record of Lake Tilla (site 14, Fig. 9.1a), with a gradual (during the African Humid
Period termination) and abrupt (during 3000 cal yr BP crisis) floristic shifts from
Guinean to Sudano-Guinean savanna (Salzmann 2000). As illustrated in Lake
Mbalang (site 1, Fig. 9.1a) (Vincens et al. 2010), the first phase of this abrupt
climatic change triggered a transition from forest to savanna taxa (Fig. 9.1c) in
some sites.
The second phase was rather short and abrupt, lasting between 2500 and
2000 cal yr BP. The SST reconstructions (Fig. 9.1e) and geological limestone
zones (Maley et al. 2012, 2018) suggest a strong climatic seasonality. During this
phase (in gray, Fig. 9.1b–e), vegetation reconstructions from pollen data show an
increasing abundance of pioneer and secondary forest trees, and even
in grasses within forest areas (Vincens et al. 1999) (Fig. 9.1b). This suggests that
forests were highly disturbed during this period, probably exhibiting mosaics of
open patches and closed forests. Some authors even suggest the opening of a north–
south savanna corridor in the Sangha River Interval (Maley and Willis 2010) that
could have permitted the migration of Bantu-speaking people. However, the savanna
corridor has been controversial (Bremond et al. 2017). The evidence of increased
human population and activity in the forest zone is dated from ~2500 cal yr BP, with
the first iron-age settlements (Fig. 9.1d) (Wotzka 2006). Human presence in the
region generally shows a bimodal pattern (Fig. 9.1d, red highlight), with two phases
of human population expansion, and an intermediate phase of depopulation, between
~1300 and ~700 cal yr BP, but of unknown origin (Morin-Rivat et al. 2014, 2017).
9.3.2 Current Vegetation Distribution
9.3.2.1 Vegetation Types
Currently, tropical forest (in green, Fig. 9.2a) is the most important vegetation type
in central Africa, covering 42% of the study area, followed by savanna (yellow,
28.4%), Miombo woodland (brown, 7%), mosaics of tropical forest and mesic
savanna (light green, 5%), and other types of forested vegetation, such as swamp
and swamp forest (3.0%), montane forest (1.3%), and mangrove (1%). The area
affected by human activity amounts to 13.3% of the study area (red, Fig. 9.2a).
Terra firme forests occupy a vast and continuous area across the Congo basin,
forming mosaics with southern savannas, such as the Nyanga river area and the
Batéké Plateau. Deforested areas (in red, Fig. 9.2a) are prevalent around major cities
such as Douala and Yaoundé in Cameroon, Brazzaville, and Ouesso in Congo,
Kinshasa, and Kisangani in DRC, and along major road networks and rivers that
connect them (Laurance et al. 2017). Deforestation is also prevalent in the densely
populated areas in the mountains of eastern DRC. Large savanna areas are located in
the northern and southern edge of central Africa, and are associated with high human
impacts, for example, in northern Cameroon, southern Congo, and DRC (Fig. 9.2a).
There is substantial floristic variation within these vegetation types. Terra firme
forests can broadly be divided into (a) evergreen forests under wet and aseasonal
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J. C. Aleman and A. Fayolle
Period termination) and abrupt (during 3000 cal yr BP crisis) floristic shifts from
Guinean to Sudano-Guinean savanna (Salzmann 2000). As illustrated in Lake
Mbalang (site 1, Fig. 9.1a) (Vincens et al. 2010), the first phase of this abrupt
climatic change triggered a transition from forest to savanna taxa (Fig. 9.1c) in
some sites.
The second phase was rather short and abrupt, lasting between 2500 and
2000 cal yr BP. The SST reconstructions (Fig. 9.1e) and geological limestone
zones (Maley et al. 2012, 2018) suggest a strong climatic seasonality. During this
phase (in gray, Fig. 9.1b–e), vegetation reconstructions from pollen data show an
increasing abundance of pioneer and secondary forest trees, and even
in grasses within forest areas (Vincens et al. 1999) (Fig. 9.1b). This suggests that
forests were highly disturbed during this period, probably exhibiting mosaics of
open patches and closed forests. Some authors even suggest the opening of a north–
south savanna corridor in the Sangha River Interval (Maley and Willis 2010) that
could have permitted the migration of Bantu-speaking people. However, the savanna
corridor has been controversial (Bremond et al. 2017). The evidence of increased
human population and activity in the forest zone is dated from ~2500 cal yr BP, with
the first iron-age settlements (Fig. 9.1d) (Wotzka 2006). Human presence in the
region generally shows a bimodal pattern (Fig. 9.1d, red highlight), with two phases
of human population expansion, and an intermediate phase of depopulation, between
~1300 and ~700 cal yr BP, but of unknown origin (Morin-Rivat et al. 2014, 2017).
9.3.2 Current Vegetation Distribution
9.3.2.1 Vegetation Types
Currently, tropical forest (in green, Fig. 9.2a) is the most important vegetation type
in central Africa, covering 42% of the study area, followed by savanna (yellow,
28.4%), Miombo woodland (brown, 7%), mosaics of tropical forest and mesic
savanna (light green, 5%), and other types of forested vegetation, such as swamp
and swamp forest (3.0%), montane forest (1.3%), and mangrove (1%). The area
affected by human activity amounts to 13.3% of the study area (red, Fig. 9.2a).
Terra firme forests occupy a vast and continuous area across the Congo basin,
forming mosaics with southern savannas, such as the Nyanga river area and the
Batéké Plateau. Deforested areas (in red, Fig. 9.2a) are prevalent around major cities
such as Douala and Yaoundé in Cameroon, Brazzaville, and Ouesso in Congo,
Kinshasa, and Kisangani in DRC, and along major road networks and rivers that
connect them (Laurance et al. 2017). Deforestation is also prevalent in the densely
populated areas in the mountains of eastern DRC. Large savanna areas are located in
the northern and southern edge of central Africa, and are associated with high human
impacts, for example, in northern Cameroon, southern Congo, and DRC (Fig. 9.2a).
There is substantial floristic variation within these vegetation types. Terra firme
forests can broadly be divided into (a) evergreen forests under wet and aseasonal
294
J. C. Aleman and A. Fayolle