Did a savanna corridor open up across the Central African forests

Did a savanna corridor open up
across the Central African forests
2500 years ago?
Jean Maley & Kathy Willis
[email protected] ; [email protected]
In order to reconstruct the vegetation dynamics in Central Africa over the past 3000
years, pollen analyses were collected from a number of sedimentary sequences spread
over the Congolian forest domain (Figure 1), from the Atlantic side (South Cameroon,
Gabon, Western Congo) to the eastern side of the Congo Basin region and the
surroundings of Victoria Lake (Maley & Brenac, 1998).
The Rain Forest s of cent ral Africa.
The main st udied sit es.
Barombi
Ossa
Mopo
Maridor
Epulu
L. Vict oria
Lopé
Osokari
Ngamakala
Sinnda
Kakamoéka
Kit ina
Old Forest s
Young Forest s
Swamp Forest s
Mount ain Forest s
Figure 1. The Congolian forest domain
Grass Savannas
Wooded Savanas
These records indicate that a significant change in the vegetation occurred
throughout the region between 2500 and 2000 BP, wrought by a major disturbance
which destroyed or strongly modified the forests. In addition, this disturbance still
exerts a major influence on present vegetation formations (Maley, 2001, 2002). In
several sites on the western and eastern sides of Congo (e.g. Barombi-Mbo and
Mayombe or Osokari and Epulu), a short period of savanna extension was triggered
by this event (Figure 2).
Synt het ic pollen diagram
of Lake Barombi Mbo, west Cameroon
Forest s
Minimum
( ca. 2 5 0 0 - 2 0 0 0 BP)
Forest s minimum
during t he Last Glacial Maximum
( ca. 2 4 0 0 0 à 1 8 0 00 cal. BP)
Age 1 4 C
( from Maley & Brenac, 19 9 8 )
Figure 2. Synthetic pollen diagram of Lake Marombi Mbo
Then, very rapidly, forests started to re-establish (Ngomanda et al., 2005), with a
flush of pioneer taxa appearing in many sites, and in particular the oil palm (Maley,
1999 ; Maley et Chepstow-Lusty, 2001), as was observed in the south of the Central
African Republic (CAR) (Figure 3).
Réserves de Nouabalé-Ndoki & de Dzangha-Sangha
Maxi. ca. 1 6 0 0 cal. BP
Ent re 1 9 8 8 et 1 9 9 5 , en parcourant t out le sud de la Cent rafrique, qui est act uellement couvert de f orêt s complèt ement
inhabit ées, le Biologist e américain Mike FAY a collect é de nombreux noyaux fossiles de Palmier à huile dans de nombreuses
coupes de sols ravinés par des ruisseaux et dans plus de 1 0 0 sit es dif f érent s. Le résult at des dat at ions au radiocarbone
ef f ect uées sur plus de 8 0 échant illons est présent é sur un diagramme de f réquence par t ranches de 1 0 0 ans ( dat es non
calibrées) . La forme de ce diagramme est t ypique d’une phase nat urelle de développement associée au remplacement
progressif d’une végét at ion pionnière par une plus mat ure. Dans la région du lac Ossa on ret rouve un pic d’ext ension maximum
du Palmier à huile dat é aussi vers 1 6 0 0 cal. BP, âge similaire à celui obt enu dans ce vast e sect eur.
( cf. Maley & Chepst ow-Lust y, 2 0 0 1 )
( Maley, 2 0 0 9 )
Figure 3
These pioneer forests persisted until 1000 – 800 years BP, then the forest recovery
continued until the present day, accompanied by an increasing importance of more
shade-tolerant taxa.
A core was recently collected in Mopo Swamp, located in Congo near the southern
frontier of CAR, close to the centre of the CoForChange study area (Brncic, Willis et
al., 2009).
The dominant pollen taxa found in this record spanning the last 2500 years BP have
revealed a vegetation history very similar to that previously outlined from the other
sites, with a brief savanna extension episode dated 2500 years BP (Figure 4). This
record confirms that there has been a greater spatial extension of the savanna
vegetation during this interval in time as previously thought.
Chronology
A pollen diagram obt ained by T. Brncic & al. ( 2 0 0 9 ) ,
in t he Mopo Bai, nort h of RD.Congo
( swampy area sout h of Dzanga-Ndoki Reserve)
was adapt ed by J. Maley.
The pollen spect rum was done on a one met er long core;
t he chronology based on several radiocarbon ( AMS) and
t hermo-luminescence dat at ions.
The bot t om of t he core dat ed around 2 5 0 0 cal. BP
corresponds t o t he beginning of t he phase wit h large
pert urbat ion and erosion evidenced in numerous
ot her places of t he Congolese Forest Domain
( Maley, 2 0 0 1 , 2 0 0 2 ; Ngomanda & al. 2 0 0 5 ) .
The variat ions of t he dominant pollen t axa show clearly :
1 ) t o t he base, a phase of large pert urbat ion
of t he habit at charact erized by a st rong ext ension
of Poaceae ( 3 6 %) ( here not aquat ic) ,
relat ed t o a short savanna ext ension.
2 ) Then rapidly one observe a st rong ext ension of several
pioneer t axa, wit h part icularly 4 0 % of Alchornea pollen
and already quit e 1 0 % of Elaeis guineensis.
Import ance of Pt eridophyt a spores are frequent ly
linked t o pert urbed environment s.
3 ) The peak of t hese diverse pioneer t axa occurred
at ca. 2 0 0 0 cal. BP, wit h part icularly 2 0 % for t he oil palm.
This palm t ree disappeared in t his area at ca. 9 0 0 cal. BP,
like t he kernels dat ed by M. Fay more t o t he nort h.
4 ) Finally, t he upper part of t he spect rum
show t he progressive spread of numerous t rees
of a more mat ure t ype which belong
t o t he present day forest habit at .
5 ) The present day swampy environment appeared
progressively bet ween 2 0 0 0 and 1 5 0 0 cal. BP.
Mat ure t axa
Pioneer t axa, 2 t ypes
Aquat ic t axa
Figure 4. Pollen Diagram in Mopo
Research undertaken in the CoForChange project shall try to validate the hypothesis,
proposed by Letouzey in 1968, then precised in 1985, and formalised and dated by
Maley in 2001 and 2002 (Figure 5), that a large savanna corridor once opened up
across the Central African forests and linked the Northern Sudanian savannas to the
Southern Batéké savannas.
Figure 5. Map of approximative areas of
residual forests
Approximative areas of residual forests
(blue-shaded areas) during the phase of
massive destruction which occurred
between 2500 and 2000 years BP. The
residual forests were mainly patchworks of
pioneer and mature forests. The blank
areas were mainly savannas. (Maley, 2001,
2002)
References
BRNCIC, T.M., WILLIS, K.J., HARRIS, D.J., TELFER, M.W., BAILEY, R.M. 2009. Fire and
climate change impacts on lowland forest composition in northern Congo during the last
2580 years from palaeoecological analyses of a seasonally flooded swamp. The Holocene, 19:
79-89.
LETOUZEY, R. 1968. Etude phytogéographique du Cameroun. Encyclopédie Biologique, 69,
508 p. (p. 227-233)
LETOUZEY, R. 1985. Notice de la carte phytogéographique du Cameroun au 1/500.000è.
Inst. Carte Intern. Végétation, Toulouse, et Inst. Rech. Agron., Yaoundé. (p. 80-81)
MALEY, J. & BRENAC, P. 1998. Vegetation dynamics, palaeoenvironments and climatic
changes in the forests of West Cameroon during the last 28,000 years. Rev. Palaeobot. &
Palyno., 99: 157-188.
MALEY, J. 1999. L'expansion du palmier à huile (Elaeis guineensis) en Afrique centrale au
cours des trois derniers millénaires : nouvelles données et interprétations. In S.Bahuchet &
al. (eds) L'Homme et la Forêt Tropicale. 237-254. Trav. Soc. Ecologie Humaine, Paris.
MALEY, J. & CHEPSTOW-LUSTY, A. 2001. Elaeis guineensis Jacq. (oil palm) fluctuations in
central Africa during the late Holocene : climate or human driving forces for this pioneering
species? Veget. Hist. Archaeobot., 10: 117-120.
MALEY, J. 2001. La destruction catastrophique des forêts d’Afrique centrale survenue il y a
environ 2500 ans exerce encore une influence majeure sur la répartition actuelle des
formations végétales. Syst. & Geogr. Plants, 71: 777-796.
MALEY, J. 2002. A catastrophic destruction of African forests about 2,500 years ago still
exerts a major influence on present vegetation formations. Bulletin Inst. Development
Studies, 33, 13-30, Brighton Univ.
NGOMANDA, A., K. NEUMANN, A. SCHWEIZER, J. MALEY. 2009. Seasonality change and
the third millenium BP rainforest crisis in southern Cameroon (central Africa). Quat. Res.,
71: 307-318.