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date: 25 September 2017

Vegetation at the Time of the African Humid Period

Summary and Keywords

An orbitally induced increase in summer insolation during the last glacial-interglacial transition enhanced the thermal contrast between land and sea, with land masses heating up compared to the adjacent ocean surface. In North Africa, warmer land surfaces created a low-pressure zone, driving the northward penetration of monsoonal rains originating from the Atlantic Ocean. As a consequence, regions today among the driest of the world were covered by permanent and deep freshwater lakes, some of them being exceptionally large, such as the “Mega” Lake Chad, which covered some 400 000 square kilometers. A dense network of rivers developed.

What were the consequences of this climate change on plant distribution and biodiversity? Pollen grains that accumulated over time in lake sediments are useful tools to reconstruct past vegetation assemblages since they are extremely resistant to decay and are produced in great quantities. In addition, their morphological character allows the determination of most plant families and genera.

In response to the postglacial humidity increase, tropical taxa that survived as strongly reduced populations during the last glacial period spread widely, shifting latitudes or elevations, expanding population size, or both. In the Saharan desert, pollen of tropical trees (e.g., Celtis) were found in sites located at up to 25°N in southern Libya. In the Equatorial mountains, trees (e.g., Olea and Podocarpus) migrated to higher elevations to form the present-day Afro-montane forests. Patterns of migration were individualistic, with the entire range of some taxa displaced to higher latitudes or shifted from one elevation belt to another. New combinations of climate/environmental conditions allowed the cooccurrences of taxa growing today in separate regions. Such migrational processes and species-overlapping ranges led to a tremendous increase in biodiversity, particularly in the Saharan desert, where more humid-adapted taxa expanded along water courses, lakes, and wetlands, whereas xerophytic populations persisted in drier areas.

At the end of the Holocene era, some 2,500 to 4,500 years ago, the majority of sites in tropical Africa recorded a shift to drier conditions, with many lakes and wetlands drying out. The vegetation response to this shift was the overall disruption of the forests and the wide expansion of open landscapes (wooded grasslands, grasslands, and steppes). This environmental crisis created favorable conditions for further plant exploitation and cereal cultivation in the Congo Basin.

Keywords: Pollen, African Humid Period (AHP), adaptive response, Green Sahara, Equatorial mountains

Introduction

When studying the causes of the dramatic drought that affected the Sahel during the 1970s, Charney (1975) pointed to the possible feedback of vegetation cover on climate. He suggested that a reduction of vegetation, with a consequent increase in albedo, contributes to significantly amplify and perpetuate the arid conditions. Furthermore, numerous authors have used the African Humid Period (AHP) (e.g., deMenocal et al., 2000) in northern Africa and the “greening of the Sahara” as benchmarks to discuss not only the role of orbitally driven insolation changes (COHMAP, 1988), but also to test several feedback mechanisms linked to changes in vegetation cover (e.g., Claussen & Gayler, 1997), as well as water surface extensions (e.g., Krinner et al., 2012) and sea surface temperatures (e.g., Braconnot, Joussaume, Marti, & de Noblet, 1999; Shanahan et al., 2009), on climate through different simulation experiments. An increasing number of model-data comparisons and paleosyntheses are aiming at validating both the models and the interpretations of the fossil records at selected key periods of the late Quaternary and at quantifying related temperature and rainfall changes (e.g., Jolly et al., 1998; Hoelzmann et al., 1998; Petit-Maire, 1999; Joussaume et al., 1999; Peyron et al., 2006). More recently, the focus was put on the timing and amplitude of the onset and termination of this dramatic change in climate and hydrology, whose effects went far beyond the north African tropics (see, e.g., Tierney & deMenocal, 2013; Shanahan et al., 2015 and references therein).

In this review, I use paleovegetation records to discuss range shifts and adaptive responses of tropical taxa to high-amplitude climate changes that occurred in the last few thousand years in north tropical Africa. For various reasons (political, geographical, and historical), the paleovegetation data coverage in this sector is extremely uneven, with some relatively well studied regions and other regions almost completely lacking data. In addition, most of the data originate from sedimentary sequences that are discontinuous, poorly dated, or both. This is particularly true in the Sahara and Sahel, where predominantly arid climate conditions led to the late Holocene desiccation of most of the lakes and wetlands, thus making continuous records of the AHP extremely rare (Watrin, Lézine, & Hély, 2009).

The AHP

An orbitally induced increase in summer insolation during the last glacial-interglacial transition enhanced the thermal contrast between land and sea and thus produced strong summer monsoons, which formed numerous lakes in regions that are arid today (COHMAP, 1988; also see Figure 1). In the Sahara, the discovery of extensive paleolakes and wetlands (Lézine, Hély, Grenier, Braconnot, & Krinner, 2011a; Lézine, Bassinot, & Peterschmitt, 2014), as well as evidence of pasturing and cereal farming in Neolithic settlements (Kuper & Kröpelin, 2006), illustrate the amplitude of the resulting environmental changes between such recent past humid period and the modern desert. Dated hydrological records used as a proxy of humidity in the Sahara and Sahel (Figure 2) show that deep, freshwater lakes progressively increased after 15.5 ka, to reach a maximum expansion at 9.5–8.5 ka. Then lake levels lowered, and lakes were replaced by wetlands. The number of lakes decreased steadily until the present day.

Vegetation at the Time of the African Humid PeriodClick to view larger

Figure 1. Dated records of lakes, swamps, and fluvial activity during the Holocene in northern Africa and the Arabian Peninsula (Lézine et al., 2014). Today, this area is among the driest regions in the world.

Vegetation at the Time of the African Humid PeriodClick to view larger

Figure 2. The number of dated records as a function of time provides an indication of the timing and amplitude of the AHP in the Sahara and Sahel. Note the asymmetrical pattern of changes, with swamps (palustrine) developing only in the second part of the period from c.8.5 ka only (Lézine et al., 2011a).

The Northward Spread of Tropical Forest Taxa and the Setting of the “Green Sahara”

Earlier studies have hypothesized that the Saharan desert disappeared and was replaced by wooded grasslands and savannas (e.g., Hoelzmann et al., 1998) forming the so-called Green Sahara. It was suggested that the Sahara/Sahel boundary was displaced north up to 23–25°N. Based on three detailed pollen sites from Sudan, Ritchie, Eyles, and Haynes (1985) and Ritchie and Haynes (1987) showed that the northern limit of the sparsely wooded desert steppes was 400 km north of its present-day position, at about 16°N, while the northern limit of the Sudanian woodlands, currently found at 14°N, would have been displaced north, to about 19°N, during the interval between 10 and 5 ka. Similarly, the regional vegetation of Chemchane, Mauritania (21°N) (Lézine, Casanova, & Hillaire-Marcel, 1990; Lézine, 1993) was Sudano-Sahelian in character, dominated by a dense graminoid cover with sparse tropical trees such as Celtis, Lannea, Rhus, Securinega virosa, and Alchornea, today found 400 to 500 km south. The presence of Sahelian taxa (such as Balanites and Mitracarpus) and of more humid types (such as Celtis, Rhus, Securinega, Hymanocardia, and Ficus) is also confirmed at places such as Gobero, Niger (17°N) (Sereno et al., 2008); Bilma, Niger (18°41N) (Schulz, Joseph, Baumhauer, Schultze, & Sponholz, 1990) Yoa, Chad (19°N) (Lézine, Zheng, Braconnot, & Krinner, 2011b); Seguedine, Niger (20°20’N) (Baumhauer & Schulz, 1984); and Taoudenni, Mali (22°30’N) (Schulz, 1991); and even northward, in the Lybian Desert, in the Wadi Teshuinat area (between 24°30 and 26°N) (Mercuri, 2008).

Watrin et al. (2009) then Hély, Lézine, and APD contributors (2014) have depicted a far more complex environment than the simple displacement of vegetation zones. Plants found today in distinct vegetation zones (tropical humid—Sudanian, and semidesertic—Sahelian and Saharan) were able to occupy a common space ranging roughly between 10 and 24°N (Figure 3). The inference is that the mosaiclike landscape that came about during the AHP led to a considerable increase of biodiversity in the Sahara and Sahel compared to those areas today.

Vegetation at the Time of the African Humid PeriodClick to view larger

Figure 3. Cooccurrence of four selected taxa from different origins today during the AHP in West and North Africa. (A) Their modern distribution: tropical humid (Celtis), from the Sahara/Sahel boundary (Salvadora), Saharan (Boscia), and from the Sahara/Mediterranean boundary (Ephedra); (B) their distribution during the AHP (from Watrin et al., 2009) versus time and latitude.

Tropical taxa probably entered the desert as gallery-forest formations along the dense network of rivers (Drake, Blench, Armitage, Bristow, & White, 2011) and water bodies (Lézine et al., 2011a), where they benefited from permanent fresh water, while the large dune fields and regs remained of the semidesert type.

As an illustration of the northward spread of tropical trees during the AHP, Figure 4 shows the behavior of Alchornea, Syzygium, and Celtis, all tropical trees that may occur in a wide range of habitats today, including gallery forests near rivers and open water bodies; and Piliostigma, which grows in woodlands and wooded grasslands. The modern range of all these taxa in western Africa does not extend beyond 16–18°N to the north. During the Holocene, these species were present at up to 20–25°N, which is 3–7° north of their northernmost geographical limit today.

The spread of tropical trees into areas currently under Sahelian and Saharan climates started roughly around 12.5 ka. Figure 3A illustrates such a northern spread pattern, although each species migrated at its own rate. Celtis was by far the most dynamic taxon, as it has been identified at up to 25°N (Mercuri & Grandi, 2001, Mercuri, 2008) and probably even northward, at Kharga, Egypt (25.26°N) (Monod, 1943; also see Figure 3C). Two main patterns of migration can be distinguished (Figure 3B):

  • The entire ranges of some taxa were markedly distinct from their current ranges: Piliostigma, for instance, has not been recorded at its present-day latitudes during the AHP, but only north of them, between 10 and 20°N.

  • Other taxa enlarged their present-day distributional ranges considerably: this is the case for Alchornea, Celtis, and Syzygium, whose limit moved north by roughly 7° latitude.

Table 1. Sites and Reference of Sites Shown in Figure 4

Site

Country

Latitude

Longitude

Proxy

Reference

Amekni

Algeria

22.786

5.519

Pollen

Guinet and Planque (1969)

Angéla-Kété

Chad

15.833

18.35

Pollen

Maley (1981)

Ari Koukouri

Niger

13.91S7

13.1

Pollen

Schulz, Pomel, Abichou, and Salzmann (1995)

Baie de Saint Jean

Mauritania

19.473

‒16.301

Pollen

None

Bilma

Niger

18.683

12.916

Pollen

Schulz (1994)

Bir Atrun

Sudan

18.166

26.65

Pollen

Ritchie (1987)

Chemchane

Mauritania

20.933

‒12.216

Pollen

Lézine (1993)

El Atrun

Sudan

18.166

27.15

Pollen

Jahns (1995)

Gobero

Niger

16.986

9.154889

Pollen

Sereno et al. (2008)

Kouka

Chad

13.1

15.633

Pollen

Maley (1981).

Lake Yoa

Chad

19.057

20.500

Pollen

Lézine et al. (2011).

Mandi

Chad

13.383

14.75

Pollen

Maley (1981)

Oyo

Sudan

19.26

26.18

Pollen

Ritchie et al. (1985)

Selima Oasis

Sudan

21.366

29.31

Pollen

Ritchie and Haynes (1987)

Taoudenni

Mali

22.5

‒4

Pollen

Cour & Duzer (1976)

Uan Afuda Cave

Libyan Arab Jamahiriya

24.868

10.500

Pollen

Mercuri (1999)

Uan Muhuggiag

Libyan Arab Jamahiriya

24.842

10.513

Pollen

Mercuri, Trevisan Grandi, Mariotti Lippi, and Cremaschi (1998)

Uan Tabu

Libyan Arab Jamahiriya

24.859

10.528

Pollen

Mercuri and Trevisan Grandi (2001)

Amekni

Algeria

22.893

5.24

Seed/Charcoal

Camps (1969)

Dogonboulo

Niger

18.3

11.55

Seed/Charcoal

Neumann (1992)

Kharga

25.44

30.558

Seed/Charcoal

Monod (1943)

Oued el Abid

Mauritania

18.90

‒12.35

Seed/Charcoal

Monod (1943)

Tichitt

Mauritania

18.46

‒9.366

Seed/Charcoal

Munson (1971)

Toungad

Mauritania

20.083

‒13.133

Seed/Charcoal

Monod (1943)

Vegetation at the Time of the African Humid PeriodClick to view larger

Figure 4. Migrational patterns of four tropical taxa (Alchornea, Celtis, Piliostigma, and Syzygium) during the AHP. (A) Date of their first appearance at a given latitude; (B) (a) their modern distribution in West and North Africa, (b): their Holocene distribution versus time and latitude (from Watrin et al., 2009). Data (extracted from the African Pollen Database) are presented under the form of probability density functions (PDFs) from the plant/pollen presence. Two probability thresholds were defined as the limit of the most probable occurrence of a pollen taxon in this spatiotemporal space: the 85% threshold displays the most similar contour compared to the limit of the presence whereas the 50% threshold points to the maximum of the distribution. (C) Distribution of Celtis pollen (red dots) and plant macro remains (charcoal, seeds) (green dots) of Holocene age in the Sahara and Sahel. Limits of the Sahara and Sahel after White (1983) (Table 1).

The Expansion of Forests Near the Equator at the Onset of the AHP

In the western Congo Basin, continuous pollen sequences covering the whole AHP are extremely rare. In this context, a postglacial recolonization of the lowland rain forest from glacial refugia that was restricted to inselbergs or along rivers, as hypothesized by Hamilton (1982) and Maley (1987), or more recently by Runge (2014), is poorly documented. Genetic studies on modern rain forest trees show discontinuities (Hardy et al., 2013), which supports the occurrence of forest fragmentation and refugia during Pleistocene climate oscillations. However, the timing of these genetic discontinuities remains unresolved. If the fragmentation of the Equatorial forest block during the last glacial period remains debated, pollen data converge to confirm earlier hypotheses (Aubréville, 1949), according to which its northern limit would have been displaced south during the last glacial period. Biome reconstructions at two sites from the northern edge of the forest in NW Cameroon, Barombi Mbo (Lebamba, Vincens, & Maley, 2012; Maley & Brenac, 1998), and Bambili (Izumi & Lézine, 2016), show that the forest biomes were strongly reduced at that time, while forests probably entirely disappeared west of the Dahomey Gap. Forests started to develop immediately following the last glacial period at 17 ka in the lowlands surrounding Lake Barombi Mbo due to exceptionally humid climate conditions related to the proximity of the Atlantic Ocean. To the east, at Bambili in the highlands (Lézine et al., 2013a), forests developed only about 2000 years later, around 14.7–15 ka. Then, forests reached their maximum development and mature stage (c.11.7–11.5 ka).

Vegetation at the Time of the African Humid PeriodClick to view larger

Figure 5. Upward migration of Olea and Podocarpus since the last glacial period in western Africa (from Lézine et al., 2013a, modified—pollen %) (Table 2).

The behavior of two of the emblematic trees of the mountain forests today, Olea and Podocarpus, give insight into the specific tolerance of tropical mountain taxa to climate change since the last glacial period. (Figure 5). Olea occurred in a wider elevational range from the lowlands, where they developed during the last glacial (Barombi Mbo), to the Cameroon highlands during the Holocene (Bambili): This taxon left the lowlands and migrated upward to form well-identified stands near Lake Tilla in Nigeria (Salzmann, Hoelzmann, & Morczinek, 2002) between 11.5 and 9.5 ka, and probably at the same time near Bilanko in the Bakété plateaus to the south (Elenga, Vincens, & Schwartz, 1991) and then expanded at Bambili from 10.3 ka onward. Elevation ranges of Olea during the Holocene also included the Adamawa plateaus at about 1,000 m asl (Vincens, Buchet, & Servant, 2010). Unlike Olea, Podocarpus was absent from the lowlands of NW Cameroon during the last glacial period, occurring only in the Bamenda plateaus near Bafounda (Kadomura & Kiyonaga, 1994). Podocarpus populations, however, were extensive south of the Equator, as evidenced by deep sea pollen records off Africa (Dupont, Jahns, Marret, & Shi, 2000), particularly in the mountains of Angola and southern Democratic Republic of Congo, where relict populations still occur (e.g., Mills, Melo, & Vaz, 2013). Podocarpus populations left the middle elevations during the glacial-deglacial transition (Elenga, Vincens, & Schwartz, 1991) and then developed in the highlands, at Bambili, after 10 ka.

Table 2. Sites and Reference of Sites Shown in Figure 5

Site

Country

Latitude

Longitude

Proxy

Reference

Bafounda

Cameroon

5.533

10.333

Pollen

Tamura (1990)

Bambili

Cameroon

5.936

10.242

Pollen

Lézine et al. (2013)

Barombi Mbo

Cameroon

4.662

9.404

Pollen

Maley and Brenac (1998)

Bilanko

Congo

‒3.516

15.35

Pollen

Elenga et al. (1991)

Mbalang

Cameroon

7.316

13.733

Pollen

Vincens et al. (2010)

Tilla

Nigeria

10.390

12.1245

Pollen

Salzmann et al. (2002)

The End of the AHP

At the end of the AHP, the majority of sites in tropical Africa recorded a shift to drier conditions, with many lakes and wetlands drying out (Gasse, 2000; Lézine et al., 2011a). Vincens et al. (1999), and then Lézine (2007), showed that the vegetation response to this shift was the overall disruption of the forests and the wide expansion of open landscapes (wooded grasslands, grasslands, or steppes). This environmental crisis took place between 4.5 and 2.5 ka according to local environmental (hydrological and morphological) conditions (Figure 6).

Table 3. Sites and Reference of Sites Shown in Figure 6

Site

Country

Latitude

Longitude

Proxy

Reference

Bafounda

Cameroon

5.533

10.333

Pollen

Tamura (1990)

Baie de Saint Jean

Mauritania

19.473

‒16.301

Pollen

None

Bal

Nigeria

13.304

10.943

Pollen

Waller and Salzmann (1999)

Bambili

Cameroon

5.936

10.242

Pollen

Lézine et al. (2013)

Barombi Mbo

Cameroon

4.662

9.404

Pollen

Maley and Brenac (1998)

Bilanko

Congo

‒3.516

15.35

Pollen

Elenga et al. (1991)

Bosumtwi

Ghana

6.5

‒1.416

Pollen

Maley and Livingstone (1983)

Coraf

Congo

‒4.75

11.85

Pollen

Elenga et al. (2001)

Diogo

Senegal

15.266

‒16.8

Pollen

Lézine (1988)

Kaigama

Nigeria

13.251

11.5675

Pollen

Waller and Salzmann (1999)

Kajemarum

Nigeria

13.303

11.024

Pollen

Waller and Salzmann (1999)

Kitina

Congo

‒4.268

12

Pollen

Elenga et al. (1996)

Lake Yoa

Chad

19.057621

20.500690

Pollen

Lézine et al. (2011)

Mbalang

Cameroon

7.316

13.733

Pollen

Vincens et al. (2010)

Mboandong

Cameroon

4.5

9.4

Pollen

Richards (1986)

Mopo Bai

Congo

2.240

16.261

Pollen

Brncic et al. (2009)

Ngamakala

Congo

‒4.071

15.383

Pollen

Elenga et al. (1994)

Nguène

Gabon

‒0.2

10.466

Pollen

Ngomanda et al. (2007)

Njupi

Cameroon

6.45

10.316

Pollen

Zogning et al. (1997)

Nyabessam

Cameroon

2.4000

10.400

Pollen

Ngomanda et al. (2009)

Ossa

Cameroon

3.800

10.75

Pollen

Reynaud-Farrera et al. (1996)

Sélé

Benin

7.15

2.433

Pollen

Salzmann and Hoelzmann (2005)

Shum Laka

Cameroon

5.85

10.05

Pollen

Kadomura and Kiyonaga (1994)

Sinnda

Congo

‒3.834

12.802

Pollen

Vincens et al. (1998)

Songolo

Congo

‒4.7585

11.859

Pollen

Elenga et al. (2001)

Tilla

Nigeria

10.390

12.1245

Pollen

Salzmann et al. (2002)

Tizong

Cameroon

7.25

13.583

Pollen

Lebamba et al. (2016)

Touba N’Diaye

Senegal

15.166

‒16.866

Pollen

Lézine (1988)

Vegetation at the Time of the African Humid PeriodClick to view larger

Figure 6. The last environmental crisis at the end of the AHP in western Africa (from Vincens et al., 1999 and Lézine, 2007, modified)

The Retreat of Tropical Plant Communities from the Saharan Desert

All the tropical elements disappeared from the gallery forest that previously expanded in the Sahara. Whether this retreat was progressive or abrupt has long been a matter of debate, mainly due to the lack of continuous sedimentary archives in the Sahara, which severely hampers our view of the end of the AHP. Based on marine records off western Africa, Shanahan et al. (2015) suggested that the end of the AHP was abrupt, at 20.8°N (and 5.5 ka), but occurred later and progressively at lower latitudes, reflecting the gradual southward migration of the tropical rain belt.

Recent studies at In-Atei, in southern Algeria (Lecuyer et al., 2016), and from the Gulf of Aden (Fersi, Lézine, & Bassinot, 2016) shed new light on the timing of the environmental degradation that resulted in the present-day semiarid landscape. The surroundings of In-Atei became increasingly dry from 7.9 ka onward, a condition that closely matches earlier results at Oyo, Sudan, where the dramatic decline in Poaceae at 7.5 ka was interpreted as reflecting the replacement of the original wooded savanna by a drier, semidesert environment (Ritchie, 1994). East of the Sahara, Fersi et al. (2016) depict the progressive drying of the environment in the surroundings of the Gulf of Aden starting at 7.5 ka and reaching a maximum at 5.5 ka.

The discovery of groundwater-fed Lake Yoa, in the hyperarid desert of northern Chad, provides a unique, continuous sedimentary sequence from 6 ka to the present that is available in the entire Saharan desert (Kröpelin et al., 2008; Lézine et al., 2011b). At Yoa, the retreat of tropical tree taxa was gradual and ended about 4 ka. Then the modern desert environment definitely took place at 2.7 ka.

The Fragmentation of the Equatorial Forest Domain

At the end of the AHP, the floristic composition, structure, and geographical distribution of the Equatorial forests were greatly affected: forests collapsed at high and middle elevations (Vincens et al., 2010; Lézine et al., 2013b; Lebamba et al., 2016) whereas secondary forest formations characterized by light-demanding trees expanded in the lowlands of the Congo basin (e.g., Reynaud-Farrera, Maley, & Wirrmann, 1996; Brncic, Willis, Harris, Telfer, & Bailey, 2009; Ngomanda, Neumann, Schweizer, Maley, 2009) at the expense of former evergreen and semideciduous forests. Most of the savannas today found within the forest massif (Schwartz, Dechamps, Elenga, Mariotti, & Vincens, 1996) or at its immediate fringe (Runge, 2002; Desjardins et al., 2013) originated in this time period. This is also the case for the “Dahomey Gap,” an open corridor composed of a mixture of woodlands and savannas that separates the upper Guinean forest block to the west from the lower one to the east between western Nigeria and Ghana (Salzmann & Hoelzmann, 2005; Hély, Braconnot, Watrin, & Zheng, 2009).

All the pollen records suggest the abrupt character of this forest disruption. At Bambili, for instance, arboreal percentages dropped by 40% in less than 300 years. It has been suggested that this forest collapse resulted from the progressive destabilization of the forest massif linked to the increased length of the dry season since the mid-Holocene.

The role of human populations in the origin of open landscapes in central Africa has been proposed (e.g., Bayon et al., 2012). However, there is no direct (neither anthropogenic nor natural) evidence for forest destruction by human populations. If it is admitted that the collapse of the rain forest created favorable conditions for further plant exploitation and cereal cultivation (Ngomanda et al., 2009) and thus facilitated the spread of agricultural communities throughout Central Africa, its origin is clearly attributed to climatic change (Neumann et al., 2012). A thorough analysis of the temporal relationship between phases of human occupation and of vegetation changes in Cameroon (Lézine et al., 2013b) shows that the forest collapse predated the spread of human settlements and the development of plant exploitation (Figure 7).

Vegetation at the Time of the African Humid PeriodClick to view larger

Figure 7. Temporal relationship between phases of human occupation and phases of forest development in Cameroon during the Holocene (Lézine et al., 2013b). The number of dated archeological records is shown in pink and of the dated remains of edible plants in light blue. Green lines correspond to arboreal pollen curves showing the collapse of the forest at 3.3 ka at Bambili in the highlands (Lézine et al., 2013a), and then at 3.0 ka at Mbalang and Tizong in the Adamawa plateaus (Vincens et al., 2010; Lebamba et al., 2016). The light green dotted line shows the development of light demanding trees within the rain forest in the lowlands from 2.4 ka onwards (here at Lake Ossa; Reynaud-Farrera et al., 1996).

Conclusion

This review makes the following points:

  • Tropical taxa widely expanded during the AHP, which shifted latitudes or elevations, expanded population size, or both. These expansions were individualistic, and not merely one of existing vegetation zones. Obviously, biodiversity considerably increased since tropical and desert taxa cooccurred.

  • The end of the AHP reveals two major adaptive responses. In the Sahara, the retreat of tropical trees was progressive, starting from the mid-Holocene and worsening after 5.5–4 ka. Near the Equator, the tropical forests progressively destabilized from the mid-Holocene, and then a threshold was crossed, leading to the abrupt collapse of the forests, its slocal disruption, or both. Increased length of the dry season seems to have been a major driver of this environmental degradation.

Acknowledgments

Thanks are due to the African Pollen Database contributors and to S. J. Ivory (Brown University) for helpful comments that improved the manuscript. This research is part of projects funded by the National Research Funding Agency in France (ANR-09-PEXT-001 C3A), the Belgian Federal Science Policy Office (BR/132/A1/AFRIFORD), and the Belmont Forum “VULPES” project. The author is supported by the Centre National de la Recherche Scientifique (CNRS), France.

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