Eiszeitalter
u.
112 — 119
Gegenwart
Hannover
47
1997
5 flg.
Paleovegetation in West Africa for 18.000 B.P. and 8.500 B.P. DIETER ANHUF*
Q u a t e r n a r y , p a l e o c l i m a t e , p a l e o v e g e t a t i o n , s a v a n n a s , dry forest, s e m i - d e c i d u o u s a n d e v e r g r e e n rainforests
A b s t r a c t : T h e investigations w e r e concentrated o n the recon
ü b e r d i e Erfassung d e r r e z e n t e n V e g e t a t i o n s b e d e c k u n g si
struction o f former vegetation patterns for the Suclanian a n d
c h e r l i c h n u r in A u s n a h m e f ä l l e n d i e n a t ü r l i c h e V e g e t a t i o n
G u i n e a n s a v a n n a s a s well a s tropical s e m i - e v e r g r e e n a n d
erfaßt wird. Es stellt sich zunächst d i e F r a g e , w e l c h e V e g e -
e v e r g r e e n rainforests. A n u m b e r o f r e s e a r c h projects c o n
t a t i o n s f o r m a t i o n H i n w e i s e a u f ein n o c h w e i t g e h e n d natur
c e r n i n g c h a n g e s in t h e v e g e t a t i o n c o v e r h a v e s h o w n that
n a h e s P f l a n z e n k l e i d liefert.
e v e n tropical r e g i o n s h a v e b e e n effected b y t h e e n o r m o u s
D a s Material, d a s e i n e v e g e t a t i o n s ö k o l o g i s c h e A n a l y s e d e r
climatic o s c i l l a t i o n s o f t h e last 2 0 , 0 0 0 y e a r s . T h e p r o b l e m
Zeitscheiben
linked to t h e j u d g e m e n t o f a c h a n g i n g vegetation is t h e in
zunächst über eine Dokumentation vorhandener
c r e a s i n g effect o f h u m a n i n f l u e n c e o n t h e a p p e a r e n c e o f
t i o n e n zu P a l ä o v e g e t a t i o n u n d P a l ä o k l i m a
the W e s t African v e g e t a t i o n c o v e r . T h u s , all investigations
stellt. Es handelt
o f t h e t e m p o r a l c h a n g e in tropical e c o s y s t e m s a r e confront
S a c h g e b i e t e n Archäologie, G e o m o r p h o l o g i e , D e n d r o c h r o n o
e d with t h e fact that a r e a s w h i c h h a v e n o t b e e n i n f l u e n c e d
logie. Paläobotanik, Palynologie, Tiefseebohrung und Isoto
by m a n c a n rarely b e found. T h e primary q u e s t i o n is w h i c h
p e n a n a l y s e . D e r n ä c h s t e Schritt enthält d i e A b l e i t u n g r e c h
formations o f v e g e t a t i o n m a y a l l o w us t o d r a w c o n c l u s i o n s
n e r i s c h e r B e z i e h u n g e n d e r a k t u e l l e n V e g e t a t i o n zu K l i m a
a p p l i c a b l e t o a nearly natural vegetation. A first step is t h e
u n d B o d e n . D i e s e M o d e l l e d i e n e n dazu, d i e r e z e n t e n ( a k
estimation o f p o t e n t i a l forest c o m m u n i t i e s u n d e r r e c e n t cli
tuellen) Analogien Vegetation/Klima auch a u f die Paläo-
18.000
und 8.500
B.P. ermöglicht,
wurde Informa
zusammenge
sich dabei um Informationen
aus den
matic c o n d i t i o n s .
z e i t s c h e i b e n a n z u w e n d e n . Vorgestellt w e r d e n K a r t e n d e r
T h e derivation o f t h e p a l e o v e g e t a t i o n o f W e s t Africa w a s
Vegetationsbedeckung
based
( K l i m a p e s s i m u m ) u n d post 8 . 5 0 0 B . P . ( n a c h e i s z e i t l i c h e s
o n varied
published
information
concerning pa
l e o c l i m a t e a n d p a l e o v e g e t a t i o n . T h i s information is b o u n d to t h e s u b j e c t s o f a r c h a e o l o g y , g e o m o r p h o l o g y ,
West-Afrikas
für
18.000
B.P.
Klimaoptimum).
dendro
c h r o n o l o g y , p a l y n o l o g y , d e e p - s e a - c o r e a n d i s o t o p e analy
Introduction
sis. B y derivation o f t h e n u m e r i c relations b e t w e e n natural vegetation a n d climate u n d e r recent conditions, according models o f analogous vegetation climate o f paleoclimatic conditions could b e drawn. Presented are paleovegetation
V e g e t a t i o n m a p s for 1 8 . 0 0 0 B . P . ( h i g h e r aridity t h a n t o d a y ) a n d 8 . 5 0 0 B . P . ( h i g h e r humidity t h a n t o d a y )
m a p s o f W e s t Africa for 1 8 . 0 0 0 B . P . (glacial climatic pessi-
were
m u m ) a n d 8.500 B.P. (postglacial climatic optimum).
m a d e the potential quasi-natural v e g e t a t i o n
to
be
drawn.
Before
these
maps
could
be
forma
t i o n s in W e s t Africa u n d e r r e c e n t c l i m a t i c c o n d i t i o n s h a d t o b e investigated.
[Paläovegetation i n West-Afrika v o r 1 8 0 0 0 u n d 8 5 0 0 J a h r e n v o r heute]
The
problem
facing nearly
all r e s e a r c h
work
on
c h a n g e s in tropical ecological systems is that the regis Kurzfassung:
Die Arbeiten
eine Rekonstruktion sudanischen
früherer
konzentrierten
sich
darauf,
Vegetationsmuster
für d i e
und guineensischen
Savannenlandschaften
s o w i e für d i e t r o p i s c h e n h a l b i m m e r g r ü n e n u n d i m m e r g r ü n e n F e u c h t w ä l d e r zu erstellen. Z a h l r e i c h e
Untersuchun
tration o f c o n t e m p o r a r y vegetation o n l y s e l d o m e n c o m p a s s e s the natural vegetation. W h i c h t y p e s o f v e g e t a t i o n indicate a m o r e o r less natural v e g e t a t i o n , however?
g e n z u m z e i t l i c h e n V e g e t a t i o n s w a n d e l h a b e n gezeigt, d a ß
The
a u c h d i e t r o p i s c h e n R e g i o n e n v o n d e n n a c h h a l t i g e n Kli
u n d e r t a k e n in t h e Ivory Coast, a c o u n t r y w h i c h c a n
m a o s z i l l a t i o n e n d e r letzten 2 0 . 0 0 0 J a h r e nicht u n b e e i n f l u ß t
b e divided in t w o floristic areas: the G u i n e a r e g i o n in
g e b l i e b e n sind. E i n z e n t r a l e s P r o b l e m b e i d e r B e u r t e i l u n g
t h e s o u t h and the S u d a n region in t h e north.
e i n e r s i c h w a n d e l n d e n V e g e t a t i o n ist d e r in d e n j ü n g e r e n
T h e tropical o m b r o p h i l e forests w h i c h d o m i n a t e t h e
Zeitabschnitten ( a b ca. 8.500 B . P . ) z u n e h m e n d
floristic
anthropo-
z o o g e n induzierte Einfluß a u f d i e A u s p r ä g u n g d e r tropi schen Pflanzendecke. S o s e h e n sich s ä m t l i c h e A r b e i t e n z u m zeitlichen W a n d e l t r o p i s c h e r Ö k o s y s t e m e mit d e m P r o b l e m konfrontiert, d a ß *) Address o f t h e author: Priv.-Doz. Dr. D . ANHUF, G e o g r a p h i s c h e s Institut d e r Universität M a n n h e i m , 6 8 1 3 1 M a n n heim
g e o - b o t a n i c a l analysis n e c e s s a r y h e r e f o r e w e r e
G u i n e a r e g i o n c a n again b e s u b d i v i d e d into
e v e r g r e e n o m b r o p h i l e forests, in w h i c h the
annual
dry s e a s o n d o e s not e x c e e d o n e to t w o m o n t h s , a n d into
semi-deciduous
ombrophile
forests
with
not
m o r e t h a n three t o four m o n t h s o f dry s e a s o n ( A N H U F & FRANKFNBFRG, 1 9 9 D The
S u d a n r e g i o n is d o m i n a t e d
by
summer-green
forests. Most t r e e s s h e d their l e a v e s d u r i n g t h e arid
P a l e o v e g e t a t i o n in W e s t Africa for 1 8 . 0 0 0 B . P . a n d 8 . 5 0 0 B . P .
season, as it lasts for m o r e than four months. T h e n o r t h w a r d d e c r e a s e in precipitation w h i c h is c o m b i n e d with the prolongation o f t h e arid s e a s o n allows yet another subdivision o f these forests into dense dry forests (forets d e n s e s s e c h e s ) a n d o p e n dry forests (forets s e c h e s ) , the latter o f w h i c h g r o w in regions in which the arid season lasts longer than half a y e a r (ANHUF 6k FRANKENBERG, 1991 )•
Estimating t h e potential forest types u n d e r consider ation o f t h e c o n t e m p o r a r y climatic conditions w a s the first s t e p o f the analysis. T h e tree as s u c h repre sents a n essential e l e m e n t for the stability o f t h e landscape within a certain forest type, as well as being a n important indicator for the W e s t African s a vanna. An important aspect o f the c h a n g e in vegeta tion in t h e s e n s e o f landscape degradation is describ e d b y t h e changing patterns o f tree types. But w h y is this question s o important? Is it n o t e n o u g h to simply equalize vegetation a n d precipita tion z o n e s in order t o define the borders o f land scape areas? T h e idea that the character, as well as the b o r d e r o f a landscape can b e drawn b y only o n e of n u m e r o u s natural factors automatically leads t o the idea o f a shrinking o r perhaps expanding o f the recent existing vegetation formations. Investigations in Senegal a n d the Ivory Coast, as well as w o r k s o f SCHULZ & POMEL ( 1 9 9 2 ) a n d
NEUMANN
( 1 9 8 8 ) h a v e s h o w n that m a n has hardly b e e n consid ered in reconstructing t h e climate a n d vegetation throughout climatic c h a n g e s . In actual fact, the land s c a p e - d e v e l o p m e n t o f W e s t Africa h a s b e e n influ e n c e d b y m a n in three main phases. As early as 7.000 B.P. a n d e v e n more s o after 4 . 0 0 0 B.P., sustainable impacts o f m a n influenced the savanna a n d forest re gions o f sub-saharan W e s t Africa. Impact o f m a n during this time c h a n g e d and formed t h e vegetation. Primarily, the hunting a n d grazing fires w e r e o f limited effect. Thereafter, increased num bers o f livestock catised essential c h a n g e s in vegeta tion. O n t h e o n e hand, this led to a m o r e o p e n land scape a n d o n the other hand it meant a reduction of tree- a n d shrub-vegetation. Furthermore it creates a different spectrum o f plant species. T r e e s and shrubs s u c h as Acacia albida and Acacia laeta w e r e s u p p o r t e d b y distribution as z o o c h o r i c e l e ments. Not o n l y grazing and burning are responsible for less d e n s e vegetation, h o w e v e r . T h e main c a u s e lies in t h e specific selection a n d support o f different trees a n d shrubs serving h u m a n nutrition. T h e s a m e applies for grasses a n d herbs. O u t o f the tree-savan nas a n d o p e n dry-forests so-called park-savannas have e m e r g e d b e c a u s e trees a n d shrubs h a v e b e e n , and still a r e , o f greatest importance for the human diet. T h e s e park-savannas c a n b e described as inte grated land u s e systems with fruit trees a n d crops. Typical trees o f park-savannas are: Balanites aegyp-
113
tiaca, Acacia, albida, Sclerocarya birrea, Adansonia digitata, Butyrospermum parkii, Parkia biglobosa, Daniellia oliveri, Lophira alata, Borassus aetbiopum and Elaeis guineensis. A further massive impact o n the vegetation o f the Sudanian a n d Guinean Z o n e c a n b e recalled as o f 3.000 - 2.500 B.P. w h e n iron ore extraction a n d iron processing began. T h e production o f iron w a s b a s e d o n charcoal: the smelting p r o c e s s required 100 kg o f w o o d to obtain 5 kg o f metal (KADOMOURA 1 9 8 9 ) . In order to create a m a p with potential quasi-natural vegetation formations in W e s t Africa, testing areas (1 h a ) w e r e c h o s e n in t h e Sudanian a n d Guinean domain, in which all tree s p e c i e s w e r e identified a n d c o u n t e d . T h e inventories in a n t h r o p o g e n i c o r z o o g e n i c influenced testing areas w e r e c o m p a r e d with t h o s e o f quasi uneffected areas (factor analysis - AN HUF 1 9 9 4 ) . T h e c o m p a r i s o n o f all the 2 2 9 savanna a n d forest test plots e n a b l e d a numerical statement o f t h e anthropogenic influenced vegetation, a n d in addition to this, a reconstruction o f t h e vegetation c o v e r without human i m p a c t (fig. 1 ) . T h e hyperombrophile forest can n o t exist in regions with m o r e than 1800 m m o f precipitation a year a n d a m a x i m u m o n e month dry period. This type o f for est c a n only b e found in the farthest southwest a n d southeast o f the Ivory Coast. Most regions with o m b r o p h i l e evergreen forests have a n annual rainfall o f approx. 1 4 0 0 / 1 5 0 0 - 1 8 0 0 mm. Also here the dry period does n o t last longer than o n e o r two months. In summary, o n e c a n say that the northern bounda ries o f t h e ombrophileforests lie at approximately 1 4 0 0 m m ( m i n i m u m ) annual precipitation. T h e continuation o f t h e o m b r o p h i l e forests is the transitional z o n e o f d e n s e semi-deciduous o m b r o phile forests which are characterized b y a humid s e a s o n lasting up to eight t o nine months. It c a n b e said that its northern e x p a n s i o n s reach areas with a m a x i m u m dry season o f four months. Further, this z o n e is recognized as t h e frontier b e t w e e n the floristic G u i n e a - Z o n e a n d t h e floristic Sudan-Zone. Within the Sudan-Zone dry forests dominate the landscape. T h e forest types following t h e semi-deci d u o u s o n e s are the d e n s e dry forests (forets denses s e c h e s ) . T h e y represent a climax vegetation within the transitional z o n e o f t h e a b o v e d e s c r i b e d floristics. T h e dry season is likely to last u p t o six months. Areas with an even l o n g e r dry season ( > 6 months) w h i c h at the s a m e time have a precipitation o f 1 0 0 0 / 1 1 0 0 0 m m p e r y e a r are considered again as a transitional z o n e leading to the o p e n dry forests (forets s e c h e s ) . In the Ivory Coast this forest type might again b e found ( e . g . Pare Nationale de la C o m o e ) in the farthest northeastern parts n e a r the b o r d e r to Burkina Faso, as the o p e n dry forests c a n survive with only four t o five humid m o n t h s a year.
114
DIETER ANHUF
Potential Vegetation under recent climatic conditions
Fig. 1: Natural vegetation cover under recent climatic conditions. Abb. 1: Naturnahe Vegetationsformation. B a s e d o n results o f research work in the Ivory Coast (Anhuf 1 9 9 4 ) , as well as on different literature, a m a p o f the potential quasi natural forest types with c o n t e m p o r a r y climate parameters w a s constructed for all o f W e s t Africa. Methods o f constructing paleovegetation
maps
Maps reflecting the c h a n g e s o f vegetation w e r e c o n structed with the u s e o f standarized m e t h o d s . T h e facts from w h i c h an e c o l o g i c a l vegetation analysis o f the previous m e n t i o n e d time-schedules could b e drawn a l l o w e d a d o c u m e n t a t i o n o f published infor mation c o n c e r n i n g paleovegetation a n d paleoclimates in c o m p a r i s o n with recent conditions. T h e infor mation u s e d for the d o c u m e n t a t i o n is b o u n d to the following subjects: g e o m o r p h o l o g y , palynology, paleobotany, deep-sea-cores, ice cores, oxygen isotope data, d e n d r o c h r o n o l o g y , prehistory, and p a l a e o n t o l o g y (fig. 2 ) . T h e resulting publications e n a b l e d the establish m e n t o f a data-base for climate a n d vegetation for the time-scales in discussion. T h e final m a p s o f pa-
laeovegetation h o w e v e r , could only b e c r e a t e d b y cross-referencing b o t h kinds o f information in order to maintain reciprocal correlations a n d therefore a v o i d gaps in the data about vegetation. Further m o r e this cross-referencing allows a calculation c o n c e r n i n g the paleo-water-budget. T h e s e calculations required that the relationship b e t w e e n climate and vegetation can b e constructed. This m o d e l could only b e b a s e d o n c o n t e m p o r a r y conditions a n d d e p e n d e n c i e s . T h e influencing parameters w e r e a s u m o f r e g i o n a l differentiated climatic ( t e m p e r a t u r e , preci pitation, humidity, e v a p o r a t i o n ) a n d soil data. T h e different values for defining a single p a r a m e t e r w e r e estimated with the h e l p o f multi-dimensional statis tical procedures. With the help o f a r e c i p r o c a l c o m parison o f p a l e o c l i m a t e and p a l e o v e g e t a t i o n infor m a t i o n a relatively acurate reconstruction o f vegeta tion c o u l d therefore b e guaranteed.
The vegetation m a p o f West Africa a r o u n d 1 8 . 0 0 0 B.P. In the east o f the Ivory Coast, n e a r the b o r d e r to G h a n a , only small relicts o f the s e m i - d e c i d u o u s rain-
Paleoclimatic Samples in West Africa 18000 BP 1 Atlantik/Westafrika/M1328
Hooghiemstra.H.(1982)
2 A6antjk/Westafrlka/M1640
Hooghiemstra.H (1982)
3 AUannWWestafrika/M1323
Hooghiemstra.H (1982)
4 AtlantikAf7estafrika/M1320
Hooghiemstra.H (1982)
5 Atlar«ik/Westafnka/M1320
Hooghiemstra.H.(1982)
6 Atlantik/M16415
Dupont,L.M.;Agwu.C.O.C.(1992)
7 Golf von Guinea, KS 84067
Fredouk, A.(1994)
6 Golf of Guinea/KS12
lezine&veronaud. 1993
10 M16776
Lutze et al.,(1966)
11 M16856
Lutze etat,(1966)
12 M16667
Lutze et at,(1966)
14 Ofuabo creek, Niger-Delta
Sowumni, A. M (1981)
16 Golf von Guinea, KW 23
Bengo, M D Maley. J. (1991)
18 Bois de BilankoiCongo
Elenga.H (1992)
19 Etang de Ngamaka!a;Congo
Elenga.H (1992)
23 Cape Barbas/M123-10-4
Aowu.C.O.C., Beug,H.J.,(1982)
28 Senegal/Valdivia 132-18-1
Aawu,C.O.C..Beug,H.J..(1982)
29 Sierra Leone/M30(KI184)
Agwu,C.O.C.,Beug,H.J.,(1982)
74 AUantik/Westafrika
Hooghiemstra,H.(19o2)
76 AtlanlikANestafrika
Hooghiemstra.H. (1982)
103 GeoB1008
DupontL.M.,(1995)
Samples •fc %
lake level variations
A
macro remains
ZZ- limnic faunal remains O
Fig. 2: Map of the locations of pollen-profiles for the LGM, published in literature. Abb. 2: Karte der veröffentlichten Pollenprofile für das LGM.
Km
pollen
Capitals
0
200 400 600 800
Scale: 1 ; 3 0 0 0 0 0 0 0
116
DIETER ANHUF
forest h a v e survived t h e last glacial m a x i m u m . In contrast, this s a m e forest type in the W e s t remained in t h e highlands o f G u i n e a as well as in the area o f C a p e Palmas at the south p e a k o f the West African continent. Even u n d e r c i r c u m s t a n c e s o f m i n o r preci pitation b e t w e e n Greenville ( 5 N / 9 ° 3 W ) and Cape Palmas the e c o l o g i c a l climatic c o n d i t i o n s allowed the survival o f a strip o f e v e r g r e e n rainforests along the coast. T h e s e conditions c a n also b e found along the Niger Delta a n d e v e n further to the east, around D o u a l a and its hinterland w h i c h again a l l o w e d the survival o f e v e r g r e e n rainforests. In the central part o f the Ivory Coast, as well as in the area o f the D a h o m e y G a p , dry forests with a high p e r c e n t a g e o f g r a m i n e e s almost r e a c h e d the G u i n e a n coast. T h e area around Accra s h o w e d o n l y o p e n tree savannas c o m p a r a b l e with the sahelian type o f today around 1 8 . 0 0 0 B . P . T h e o r o g r a p h i c situation o f the high lands o f G u i n e a supported the survival o f rather h u m i d forest formations e v e n in its northern parts (Fouta Djalon). B e c a u s e o f the general reduction o f temperature o f four to six degrees, the l o w mountain range o f the G u i n e a n Highlands w a s effected by a m a x i m u m rainfall area (LAUER 1 9 8 9 ) . T h e r e f o r e , the e v e r g r e e n and the d e n s e dry forest types presented 0 1
in the m a p should rather b e described as elfin and m o u n t a i n forests (fig.3). At 1 8 . 0 0 0 B . P . . the transition from the o p e n dry for ests, today found in t h e central and southern Saheli an areas, to the o p e n tree savannas w a s shifted to 13°N at the western coast, almost to 1 0 ° N in the northeastern e d g e o f the Ivory Coast a n d to 1 2 ° N in the region o f Kaduna (Nigeria) s o that it lay further to the south than today ( 3 - 4 ° ) . T h e transition from the grass savannas to the diffuse vegetation o f the southern Saharan b o r d e r lay at 15°N in S e n e g a l and at nearly 14°N in the central part o f todays Sudanian Z o n e (north o f Kano/Nigeria). T h i s d e s c r i b e d northern frontier o f the grass savanna in W e s t Africa is c o m p a r a b l e to the southern border o f m o v i n g sand d u n e s in the P l e i s t o c e n e (TALBOT 1 9 8 4 ) . The
Vegetation m a p o f West Africa a r o u n d 8 . 5 0 0 B.P.
The climatic p e s s i m u m in West Africa e n d e d a r o u n d 1 2 . 5 0 0 B . P . T h e marine pollen cores along W e s t Af rica a r c p r o o f for the climatic change w h i c h must h a v e b e e n very e n o r m o u s (AGWU & B E U G 1 9 8 2 , R o s SIGNOL-STRICK & D U Z E R
N
Fig. 3: V e g e t a t i o n m a p s o f West Africa a r r o u n d 18.000 B.P.
Abb.3: Karte der Vegetationsformationen Westafrikas um 18.000 IIP
1979
A + B , HOOGHIEMSTRA
2 TAOUDENNI. MALI
SCHULZ. E ( 1 9 8 7 )
8 ATLANTIK / M 1 6 4 1 5
DUPONT,L M.;AGWU.C.O.C.,(1992)
9 LAKE BOSUMTWI, GHANA
MALEY, J . ( 1 9 9 1 )
1 3 OURSI, BURKINA FASO
Paleoclimatic Samples in West Africa 8500 BP
BALLOUCHE, A., NEUMANNN, K. ( 1 9 9 4 )
1 4 SEBKHA OF CHEMANCE, MAURETANIEN
LEZINE, A-M. ( 1 9 8 7 )
17 OF UA BO CREEK, NIGER-DELTA
SOWUMNI, A. M. ( 1 9 8 1 )
1 5 CHAINE DE GOBNANGOU/BURKINA FASO
NEUMANN,K.; BALLOUCHE.A ( 1 9 9 2 )
1 6 GOLF VON GUINEA, KW 2 3
BENGO. M. D , MALEY, J . ( 1 9 9 1 )
1 8 BOIS DE BILANKO;CONGO
ELENGA.H ( 1 9 9 2 )
1 9 ETANGDE NGAMAKALA;CONGO
ELENGA.H. ( 1 9 9 2 )
2 0 LAKE CHAD
ADAMS & TETZLAF,(1983)
21 LAKE CHAD
LEZINE, A-M
2 5 NIGER/ERG VON BILMA/DIBELLA
BAUMHAUER.R ,(1987)
31
NIGERIA/GAJIGANRTA
BALLOUCHE. NEUMANN, ( 1 9 9 4 )
32
NIGERIA/LANTEWA
(1987)
BALLOUCHE, NEUMA NN, ( 1 9 9 4 )
3 6 SEGUEDINE, NIGER
SCHULZ, E, ( 1 9 8 7 )
6 2 TIBESTI/CHAD
GRUNERT,J.,(1972)
8 8 TROU AU NATRON
BOTTCHER.U , ERGENZINGER,P.J.,(1972)
8 9 BEGOUR KRATER
BOTTCHER.U.. ERGENZINGER.P. J . , ( 1 9 7 2 )
9 0 ENN ZOUMNTOUANOFOU
BOTTCHER.U., ERGENZINGER,P.J.,(1972)
9 2 MOUSKORBE
BOTTCHER.U , ERGENZINGER,P.J , ( 1 9 7 2 )
9 3 YEBBI BOU
BÖTTCHER,U., ERGENZINGER.P J . , ( 1 9 7 2 )
9 4 ENN ZOUMRI/ORE
BOTTCHER.U , ERGENZINGER,P. J . , ( 1 9 7 2 )
9 5 ENN. TABIROU
BOTTCHER.U , ERGENZINGER, P. J , ( 1 9 7 2 )
9 6 DJEBEL NERO
BOTTCHER.U , ERGENZINGER.P.J.,(1972)
9 7 TARSO
BOTTCHER.U.. ERGENZINGER,P.J..(1972)
121 EL RHIMIYA
PETIT-MAIRE.N.(1991)
1 2 4 AMEKRU
PETIT MA.RE.N . ( 1 9 9 1 )
1 2 6 GABRONG
PETIT MAIRE.N , ( 1 9 9 1 )
1 3 2 ACHELOUMA
PETIT-MAIRE.N.,(1991)
1 4 2 BEGOUR
PETIT-MAIRE.N..(1991)
1 4 3 ADRARBOUS
PETRT-MAIRE N (1 9 9 1 }
1 5 0 AGADEM
PETIT-MAIRE.N . ( 1 9 9 1 )
Samples
•jif
Km
pollen
151 TEMET
PETIT-MAIRE,N.,(1991)
0
lake level variations
1 5 4 NEMRA
PETIT-MAIRE.N , ( 1 9 9 1 )
A
macro
1 5 5 AMEKNI
PETIT-MAIRE.N . ( 1 9 9 1 )
™
limnic faunal
O
Capitals
1 5 8 DOGONBOULO
PETRT-MAIRE,N.,(1991)
1 6 2 TIN OUAFFADENE
PETIT-MAIRE.N,(1991)
1 6 4 TAGALAGA
PETIT-MAIRE.N.,(1991)
Fig. 4: M a p o f the l o c a t i o n s o f pollen-profiles for 8.500 B.P., p u b l i s h e d in literature. Abb. 4: Karte der veröffentlichten Pollenprofile für 8.500 B.P.
remains remains
0
200 400 600 800
Scale:
1:30000000
118
DIETER ANHUF
1 9 8 8 , D U P O N T , B E U G , STALLING & TIEDEMANN 1 9 8 9 , Du-
PONT & Acwu 1 9 9 2 ) (fig.4). At first, t h e c h a n g e s in humidity in W e s t Africa took p l a c e in the a t m o s p h e r i c circulation. S e c o n d l y , the regional water cycle a c c e l e r a t e d b y having a d e n s e vegetation cover a n d a therefore higher evapotranspiration in the landscape. B a s e d o n investigations in t h e Ivory Coast (ANHUF 1 9 9 4 ) , a n extension o f moist evergreen rainforest ( A N H U F & FRANKENBERG 1 9 9 1 ) r e a c h i n g a latitude o f 7
- 73°°N ( M a n ) in the w e s t o f the Ivory Coast a n d the area o f S o k o t o in T o g o c a n b e assumed to h a v e o c c u r e d (fig.5). In the highlands o f Guinea, t h e s e s a m e forest types can e v e n b e e x p e c t e d up to t h e southern b o r d e r o f Guinea Bissau ( 1 1 ° N ) . Also in t h e eastern parts o f the Guinea Coast t h e w e t e v e r g r e e n rainfor est e x p a n d e d u p towards t h e north, u p t o Lokoja and Makurdi in Nigeria. Continuing to t h e north, a
are again followed b y t h e d e n s e dry forests. I n t h e w e s t o f t h e continent t h e b o r d e r o f the semi-humid, s e m i - d e c i d u o u s rainforests t o the d e n s e dry forests lies in t h e area o f today's G a m b i a . In contrast, t h e s a m e b o r d e r is lowered t o 1 1 ° N in the East. B a u c h i , for e x a m p l e , is assumed t o h a v e lain in t h e transitional area o f t h e semi-deciduous rainforest to t h e d e n s e dry forest. At the s a m e time, the northern frontier o f G u i n e a n flora in t h e W e s t African s e c t o r is r e a c h e d at around 8 . 5 0 0 B . P . D e n s e a n d o p e n dry forests dominate t h e floral z o n e o f Sudan. At about 8 . 5 0 0 B . P . , the b o r d e r from d e n s e t o t h e o p e n dry forests ran along t h e line o f Linguere, Nara, a bit north o f Mopti, Tillabery, a n d Zinder, as well as south o f N G u i g m i at Lake Chad. T h e e d a p h i c c a u s e d grassland o f the Niger-Inlanddelta a r e also thought t o h a v e had e x p a n d e d greatly ( P E T I T - M A I R E , FABRE, CAFCBONEL, SCHULZ, A U C O U R 1 9 8 7 ) .
Vegetation map o f West Africa around 8500 B.P
Fig. 5: Vegetation m a p o f W e s t Africa a r r o u n d 8 . 5 0 0 B.P.
Abb. 5: Karte der Vegetationsformationen Westafrikas um 8.500 B.P. smaller b a n d o f moist e v e r g r e e n rainforests c a n b e found. This strip leads t o t h e humid semi-deciduous rainforests situated n e a r Bissau ( 1 2 ° N), K a n k a n in G u i n e a , north o f B o u a k e in the Ivory Coast, along the line o f Cotonoti, Ibadan, Illorin, Kaduna as well as a l o n g the southern ridge o f the Jos-Plateau. T h e s e
T h e transition from t h e o p e n dry forests t o t h e tree savanna lay at 19° N. in t h e central part o f W e s t Afri ca, n a m e l y the triangle b e t w e e n the Adrar d e s Iforas in the W e s t , the Hoggar in the North a n c f the Air in the East. T h e northern Sudanian/Sahelian tree a n d grass s a v a n n a s were p u s h e d forward to t h e north t o
P a l e o v e g e t a t i o n in W e s t Africa for 1 8 . 0 0 0 B . P . a n d 8 . 5 0 0 B . P .
the T r o p i c o f C a n c e r (SCHULZ 1987). T h e result o f re s e a r c h in the most w e s t e r n parts o f Africa (westcoast o f S e n e g a l ) also m a t c h this reconstruction (ANHUF & FRANKENBERG 1 9 9 3 ) .
T h e transition from t h e Sahara to t h e Sahelian flora w a s o n l y very sparse a r o u n d 8.500 B . P . In c o n s e q u e n c e , the region o f Bilma s h o w s clear S a haran s p e c i e s , s u c h as the Maerua/Acacia-savanna. T h r o u g h the analysis o f charcoal, NEUMANN ( 1 9 8 8 ) was a b l e to reconstruct a Sudanian vegetation only a few kilometers s o u t h o f Fachi. Furthermore, the re sults o f r e c e n t drilling c o r e s northeast o f Niger, SCHULZ et al. ( 1 9 9 0 ) a n d POMEL & SCHULZ ( 1 9 9 2 ) s h o w
that t h e r e must h a v e b e e n a direct c o m b i n a t i o n o f Saharan and Sudanian vegetation at the time o f the H o l o c e n e . Palynological and anthracological finds h a v e p r o v e n s o (NEUMANN 1988). T h e r e f o r e , it is as s u m e d that at that time the vegetation must h a v e b e e n a c o m b i n a t i o n o f the northern parts o f the Su danian dry forests or tree savannas and o f the Saharan s a v a n n a vegetation. SCHULZ & POMEL ( 1 9 9 2 ) identi fied this type o f Saharan savanna vegetation as t h e Acacia-,
Maerua-,
Cappari.s-savanna.
Between
19°
and 2 0 ° N there w a s a gradual c h a n g e to dominant S a h a r a n savannas, w h e r e a s at 20° N already there w a s a n o b v i o u s b o r d e r o f diffuse tree/grass savanna to S a h a r a n desert vegetation. T h e desert vegetation is s u p p o s e d to h a v e b e e n more d e n s e than today.
Acknowledgements
119
References AGWU, CO. & BEUG, H.-J. (1982): Palynological studies of marine sediments of the West African coast. Meteor-Forschungsergeb nisse, Reihe C, Nr. 36, p.1-30. ANHUF, D. (1994): Zeitlicher Vegetations- und Klimawandel in der Cote d'Ivoire. Erdwissenschaftliche Forschung. 30: 7-299, Stuttgart. - & FRANKKNBF.RG, P. (1991): Die natürlichen Vegetationszonen des westlichen Afrika. Die Erde. 122: 243-265 - (1993): Sudanische Reliktwälder im westlichen Senegal. Trie rer Geographische Studien. 9: 19-30. DUPONT, L.M. & AGWU, C.O.C. (1992): Latitudinal shifts of forest and savanna in N.W, Africa during the Brunches chron: further marine palynological results from site M 16415 (9°N 19°W). Vegetation History and Archaeobotany. 1: 163-175. HOOGHIEMSTRA, H. (1988): Changes in major wind belts and vegeta tion zones in NW Africa 20.000 yr B.P., as deduced from a ma rine pollen record near Cap Blanc. Review of Paleobotany and Palynology. 55: 101-140. KADOMURA, H. (1989): Savannization in tropical Africa. In: KADOMURA, H. (Ed.), Savannization processes in tropical Africa. Depart ment of Geography, Faculty of Science - Tokyo Metropolitan University. Occasional Papers. 17: 3-15. LAUER, W. (1989): Climate and weather, p. 7-53. In: LIETH, Nr. & WERGER.
M.J.A. (Eds.), Tropical Rain Forests Ecosystems. Amsterdam, Oxford, New York, Tokyo. NEUMANN, K. (1988): Die Bedeutung von Holzkohleuntersuchungen für die Vegetationsgeschichte der Sahara - das Beispiel FachUNiger. Würzburger Geographische Arbeiten. 69: 71-85 PETIT-MAIRE, N.. FABRE, J . . CARBONFL, P.. SCHULZ, E., AUGOUR, A. M.
(1987): La depression de Taoudenni (Sahara malien) ä l'Holocene. Geodynamique. 2/2: 154-159. ROSSIGNOL-STRICK, M. & DUZER, D. (1979): West African vegetation and climate since 22.500 B.P. from deep sea cores palynology. Pollen Spores. 21/1-2: 105-134. SCHULZ, E. (1987): Die holozäne Vegetation der zentralen Sahara (N-Mali, N-Niger, SW-Libyen). Paleoecology of Africa. 18: 143161. SCHULZ, E., JOSEPH, A., BAUHAUER, R., SCHULTZE, E., SPONHOLZ, B.
This w o r k was s u p p o r t e d by the G e r m a n Ministry o f R e s e a r c h and T e c h n o l o g y ( B M F T ) ; Project Nr.: 0 7 KFT 5 7 / 9 . T h e article was p r e s e n t e d at the XTV. International INQUA Congress, Berlin, August 3 - 10, 1995.
(1990): Upper pleistocene and holocene history of the Bilma region (Kawar, NE-Niger) - Recent data in african earth scien ces. CIFEG Oec. Publ. 22: 281-284. SCHULZ, E. & POMEL, S. (1992): Die anthropogene Entstehung des Sä he! Würzburger Geographische Arbeiten. 84: 263-288. TALBOT, M.R. (1984): Late Pleistocene rainfall and dune building in the Sahel, p.203-214. In: COETZEE, J.A. & VAN ZINDEREN BARKER,
E.M. (Eds.), Paleoecology of Africa. 13- Rotterdam, Boston. Manuskript e i n g e g a n g e n am 0 3 0 9 - 1 9 9 6