rainfall.

interaction of the Westafrican monsoon and the tradewinds – see below. However, the most important feature is the general lack of water – a fact, which all living

Sahara in units, which are easy to recognise – also from ancient descriptions.

*Sahara est. omnis divisa in partes tres.......*

South of Atlas Mts. there is a double change in landscape. It is from grassland (steppe) to shrub land and from the Mediterranean realm to the Saharan one (see **Figure 1**, 11–15). Vegetation is still diffuse, but rarely exceeds 30% of soil cover, and the greater part of biomass is below the surface. Saharan floristic elements like *Fagonia arabica, Rhantherium suaveolens, Gymnocarpus decander* or *Stipagrostis pungen*s on dunes dominate in the small- or dwarf shrub lands. It is the northern

The authors [33–37] claimed that the double stress by frost and drought impedes a tree development. However, the double strategy of life in the Sahara is already visible. Only a restricted number of organisms are equipped against drought and frost. On the other hand, there is the strategy of mass and accident. Aleatoric rainfall may activate the seed bank of herbs and grasses. These therophytes must fulfil their

These accidental floras are an important resource for nomadic animal keeping. Anyway, we must not forget that *Acacia raddiana*-stands still exist in southern Tunisia (Dj. Bou Hedma) or in southwestern Morocco. Perhaps, future records will

The southern limit of semidesert is easy to recognise. Around 30°N (31° N in the East or along the Atlantic coast of Morocco), it changes from diffuse stands of *Calligonum* or *Ephedra* to another mode, (conracted or linear) of the *Acacia-*

The desert is extremely difficult to define because of the emotional component of the term (see **Figure 1**, 16-19, **Figures 4, 5**). Here, we follow the definition of Monod [44]. He stressed the difference of diffuse modes of semidesert or savanna to the contracted one – the desert. The desert is the region where permanent life is only possible in favourable places such as wadis (dry valleys) or depressions where groundwater and run off are available. Thus, permanent vegetation is contracted or linear.

It follows the oasis system, as few places, where the basic needs are guaranteed. There are several modes to cope with the scarce water resources such as the Acaciastrategy. Aleatoric rainfalls may induce germination of the seeds – perhaps already prepared by the intestines of animals. After germination, all resources are mobilised to develop a tap root to reach ground water. In that case, the plant gets independent

convince us to rethink the dynamic of the northern Sahara [38–42].

*Panicum*-type. This characterises the change from semidesert to desert.

For a useful partition of the Saharan area, we need criteria, which are applicable to the whole area. Moreover, they must summarise the ecological effects of the respective region and in principle it must be visible and recognisable even in a reduced form, and it is not useful to choose volatile elements. In that way, the vegetation is the most appropriate way to characterise the whole region and to divide it in several parts. Thus, it has an indicator function. On a second level, the plant cover shall be described by its floristic content. In addition, the plant cover can be understood from detailed descriptions – even by non-specialists. And we should not forget that vegetation is the most important resource for various organisms. In that way, we will describe and divide the

**68**

**4.2 The desert**

*Aspects of the desert. (A) Contracted vegetation in the Wadi Achelouma, northeastern Niger. (B) Achab in the Ténéré, northern Niger. (C) Wild cereal fields in the southwestern foreland of the Air Mts., N-Niger. From [27] modified.*

**Figure 5.**

*The modes of altitudinal change in the Sahara. (A) From desert to semidesert (Ahaggar/Algeria and Tibesti/ Chad) (B) From desert to savannah (Air Mts./Niger). From [43], modified.*

from climate. However, there is the other strategy of life – that of achabs, already discussed in regard to the semidesert. The seed bank rapidly reacts on aleatoric rainfalls with a short time-flora. It may be the case once in 3 years or several times a year. There are also wild cereals as part of the achabs – an important resource for human food (see **Figure 4C**).

The contracted vegetation, mainly of the *Acacia-Panicum*- (tree-tussock grass) type, is typical for the wadis of mountain areas and their forelands (see **Figure 1**, 16, 17). Large wadis in the forelands – especially in the Southwest of Adrar des Iforas and Air Mts. might touch for a short distance but they separate afterwards. In that way the impression of a diffuse plant cover may exist. It is perfectly demonstrated by Voss et al. [45, 46] for the western forelands of the Adrar des Iforas in northern Mali.

### **4.3 Altitudinal change**

As in other regions, the plant cover changes with altitude in the Sahara (see **Figure 1**, 12, 22, **Figures 5, 6**). There is an altitudinal change of vegetation in the High Mts. of the Sahara (**Figure 5**). In the Ahaggar Mts/South Algeria, the characteristic *Acacia-Panicum* vegetation of the desert wadis changes from about 2000 m into a diffuse *Artemisia*-shrub vegetation-a semidesert of a Mediterranean affiliation. In small gorges, some tree groups of *Olea lapperinii* or *Pistacia atlantica* exist. The Tibesti Mts. show similar features, however, on the highest peaks, some stands of *Erica arborea* survived (**Figure 7**). This is the Mediterranean type of altitudinal change [48].

The Air Mountains are different. Above 1800 m the contracted *Acacia-Panicum*plant cover changes to a diffuse *Acacia-Commiphora-Rhus*-savanna (savanna seen as a tree grass – vegetation under a tropical climate). Thus, it is a Sahelian type of altitudinal change. **Figure 6** gives a general overview of the Air Mts. (A) with the locations of the change to High-Mts.-savannas and the upper catchment of the wadi Anou Mekkerene (**Figure 8** see below, see also [49]).

### **Figure 6.**

*Vegetation of the Air Mts./Niger and the limits of the desert as an example for the southern Sahara. Also shown are Upper Wadi Anou Mekkerene (A, see also* **Figure 8***) and the Air-Ténéré-National Park (B) with its Addax sanctuary (C). From [47], modified.*

### **Figure 7.**

*The southern limit of the desert and the Saharan savanna. (A) The passage from desert to (Saharan) savanna at the Tigidit escarpment, northern Niger. (B) The change from the linear desert vegetation to the savanna a the Belgaschifari well NE-Niger. (C) The general aspect of the Saharan Acacia-*Panicum savanna. *From [13], modified and complemented*.

### **4.4 The southern limit of the Sahara**

In the southern forelands of the Air Mts. around 16°N/16°30′N, the aspect changes again in two steps. The first step is visible by a diffuse *Maerua crassifolia-Acacia ehrenbergiana*-savanna on the fissured sandstone-plateaus of Tigidit and also Agadem-Homodji in Southeast Niger (see **Figure 1**, 21). These savannas depend on the cistern effects of the fissures, which collect and hold water from runoff and dew [27]. On the plains, however, one observes a densification of the tree lines and the transition into a savanna of the same elements within a short distance (see **Figure 1**, 20).

**71**

**Figure 8.**

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example…*

Thus, there is the definite transition from the desert to savanna within the Saharan realm. Similar features are confirmed for northern Mauretania and Mali [50, 51]. In northern Chad, this transition is modified by substrata [52]. Large inundation plains are quasi devoid of plants, which appear only on sand ridges. On the sandy plains at about 16°N, the change into a tree-tussock grass savannah occurs similarly as it is the case for Niger. Akthar-Schuster [53] reports a comparable transition belt for the northern Sudan too. This boundary is the most disputed limit between landscape

*Wadi Anou Mekkerene and the Agalak-Aroyane Mts. of the central Air Mts. The area of the Guide pasture reserve is indicated. The difference of the diffuse mountainous savannas and the linear desert vegetation is clearly visible as well as the densification of the alluvial vegetation following down the wadis. From [43], modified.*

zones, as it caused the misunderstanding of degradation-desertification, etc.

nah, takes place within the Saharan realm.

**4.5 The climatic implications**

Finally, the Sahara is a tripartite landscape system, where the desert takes the greatest part but has its borders to the semidesert in the North and the savanna in the South. Thus, the main change in the landscape system, that of desert to savan-

*…. quarum unam dominat semideserta, aliam deserta et tertiam savanna saharica.*

At this point of description, we should also deal with climatic conditions. In the aftermath of Dubief [54], the main boundaries are often paralleled to - or defined by mean annual precipitation. However, there are also dew, runoff and especially the access to groundwater which determines plants and vegetation. So, various components are summed up. Note, that two main systems interact: the summer rains of the monsoon and the Mediterranean winter rains and trade winds. We also have to consider the aleatoric rainfalls during the whole year derived from monsoon or cold airdrops from the North. They are responsible for achabs and the short time floras demonstrate their existence. The northern boundary of the Sahara is usually assigned to an annual precipitation of about 100 mm – mainly in winter. More to the

*DOI: http://dx.doi.org/10.5772/intechopen.87030*

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example… DOI: http://dx.doi.org/10.5772/intechopen.87030*

### **Figure 8.**

*Plant Communities and Their Environment*

**70**

**Figure 6.**

**Figure 7.**

**4.4 The southern limit of the Sahara**

*Addax sanctuary (C). From [47], modified.*

distance (see **Figure 1**, 20).

*modified and complemented*.

In the southern forelands of the Air Mts. around 16°N/16°30′N, the aspect changes again in two steps. The first step is visible by a diffuse *Maerua crassifolia-Acacia ehrenbergiana*-savanna on the fissured sandstone-plateaus of Tigidit and also Agadem-Homodji in Southeast Niger (see **Figure 1**, 21). These savannas depend on the cistern effects of the fissures, which collect and hold water from runoff and dew [27]. On the plains, however, one observes a densification of the tree lines and the transition into a savanna of the same elements within a short

*The southern limit of the desert and the Saharan savanna. (A) The passage from desert to (Saharan) savanna at the Tigidit escarpment, northern Niger. (B) The change from the linear desert vegetation to the savanna a the Belgaschifari well NE-Niger. (C) The general aspect of the Saharan Acacia-*Panicum savanna. *From [13],* 

*Vegetation of the Air Mts./Niger and the limits of the desert as an example for the southern Sahara. Also shown are Upper Wadi Anou Mekkerene (A, see also* **Figure 8***) and the Air-Ténéré-National Park (B) with its* 

*Wadi Anou Mekkerene and the Agalak-Aroyane Mts. of the central Air Mts. The area of the Guide pasture reserve is indicated. The difference of the diffuse mountainous savannas and the linear desert vegetation is clearly visible as well as the densification of the alluvial vegetation following down the wadis. From [43], modified.*

Thus, there is the definite transition from the desert to savanna within the Saharan realm. Similar features are confirmed for northern Mauretania and Mali [50, 51]. In northern Chad, this transition is modified by substrata [52]. Large inundation plains are quasi devoid of plants, which appear only on sand ridges. On the sandy plains at about 16°N, the change into a tree-tussock grass savannah occurs similarly as it is the case for Niger. Akthar-Schuster [53] reports a comparable transition belt for the northern Sudan too. This boundary is the most disputed limit between landscape zones, as it caused the misunderstanding of degradation-desertification, etc.

Finally, the Sahara is a tripartite landscape system, where the desert takes the greatest part but has its borders to the semidesert in the North and the savanna in the South. Thus, the main change in the landscape system, that of desert to savannah, takes place within the Saharan realm.

*…. quarum unam dominat semideserta, aliam deserta et tertiam savanna saharica.*

### **4.5 The climatic implications**

At this point of description, we should also deal with climatic conditions. In the aftermath of Dubief [54], the main boundaries are often paralleled to - or defined by mean annual precipitation. However, there are also dew, runoff and especially the access to groundwater which determines plants and vegetation. So, various components are summed up. Note, that two main systems interact: the summer rains of the monsoon and the Mediterranean winter rains and trade winds. We also have to consider the aleatoric rainfalls during the whole year derived from monsoon or cold airdrops from the North. They are responsible for achabs and the short time floras demonstrate their existence. The northern boundary of the Sahara is usually assigned to an annual precipitation of about 100 mm – mainly in winter. More to the centre of the Sahara mean values are fictional. Rainfall becomes aleatoric and accident is the main component in the ecosystem. The southern limit of the semidesert may be attributed to about 50 mm/y and the southern border of the desert within the Sahara is more or less parallel to 150 mm/y of summer rain. As mentioned above, both limits largely depend on the combination of rainfall, runoff, dew and storage of humidity in soil. Anyhow, these clear boundaries are among the few pure climatic ones. They are visible across the whole continent.

### **4.6 Life strategies**

As mentioned above, there are two basic strategies to cope with the uncertain resources. These are the 'achab-strategy,' to answer with a mass of unprotected organisms to aleatoric resources – here rainfall. They fulfil their life cycle with these limited resources before returning back to the dormant state in the seeds. The '*Acacia*-strategy' includes the use of tap-and flat roots and vegetative/generative propagation. Useful rainfall is exploited by the germinating of seeds. The saplings grow in the first years below the surface and develop taproots until they reach a groundwater lens or horizon. Afterwards, they grow above the surface, develop lateral roots and are more or less independent from the actual climate.

### **4.7 Differences in concepts and analyses/interpretations**

Different concepts may produce different interpretations. The vegetation map (**Figure 1**) differs in several points from the concepts of other colleagues especially in type and position of the southern boundary of the Sahara. We do not follow the interpretation given by Medail-Quezel [24] or White [55] for the North-extensions of the Sahel in the southwester forelands of the Adrar des Iforas (N-Mali) and of the Air Mts. (Niger) as well as for the southern half of the Air Mts. [56].

The forelands are not seen as part of the Sahel but as regions of enlarged wadis see [45, 46]. The Air Mts. are considered as Saharan desert-mountains with a Sahelian altitudinal change – as for example, the Ahaggar Mts. or the Tibesti, which do not belong to the Mediterranean out of their high altitude vegetation. White [54] takes the northernmost savannas as part of the Sahel. Another point is the statistical approach as shown by Linder et al. [57]. They define various borderlines of Sahel versus Sahara out of all zoological and floristic elements. Most of those boundaries reach several hundreds of km more to the North – into the region of plain desert. This represents the principal difference of field analysis and pure statistical analysis without any ground check. Another point is the difference and extension of the Sudan- and Guinea-zones. The concepts of the Kew and Toulouse schools [58, 59] differ at the Nigeria-Cameroon border. In that case, we follow the 'Toulouse' school.

## **5. The Sahel and its savannas**

A few km to the South, the aspect of landscape changes again (see **Figure 1**, 23-33, **Figures 9, 10**). The savanna remains but the floristic composition differs. Beside *Acacia, Commiphora* becomes characteristic and the annual grasses like *Aristida mutabilis*or *Cenchrus biflorus* are dominant. It is the definite change from the Sahara to the Sahel. Phytosociologically, it is defined by the transition from the*Acacio-Panicion* to the *Acacio-Aristidion* [47].

As **Figures 9** and **10** demonstrate, the Sahelian savannas are intensively exploited. The northern ones are pasture areas, and millet growing dominates in the *Acacia-Piliostigma* savannas. These are cultural landscapes and they demonstrate

**73**

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example…*

*Aspects of the Sahel. (A) The Sahelian savanna on the Tigidit plateau, Central Niger* Acacia, Commiphora, Maerua *and annual grasses. (B) Millet fields near Birni-n-Konni, southern Niger. (C) Animal keeping near Abalak, northern Niger. (D) Desertification. The overexploited area of Ader, near Koutous, central Niger.* 

the variety of degradation. The southern Sahel is also a region of the old rooted agroforestry systems – the parks [60–62]. They are dual and integrated systems of animal (cattle) keeping and agriculture. The main feature is the two storey aspects of trees of a restricted species composition and only one or two generations. The Gao (*Faidherbia*)-parks, however, often show several generations of shrubs and trees. The intention of these parks is the production of vegetal or animal fat and agrarian products. They have been constituted by selection from a pre-existing vegetation (*Vitellaria* and *Parkia*-parks), by tolerance and assistance- as for the *Faidherbia*-parks – or by former defence plantation as it is the case of *Borassus*-parks

*The aspects of agro-forestry. (A)* Faidherbia albida*-Park for animal keeping and agriculture in southern Niger. (B) Karité-Park (*Vetiveria paradoxa*) in northern Togo. Tree cultivation for fat and agriculture with the general employment of fire. (C) Ronier palm-Park (*Borassus aethiopum*), southern Niger, for various* 

Either it is a tool to clear land for new fields – few areas where fallow – either shifting cultivation is still practised or it is used for cleaning or sanitary purposes [63–66]. The Sahel is a savanna region and climatically it is influenced by tropical summer rains (monsoon) with a gradient from about 800 to 150/ 200 mm/y and

For long periods, the Sahel was only regarded as a transition zone to the real (Sudanian)-savannas [33, 50]. From the 1970s, this region was accepted as one of the consistent savannas [67] even widely transformed to cultural landscapes [68].

Type and dynamic of landscape may often be read and understood from its history (see **Figures 11–16**). A series of more or less precise descriptions is on our

[61]. Fire is still a part of the agricultural management.

*exploitations of the trees and agriculture. Former defence parks. Drawing Schulz.*

**6. Lessons from the past. The last 200 years**

with a rainy season of 3–5 months.

*DOI: http://dx.doi.org/10.5772/intechopen.87030*

**Figure 9.**

**Figure 10.**

*Drawing Schulz.*

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example… DOI: http://dx.doi.org/10.5772/intechopen.87030*

### **Figure 9.**

*Plant Communities and Their Environment*

**4.6 Life strategies**

climatic ones. They are visible across the whole continent.

centre of the Sahara mean values are fictional. Rainfall becomes aleatoric and accident is the main component in the ecosystem. The southern limit of the semidesert may be attributed to about 50 mm/y and the southern border of the desert within the Sahara is more or less parallel to 150 mm/y of summer rain. As mentioned above, both limits largely depend on the combination of rainfall, runoff, dew and storage of humidity in soil. Anyhow, these clear boundaries are among the few pure

As mentioned above, there are two basic strategies to cope with the uncertain resources. These are the 'achab-strategy,' to answer with a mass of unprotected organisms to aleatoric resources – here rainfall. They fulfil their life cycle with these limited resources before returning back to the dormant state in the seeds. The '*Acacia*-strategy' includes the use of tap-and flat roots and vegetative/generative propagation. Useful rainfall is exploited by the germinating of seeds. The saplings grow in the first years below the surface and develop taproots until they reach a groundwater lens or horizon. Afterwards, they grow above the surface, develop

Different concepts may produce different interpretations. The vegetation map (**Figure 1**) differs in several points from the concepts of other colleagues especially in type and position of the southern boundary of the Sahara. We do not follow the interpretation given by Medail-Quezel [24] or White [55] for the North-extensions of the Sahel in the southwester forelands of the Adrar des Iforas (N-Mali) and of the

The forelands are not seen as part of the Sahel but as regions of enlarged wadis

A few km to the South, the aspect of landscape changes again (see **Figure 1**, 23-33, **Figures 9, 10**). The savanna remains but the floristic composition differs. Beside *Acacia, Commiphora* becomes characteristic and the annual grasses like *Aristida mutabilis*or *Cenchrus biflorus* are dominant. It is the definite change from the Sahara to the Sahel. Phytosociologically, it is defined by the transition from

As **Figures 9** and **10** demonstrate, the Sahelian savannas are intensively exploited. The northern ones are pasture areas, and millet growing dominates in the *Acacia-Piliostigma* savannas. These are cultural landscapes and they demonstrate

see [45, 46]. The Air Mts. are considered as Saharan desert-mountains with a Sahelian altitudinal change – as for example, the Ahaggar Mts. or the Tibesti, which do not belong to the Mediterranean out of their high altitude vegetation. White [54] takes the northernmost savannas as part of the Sahel. Another point is the statistical approach as shown by Linder et al. [57]. They define various borderlines of Sahel versus Sahara out of all zoological and floristic elements. Most of those boundaries reach several hundreds of km more to the North – into the region of plain desert. This represents the principal difference of field analysis and pure statistical analysis without any ground check. Another point is the difference and extension of the Sudan- and Guinea-zones. The concepts of the Kew and Toulouse schools [58, 59] differ at the Nigeria-Cameroon border. In that case, we follow the 'Toulouse' school.

lateral roots and are more or less independent from the actual climate.

Air Mts. (Niger) as well as for the southern half of the Air Mts. [56].

**4.7 Differences in concepts and analyses/interpretations**

**72**

**5. The Sahel and its savannas**

the*Acacio-Panicion* to the *Acacio-Aristidion* [47].

*Aspects of the Sahel. (A) The Sahelian savanna on the Tigidit plateau, Central Niger* Acacia, Commiphora, Maerua *and annual grasses. (B) Millet fields near Birni-n-Konni, southern Niger. (C) Animal keeping near Abalak, northern Niger. (D) Desertification. The overexploited area of Ader, near Koutous, central Niger. Drawing Schulz.*

#### **Figure 10.**

*The aspects of agro-forestry. (A)* Faidherbia albida*-Park for animal keeping and agriculture in southern Niger. (B) Karité-Park (*Vetiveria paradoxa*) in northern Togo. Tree cultivation for fat and agriculture with the general employment of fire. (C) Ronier palm-Park (*Borassus aethiopum*), southern Niger, for various exploitations of the trees and agriculture. Former defence parks. Drawing Schulz.*

the variety of degradation. The southern Sahel is also a region of the old rooted agroforestry systems – the parks [60–62]. They are dual and integrated systems of animal (cattle) keeping and agriculture. The main feature is the two storey aspects of trees of a restricted species composition and only one or two generations. The Gao (*Faidherbia*)-parks, however, often show several generations of shrubs and trees. The intention of these parks is the production of vegetal or animal fat and agrarian products. They have been constituted by selection from a pre-existing vegetation (*Vitellaria* and *Parkia*-parks), by tolerance and assistance- as for the *Faidherbia*-parks – or by former defence plantation as it is the case of *Borassus*-parks [61]. Fire is still a part of the agricultural management.

Either it is a tool to clear land for new fields – few areas where fallow – either shifting cultivation is still practised or it is used for cleaning or sanitary purposes [63–66]. The Sahel is a savanna region and climatically it is influenced by tropical summer rains (monsoon) with a gradient from about 800 to 150/ 200 mm/y and with a rainy season of 3–5 months.

For long periods, the Sahel was only regarded as a transition zone to the real (Sudanian)-savannas [33, 50]. From the 1970s, this region was accepted as one of the consistent savannas [67] even widely transformed to cultural landscapes [68].

## **6. Lessons from the past. The last 200 years**

Type and dynamic of landscape may often be read and understood from its history (see **Figures 11–16**). A series of more or less precise descriptions is on our

### **Figure 11.**

*The history of the southern limit of the desert and the Sahara at the Tigidit plateau, northern Niger (from [69], modified).*

#### **Figure 12.**

*The history of the southern limit of the desert and the Sahara at the Belgashifari well, NE Niger (from [69], modified).*

**75**

**(see above)**

**Figure 13.**

*nineteenth century. Cartography Schulz.*

*6.1.1 The Tigidit cuesta (16°25*′*N, 7°55*′*E)*

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example…*

disposition centred on the traditional transsaharan trade routes from the 1820s on. For the present case, the historic 'Borno-Road' – Tripolis-Kukawa and its deviation via Ghadames-Rhat-Agadez – served as a perfect source of information. It was the most frequented caravan-route in the nineteenth and early twentieth century, whereas the Tombouctou-Fez (Morocco) road was already less used. From 1822 on, we have for every 30 years a report of the voyagers [20, 71–76] on the nature of the landscapes. As wells were crucial points for the caravans, they also served as reference points in all the reports. Vegetation has always been an important topic in their reports which relied on the vernacular names of plant species – in Arabic or in other languages. Thus, we have a suitable base to reconstruct the plant cover for the nineteenth and for the first half of the twentieth centuries as we can use the indicator values of the modern vegetation.

*Northern and western Africain the nineteenth century. Vegetation maps and a reconstruction of precipitation based on the reports of the early explorers [70]. The importance of the achabs is visible in the second half of the* 

**6.1 At first, we will present the landscape changes at the desert-savanna-**

transition and the Saharan savanna was much more extended [13].

*6.1.2 The transition at the Belgashifari well (16°2 N, 13°14*′*E)*

**transition: at the reference points Tigidit cuesta and at the Belgashifari well** 

As mentioned above, the contrast between the contracted mode of the *Acacia-Panicum-*vegetation (desert) in the foreland of the cuesta and its diffuse mode (savanna) on its top is clearly visible (cf. **Figure 17**). The dots depict the extension of the Saharan savanna and the change to those of the Sahel. At 1937, the situation was similar but the belt of the *Acacia-Panicum-*savanna was smaller and the extension of the Sahelian *Commiphora*-savanna was greater [20]. In the middle of the nineteenth century, the situation was different. A large grass cover masked the main

In 1984, the change from contracted to diffuse (permanent) vegetation was as clear as at Tigidit (see above). However, the Saharan savanna was much more extended (see **Figure 12**). In 2014, the situation was comparable, but trees were much more scarce. It was in 1822, when Denham [71] gave the first of the historical descriptions: he reported the change from desert to savanna near its present position. After a belt of a lush savanna, he described a clear change to a dense savanna.

*DOI: http://dx.doi.org/10.5772/intechopen.87030*

*Vegetation Dynamics. Natural versus Cultural and the Regeneration Potential. The Example… DOI: http://dx.doi.org/10.5772/intechopen.87030*

**Figure 13.**

*Plant Communities and Their Environment*

**74**

**Figure 12.**

*modified).*

**Figure 11.**

*[69], modified).*

*The history of the southern limit of the desert and the Sahara at the Tigidit plateau, northern Niger (from* 

*The history of the southern limit of the desert and the Sahara at the Belgashifari well, NE Niger (from [69],* 

*Northern and western Africain the nineteenth century. Vegetation maps and a reconstruction of precipitation based on the reports of the early explorers [70]. The importance of the achabs is visible in the second half of the nineteenth century. Cartography Schulz.*

disposition centred on the traditional transsaharan trade routes from the 1820s on. For the present case, the historic 'Borno-Road' – Tripolis-Kukawa and its deviation via Ghadames-Rhat-Agadez – served as a perfect source of information. It was the most frequented caravan-route in the nineteenth and early twentieth century, whereas the Tombouctou-Fez (Morocco) road was already less used. From 1822 on, we have for every 30 years a report of the voyagers [20, 71–76] on the nature of the landscapes. As wells were crucial points for the caravans, they also served as reference points in all the reports. Vegetation has always been an important topic in their reports which relied on the vernacular names of plant species – in Arabic or in other languages. Thus, we have a suitable base to reconstruct the plant cover for the nineteenth and for the first half of the twentieth centuries as we can use the indicator values of the modern vegetation.
