The impact of palaeoclimatic changes

The presence of coastal savannas c. 100 km south from the savanna belt in Côte d'Ivoire (e.g. in Dabou, Grand-Bassam or Grand-Lahou) and the persistence of rainforest outliers in the north of V-Baoulé (e.g. in Bamoro), indicate that the northern limit of the continuous Guinean rainforest has been fluctuating during the late Quaternary. Aubréville (1949) and Mangenot

(1955) hypothesised that these fluctuations could be congruent with the glaciations of the northern hemisphere, with cooler episodes corresponding to a drier climate (Aubreville 1962). With the development of various techniques such as palynology, sea level interpretation, diatoms and phytolithes, many data have been accumulated on past vegetation and climatic conditions and have confirmed these hypotheses. With the exception of Lake Bosumtwi in Ghana, there is a lack of paleobotanical data in West Africa due to the scarcity of lakes in which pollen accumulation can be studied. Reconstruction of paleoclimate is thus difficult and we have to refer to the general scheme for whole tropical Africa, as summarised below (Maley 1990, Thomas & Thorp 1992):

• From 30,000 to 12,000 years BP: a dry and cold phase, with its maximum between 20,000 and

15,000 years BP. The climatic swing has been estimated to be c. 8° latitude to the south. At this time, forest in Africa was restricted to a limited number of refuges, the main one in West Africa being located between present Liberia and Guinea (Maley 1989, Wieringa & Poorter chapter 6). Pollen types have been found of Afromontane species such as Olea hochstetteri or Podocarpus sp. (Fredoux 1994, Salzmann 2000). These species are nowadays absent from West Africa and only found on Mt Cameroon, indicating that this drier episode was also cooler, the temperature fall being estimated to be (3) 4-6 (7)°C.

• From 12,000 to 7000 BP: humid phase, extension of the forest to approximately its present limits.

• From 7000 to 3500 BP: warmer and wetter climate than at present. The forest extends to its maximum range.

• From 3500 BP to date: progressive decrease of temperature and humidity to meet its present values, the forest retreats to its present position.

It is worth mentioning that another dry maximum event has been documented for Central Africa c. 3000 BP. During the last millennium the forest has been in another phase of expansion (Vincens et al. 1999). From work on lake levels in Lake Bosumtwi (Maley 1987) and recent unpublished work in the Dahomey Gap, it appears that this dry episode also took place in West Africa (Salzmann & Neumann, unpublished results).

Palynological data have a rather coarse temporal resolution. This has to be taken into account when one tries to bridge the gap between these data and more recent historical records. For the last centuries, it is not clear whether an observed afforestation trend reflects a minor oscillation (decadal or secular) or can be fitted into the general scheme presented above (millennial).

These paleoclimatic oscillations and the subsequent displacement of forest and savanna have implications at several levels. First, it can explain the relative low diversity of West African forest flora in comparison to Central Africa; many species could have become extinct during the dry periods. Second, it gives very interesting insight into

Figure 3.3 The forest-savanna mosaic as seen from the ground in Lamto, Côte d'Ivoire. A) A clear-cut forest edge between a gallery forest and a herbaceous savanna, B) a smooth forest edge between a plateau forest (back, right) and a dense woody savanna (front, left).

the connections between Upper Guinea and the Congolian basin during humid periods, which might explain the disjunct distribution of certain species (see also Holmgren et al.chapter 7). Third, the present separation by the Dahomey Gap and the past separation of the Upper Guinea forest block itself by the extension of V-Baoule down to the sea could have led to allopatric speciation resulting in sister species on both sides of the separation (see Kouame et al.chapter 5 for implications for species composition). Fourth, the present extension of the Guinean savannas inside areas with climatic conditions favourable to forest can certainly be interpreted as the resilience of past distribution. A recent modelling study has shown that both savanna and forest could persist under the same conditions. By varying the dry season precipitation the model shows hysteresis between forest and savanna (Da Silveira & Sternberg 2001).

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