*6.3.2 The Guidimouni sediment record*

The sediment core was taken in 2013 in order to reconstruct the recent landscape and vegetation history [69]. The lakes of the Guidimouni depression are shallow lakes or ponds, and they are not more than 2 m in depth. However, their surface varies much during the year. In drought periods, the lakes may dry out (see [83]). Thus, one has also to think on the risk of disturbance by wind and breaking waves and also of deposition gaps caused by desiccation. A 70 cm-long tube could be enforced into the sediments of the western lake. The sediment record consists of silty or sandy gyttias with a variable content of organic matter. Four thin sections were made in the Mineralogy Department of Szeged University, Hungary. They should help to understand the sedimentation processes and also detect possible zones of reworked sediments.

At a first look, the sediments seem to be uniform or amorphous. However, at 400× magnification, it was possible to discriminate into two mayor features, which are explained by **Figure 16**. Under a disturbed section of about 12 cm, the deposits are organised in fine – millimetric – layers, which are separated by algae/bacteria films, respectively, by their jellies. These are always densely coloured by Fe-oxides. The uppermost sediment is mixed and does not show a distinct structure, but it depicts the presence of diatoms. Thus, the sedimentation starts with an inwash/ inflow of sandy-silty material and alterated organic matter. On this layer, a film of algae/bacteria-jelly is formed indicating a eutrophic and energy-rich shallow water body. It fixes the sediment beneath. Small arrows indicate the positions of these films. However, the water-rich and unstable layers of the upper cm are exposed to wave action, slumping phenomena or other disturbances. So, they may be contorted, displaced or mixed again. The upper two columns of **Figure 16** show these phenomena. The central part of the record (about 20–53 cm), however, is mainly made of sands or silt, but still separated by the algae layers. There is information that this part belongs to the drought period of the 1970/80s. During this time, the lakes became almost dry as reported by locals (Adamou, frdl. comm.). Anyway, a certain amount of water still must have persisted to allow the formation of the algae/bacteria films. The lowest thin section depicts an in-wash of weathered middle and coarse sand and a dense organic rich gyttia, which again is divided by algae/bacteria layers. The general formation of bacteria/algae films will counteract the disturbance effects of waves in the shallow water. Considering these facts, a sampling with a distance less than 5 cm seemed not to be useful – out of the disturbance risk.

An important feature is the regular presence of charred material. It is made of grass coal-flitters consisting of cuticulae, leaves or parenchyma remains. Charcoal from wood seems to be very rare. These flitters are kept in the thin layers and are oriented along the algea-films. Thus, during the time of the deposition of the record, fire always was an important part of landscape dynamics. At present, the inhabitants regularly use fire to clear the dune area and the reeds in order to prepare their fields. So, it is likely to adopt this model also for the past. It is indicated by the regular presence of grass-coal flitters. Coarse ones will not have been transported over long distances.

### *6.3.3 Vegetation history of the last 100 years*

The detection of the stabilising bacteria/algae films visible in the thin sections allowed exploiting the record for pollen analysis. The diagram (**Figure 16**) was

**81**

**realitiy**

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

constructed on the base of all pollen but aquatics were excluded. The most of the arboreal and non-arboreal elements show only values of less than 1%. Thus, they are only represented for their presence in the diagram. The pollen diagram is characterised by the elements of an open Sahelian savanna of the *Acacia-Balanites*-type. Dominant are grasses and aquatics (*Typha,* Cyperaceae). Cerealia are persistent.

Three pollen zones could be discriminated on the base of the variation *Typha*-Gramineae for the one and for the other on the base of the diversity of floristic

PZ I. 65–40 cm: The aquatics have high values against the low values of grasses. Arboreal pollen shows a relatively high diversity including some Sudanian/Sahelian

PZ II. 40–23 cm: The part of the aquatics is reduced by rising values of grasses.

PZ III. 23–0 cm: There is a rise of the aquatics against reduced values of grasses.

The charcoal record, which mainly consists of grass coal, depicts the general presence of fire in the region as it is. It still today comprises flaming of the reeds in order to get place for new fields and also slash and burn on the dune slopes. The sharp rise in PZ III represents an accelerated burning for new fields after the end of

The nature of the sediments will not allow a radiocarbon dating. However, the presence of *Eucalyptus* and *Casuarina* (see the arrows in the diagram) shows that the sequence is not older than the beginning of the twentieth century. The colonial authorities of Nigeria planted both tree species as ornamental or afforestation elements as well as roadside-trees [84, 85]. Their pollen takes part in the long distance transport. Thus, in combination with sediment modes and the fact that PZ II is apparently contemporaneous to the desiccation of the lake during the drought of the 1970s up to the beginning of the 1980s, the base of the core might be deposited

The only comparable record reaching to the present time is that of Oursi in Burkina Faso [86], which shows a similar open vegetation due to extensive agriculture and animal breeding. However, the record of grass coal stands unique also compared to the upper parts of the Manga lake records [87–89]. But tiese lakes did not provide suitable sediments to follow them up to the present. This record is the first to discriminate between the two main elements in the charred material (grasses

**7. Regeneration. A confusion of concepts, different observations and** 

The discussion about regeneration is controversial. There is the position of a definite or long-time degradation, which Miehe et al. [90] explain by a short time of resilience and then a declining to a lower ecological equilibrium. This corroborates the conclusion from the Guidimouni-record as presented above. Hahn and Kusserow [91] and Kusserow [92] report a severe degradation of the Sahel from remote sensing

The arrows point to the *Casuarina*- and *Eucalyptus* curves.

The diversity of arboreal and non-arboreal elements is reduced too.

Trees and shrubs recover but do not reach to the diversity of PZ I.

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

elements (*Guiera, Khaya*, Combretaceae).

elements:

*6.3.4 Charcoal*

the drought period.

during the l920 years.

and trees) – at least for the Sahel.

**6.4 Time frame**

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

constructed on the base of all pollen but aquatics were excluded. The most of the arboreal and non-arboreal elements show only values of less than 1%. Thus, they are only represented for their presence in the diagram. The pollen diagram is characterised by the elements of an open Sahelian savanna of the *Acacia-Balanites*-type. Dominant are grasses and aquatics (*Typha,* Cyperaceae). Cerealia are persistent.

The arrows point to the *Casuarina*- and *Eucalyptus* curves.

Three pollen zones could be discriminated on the base of the variation *Typha*-Gramineae for the one and for the other on the base of the diversity of floristic elements:

PZ I. 65–40 cm: The aquatics have high values against the low values of grasses. Arboreal pollen shows a relatively high diversity including some Sudanian/Sahelian elements (*Guiera, Khaya*, Combretaceae).

PZ II. 40–23 cm: The part of the aquatics is reduced by rising values of grasses. The diversity of arboreal and non-arboreal elements is reduced too.

PZ III. 23–0 cm: There is a rise of the aquatics against reduced values of grasses. Trees and shrubs recover but do not reach to the diversity of PZ I.

### *6.3.4 Charcoal*

*Plant Communities and Their Environment*

*6.3.2 The Guidimouni sediment record*

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.

The sediment core was taken in 2013 in order to reconstruct the recent landscape and vegetation history [69]. The lakes of the Guidimouni depression are shallow lakes or ponds, and they are not more than 2 m in depth. However, their surface varies much during the year. In drought periods, the lakes may dry out (see [83]). Thus, one has also to think on the risk of disturbance by wind and breaking waves and also of deposition gaps caused by desiccation. A 70 cm-long tube could be enforced into the sediments of the western lake. The sediment record consists of silty or sandy gyttias with a variable content of organic matter. Four thin sections were made in the Mineralogy Department of Szeged University, Hungary. They should help to understand the sedimentation processes and also detect possible zones of reworked sediments. At a first look, the sediments seem to be uniform or amorphous. However, at 400× magnification, it was possible to discriminate into two mayor features, which are explained by **Figure 16**. Under a disturbed section of about 12 cm, the deposits are organised in fine – millimetric – layers, which are separated by algae/bacteria films, respectively, by their jellies. These are always densely coloured by Fe-oxides. The uppermost sediment is mixed and does not show a distinct structure, but it depicts the presence of diatoms. Thus, the sedimentation starts with an inwash/ inflow of sandy-silty material and alterated organic matter. On this layer, a film of algae/bacteria-jelly is formed indicating a eutrophic and energy-rich shallow water body. It fixes the sediment beneath. Small arrows indicate the positions of these films. However, the water-rich and unstable layers of the upper cm are exposed to wave action, slumping phenomena or other disturbances. So, they may be contorted, displaced or mixed again. The upper two columns of **Figure 16** show these phenomena. The central part of the record (about 20–53 cm), however, is mainly made of sands or silt, but still separated by the algae layers. There is information that this part belongs to the drought period of the 1970/80s. During this time, the lakes became almost dry as reported by locals (Adamou, frdl. comm.). Anyway, a certain amount of water still must have persisted to allow the formation of the algae/bacteria films. The lowest thin section depicts an in-wash of weathered middle and coarse sand and a dense organic rich gyttia, which again is divided by algae/bacteria layers. The general formation of bacteria/algae films will counteract the disturbance effects of waves in the shallow water. Considering these facts, a sampling with a distance

less than 5 cm seemed not to be useful – out of the disturbance risk.

*6.3.3 Vegetation history of the last 100 years*

An important feature is the regular presence of charred material. It is made of grass

The detection of the stabilising bacteria/algae films visible in the thin sections allowed exploiting the record for pollen analysis. The diagram (**Figure 16**) was

coal-flitters consisting of cuticulae, leaves or parenchyma remains. Charcoal from wood seems to be very rare. These flitters are kept in the thin layers and are oriented along the algea-films. Thus, during the time of the deposition of the record, fire always was an important part of landscape dynamics. At present, the inhabitants regularly use fire to clear the dune area and the reeds in order to prepare their fields. So, it is likely to adopt this model also for the past. It is indicated by the regular presence of grass-coal flitters. Coarse ones will not have been transported over long distances.

**80**

The charcoal record, which mainly consists of grass coal, depicts the general presence of fire in the region as it is. It still today comprises flaming of the reeds in order to get place for new fields and also slash and burn on the dune slopes. The sharp rise in PZ III represents an accelerated burning for new fields after the end of the drought period.

### **6.4 Time frame**

The nature of the sediments will not allow a radiocarbon dating. However, the presence of *Eucalyptus* and *Casuarina* (see the arrows in the diagram) shows that the sequence is not older than the beginning of the twentieth century. The colonial authorities of Nigeria planted both tree species as ornamental or afforestation elements as well as roadside-trees [84, 85]. Their pollen takes part in the long distance transport. Thus, in combination with sediment modes and the fact that PZ II is apparently contemporaneous to the desiccation of the lake during the drought of the 1970s up to the beginning of the 1980s, the base of the core might be deposited during the l920 years.

The only comparable record reaching to the present time is that of Oursi in Burkina Faso [86], which shows a similar open vegetation due to extensive agriculture and animal breeding. However, the record of grass coal stands unique also compared to the upper parts of the Manga lake records [87–89]. But tiese lakes did not provide suitable sediments to follow them up to the present. This record is the first to discriminate between the two main elements in the charred material (grasses and trees) – at least for the Sahel.
