*6.3.1 The physical situation of the Guidimound depression*

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 less permanent water body.

**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

*Plant Communities and Their Environment*

**6.2 Mapping the past**

**Figure 17.**

*6.2.1 Climate history*

*6.2.2 The vegetation*

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

*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.*

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.

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

in the Second World War destroyed the last preserved plant specimens [77].

**78**

some *Adansionia* trees were planted. A comparable picture was given in 1936 by Aubreville et al. [20]. However, the savannas on the dunes were not as dense as Barth described it but some Sudanian trees were still present, such as *Daniellia sp.*This situation was one of the strongest arguments against the idea of an enchroaching desert.
