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

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 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)*

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.

**Figure 14.**

*The present situation and the historical development of the Guidimouni depression/SE-Niger (from [69], modified).*

Thirty years later, Barth [72] saw again the desert-savanna-boundary in a similar position as at present; however, he noted a dense herb and grass cover and an important tree-vegetation in the dune depressions. Rohlfs [74] described a dense grass and herb cover that masked the main transition, and for the South of Belgashifari well, he noted a dense savanna with Sudanian trees in the dune valley. Nachtigal [73] confirmed this mosaic too. Thirty years later, Vischer [75] described a loose grass and herb cover with the desert boundary near the present position.

**77**

**Figure 16.**

**Figure 15.**

*[69], modified).*

*The Guidimouni pollen record/ SE-Niger (from [69], modified).*

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

However, the tree cover south of it was less dense than described by his predecessors. In conclusion, we state that the main boundaries did not change their position very much, but during the 1860s, the plant cover was much more dense and diversified with a remarkable Sudanian tree vegetation reaching far to the North in the

*The sediment structure of the Guidimouni record/Southeast Niger. It demonstrates the stability of the sediments by the formation of algea-layer sand also the steady presence offire asproved by the charred material (from* 

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

dune valleys.

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

However, the tree cover south of it was less dense than described by his predecessors. In conclusion, we state that the main boundaries did not change their position very much, but during the 1860s, the plant cover was much more dense and diversified with a remarkable Sudanian tree vegetation reaching far to the North in the dune valleys.

### **Figure 15.**

*Plant Communities and Their Environment*

**76**

**Figure 14.**

*modified).*

Thirty years later, Barth [72] saw again the desert-savanna-boundary in a similar position as at present; however, he noted a dense herb and grass cover and an important tree-vegetation in the dune depressions. Rohlfs [74] described a dense grass and herb cover that masked the main transition, and for the South of Belgashifari well, he noted a dense savanna with Sudanian trees in the dune valley. Nachtigal [73] confirmed this mosaic too. Thirty years later, Vischer [75] described a loose grass and herb cover with the desert boundary near the present position.

*The present situation and the historical development of the Guidimouni depression/SE-Niger (from [69],* 

*The sediment structure of the Guidimouni record/Southeast Niger. It demonstrates the stability of the sediments by the formation of algea-layer sand also the steady presence offire asproved by the charred material (from [69], modified).*

**Figure 16.** *The Guidimouni pollen record/ SE-Niger (from [69], modified).*

### **Figure 17.**

*The strategies of life in the desert. (A) Achab. Development of therophytes after aleatoric rainfall and the formation of a root carpet. (B) Vegetative and generative strategy of* Acacia*. Deep and lateral roots with root suckers, seed germination and development of deep roots for groundwater. Drawing Schulz.*

### **6.2 Mapping the past**

These reports made it possible to establish vegetation maps for the first and for the second half of the nineteenth century. The small maps give the expedition routes. It is clear that the information does not cover the whole area, and the interpretation is certainly limited. The maps rely on the written reports, and the descriptions for the central part of the region were the most precise ones. For the southern parts of the visited regions, the descriptions are mostly based on trees, which were mentioned by their vernacular names. Due to several robberies or damages during the transport, most collected plants were lost. Finally, the bombing and burning of the Berlin Herbarium in the Second World War destroyed the last preserved plant specimens [77].

### *6.2.1 Climate history*

Nicholson et al. [70] reconstructed the mean precipitation for the last two centuries based on the landscape descriptions of early voyagers, early measurements and interpretations out of lake level- or sediment records. The diagram (**Figure 13**) is also marked for the expeditions of the early voyagers. The record depicts a long drought period from the beginning of the nineteenth century to about 1850 with a short humid spell around 1820. During the 1850s, some humid years occured followed by a series of dry years up to the 1870s. Afterwards, a humid period lasted with some interruptions until 1910. After another dry period up till 1920, the twentieth century, a long humid period occurred until the end of the 1960s. Suddenly, the climate changed to a long series of droughts, which only at the end of the 1980s seemed to diminish.

### *6.2.2 The vegetation*

Mapping is based on the present vegetation map (**Figure 1**) and documents the differences, which could be read from the historical reports. The two maps show

**79**

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

similarities and differences. They correspond in their regular presence of trees in the northern semidesert and in the position of the southern boundary of the Sahara. The Sahel region was described for extended *Acacia*-thickets and forests in the northern part and Combretaceae-savannas in the South. The Sudanian region was similar to the present one; it was, however, denser. A further differentiation was not possible. But the presence of parks was regularly mentioned. The rain forests were much more extended and consequently the Guinean zone much more restricted – compared to the present situation. The most important information lies in the regular presence of achabs in the Sahara during the second half of the nineteenth century. We conceive quasi-permanent pastures by repeated rainfall in the Sahara up to 25°N. Certainly, this was the base of a strong nomad economy providing the base for their dominance over sedentary people. All voyagers agreed on the rich and diverse game in Sahel and Sudan. They mentioned in particularthe large elephant populations. Together with the large extension of *Acacia*-forests-thickets, we have to think on elephant-landscapes [78–82]. Elephants are known as landscape engineers. They produce a twofold landscape. For the one they transform forests to medium high thickets by positive and negative selection of tree species and the elimination of high trees and for the other they also create new structures. They open the forests for their tracks giving chance to grasses, herbs or shrubs with a ruderal behaviour. In this way, they form thickets and provide dry and combustible material. So, elephants also create fire-prone landscapes too. Elephants are bound to water and they make or enlarge pools for drinking, bathing and also the uptake of minerals. As they are social animals with a long life period, they school their young generations to maintain and preserve these types of landscapes. The *Acacia*-thickets around lake Chad or in northern

Cameroon give an – even poor – model of these former landscapes.

**Guidimouni – a key locality for the Sahelian savannas**

was possible to core the upper part of the lake sediments [76].

*6.3.1 The physical situation of the Guidimound depression*

less permanent water body.

**6.3 The landscape history of the Sahelian savannas during the last 100 years.** 

A long interdune depression in SE-Niger (13°42′N/9°32′E) represents the situation of the Middle Sahalian savannas (see **Figure 1**). The region is part of that area, which was supposed to be endangered by an enchroaching desert [19]. The depression has two lakes which are fed by fresh water sources assuring a more or

**Figure 14** depicts the present situation of the depression and its recent history. The upper diagram shows the whole depression in its present situation. A degraded Middle-Sahelian savanna surrounds the lake, mainly consisting of *Acacia*- and *Balanites*-trees, *Leptadenia*-bushes and grasses. At present, it is still a regular habit to burn the reed in spring in order to have space for gardens and fields. In parallel, the *Leptadenia*-bushes on the dunes are cut and the branches afterwards burned (slash and burn) to prepare new fields after a fallow period of several years. Soils in this region belong to the arenosol-, regosol- or chambic-arenosol groups [83]. In 1848, Barth [72] described the Guidimouni depression as densely vegetated by grasses and herbs the dunes bearing an *Acacia-Commiphora-Leptadenia*-savanna. The depression itself had an *Hyphaene-Phoenix*-belt around the *Typha*-reeds. Also

In addition to the historical descriptions (see above), we also dispose on physical archives,which describe the landscape evolution during the last 100 years. They come from southeast Niger. The dune depression of Guidimouni in southeastern Niger has been described several times in the last 200 years [20, 72]. Moreover, it

*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*

similarities and differences. They correspond in their regular presence of trees in the northern semidesert and in the position of the southern boundary of the Sahara. The Sahel region was described for extended *Acacia*-thickets and forests in the northern part and Combretaceae-savannas in the South. The Sudanian region was similar to the present one; it was, however, denser. A further differentiation was not possible. But the presence of parks was regularly mentioned. The rain forests were much more extended and consequently the Guinean zone much more restricted – compared to the present situation. The most important information lies in the regular presence of achabs in the Sahara during the second half of the nineteenth century. We conceive quasi-permanent pastures by repeated rainfall in the Sahara up to 25°N. Certainly, this was the base of a strong nomad economy providing the base for their dominance over sedentary people.

All voyagers agreed on the rich and diverse game in Sahel and Sudan. They mentioned in particularthe large elephant populations. Together with the large extension of *Acacia*-forests-thickets, we have to think on elephant-landscapes [78–82]. Elephants are known as landscape engineers. They produce a twofold landscape. For the one they transform forests to medium high thickets by positive and negative selection of tree species and the elimination of high trees and for the other they also create new structures. They open the forests for their tracks giving chance to grasses, herbs or shrubs with a ruderal behaviour. In this way, they form thickets and provide dry and combustible material. So, elephants also create fire-prone landscapes too. Elephants are bound to water and they make or enlarge pools for drinking, bathing and also the uptake of minerals. As they are social animals with a long life period, they school their young generations to maintain and preserve these types of landscapes. The *Acacia*-thickets around lake Chad or in northern Cameroon give an – even poor – model of these former landscapes.
