change within RCP 2.6 is expected to be less intense and extensive, with only 1.6%
of forest areas (located at the western part of central Africa) expected to be affected
by rainfall change (Fig. 9.5c). On the contrary, according to RCP 8.5, as much as
62% of forests and 47% of savannas will be impacted by a decrease (or increase) in
annual rainfall, with a massive drying trend over central Africa, and a localized
increase in rainfall in the extreme south (Fig. 9.5d).
9.4 Discussion
9.4.1 Changes in Central African Forests and Savannas
The evidence reviewed in this study highlights that since the Last Glacial Maximum
(21,000 cal yr BP), the area covered by tropical forests experienced a succession of
dry and humid periods, which resulted in forest expansion and contraction (Maley
1989, 1996; Vincens et al. 1999) (Sect. 9.3.1). In contrast, forests in Amazonia
remained much more stable during that same period (Anhuf et al. 2006). Savannas
were more widespread and changed both in terms of extent and composition
following climate fluctuations. However, for some areas, such as Lake Bambili in
Cameroon (Site 4, Fig. 9.1a), there is accumulated evidence that savanna vegetation
has been maintained since the LGM, indicating a very stable state at this relatively
high altitude (>2000 m above sea level).
When analyzing the current extent and distribution of forest and savanna
according to the most recent vegetation types available for central Africa
(Verhegghen et al. 2012), we confirm that forests dominate the Congo basin, are
found under moist and wet climates, and experience only few fire events (Sect.
9.3.2). In contrast, savannas occupy drier areas in the northern and southern edge of
central Africa, and correspond to the Sudanian and Zambezian Savanna Regions
(White 1983). These savannas occur in areas of relatively high rainfall and experience frequent fires (Lehmann et al. 2014; Osborne et al. 2018). Forests and savannas
can both occur at intermediate rainfall (1000–2500 mm year
À1 ) (Sect. 9.3.2). This
seems to be the outcome of feedback mechanisms between canopy closure and fire,
past climatic changes, and, to a lesser extent, human occupation, as shown at the
continental (Sankaran et al. 2005; Aleman and Staver 2018) and the global (Staver
et al. 2011b) scales.
As the ecological functioning of forests and savannas depend on factors related to
both climate and land use, we then examined such ongoing changes in areas
currently covered by forests and savannas (Sect. 9.3.3). Over the past decades,
woody encroachment has been reported in savannas worldwide (Stevens et al.
2017), probably due to land use change and/or CO 2 fertilization effects. More
recently (i.e., between 2000 and 2015), there has been extensive tree cover loss in
the forest area (Hansen et al. 2013) and forest fragmentation due to industrial logging
(Laporte et al. 2007). This has been associated to hunting (Poulsen et al. 2011;
Abernethy et al. 2013) (Sect. 9.3.3).
9 Long-Term Vegetation Change in Central Africa: The Need for an Integrated. . .
301
of forest areas (located at the western part of central Africa) expected to be affected
by rainfall change (Fig. 9.5c). On the contrary, according to RCP 8.5, as much as
62% of forests and 47% of savannas will be impacted by a decrease (or increase) in
annual rainfall, with a massive drying trend over central Africa, and a localized
increase in rainfall in the extreme south (Fig. 9.5d).
9.4 Discussion
9.4.1 Changes in Central African Forests and Savannas
The evidence reviewed in this study highlights that since the Last Glacial Maximum
(21,000 cal yr BP), the area covered by tropical forests experienced a succession of
dry and humid periods, which resulted in forest expansion and contraction (Maley
1989, 1996; Vincens et al. 1999) (Sect. 9.3.1). In contrast, forests in Amazonia
remained much more stable during that same period (Anhuf et al. 2006). Savannas
were more widespread and changed both in terms of extent and composition
following climate fluctuations. However, for some areas, such as Lake Bambili in
Cameroon (Site 4, Fig. 9.1a), there is accumulated evidence that savanna vegetation
has been maintained since the LGM, indicating a very stable state at this relatively
high altitude (>2000 m above sea level).
When analyzing the current extent and distribution of forest and savanna
according to the most recent vegetation types available for central Africa
(Verhegghen et al. 2012), we confirm that forests dominate the Congo basin, are
found under moist and wet climates, and experience only few fire events (Sect.
9.3.2). In contrast, savannas occupy drier areas in the northern and southern edge of
central Africa, and correspond to the Sudanian and Zambezian Savanna Regions
(White 1983). These savannas occur in areas of relatively high rainfall and experience frequent fires (Lehmann et al. 2014; Osborne et al. 2018). Forests and savannas
can both occur at intermediate rainfall (1000–2500 mm year
À1 ) (Sect. 9.3.2). This
seems to be the outcome of feedback mechanisms between canopy closure and fire,
past climatic changes, and, to a lesser extent, human occupation, as shown at the
continental (Sankaran et al. 2005; Aleman and Staver 2018) and the global (Staver
et al. 2011b) scales.
As the ecological functioning of forests and savannas depend on factors related to
both climate and land use, we then examined such ongoing changes in areas
currently covered by forests and savannas (Sect. 9.3.3). Over the past decades,
woody encroachment has been reported in savannas worldwide (Stevens et al.
2017), probably due to land use change and/or CO 2 fertilization effects. More
recently (i.e., between 2000 and 2015), there has been extensive tree cover loss in
the forest area (Hansen et al. 2013) and forest fragmentation due to industrial logging
(Laporte et al. 2007). This has been associated to hunting (Poulsen et al. 2011;
Abernethy et al. 2013) (Sect. 9.3.3).
9 Long-Term Vegetation Change in Central Africa: The Need for an Integrated. . .
301