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Phytoliths : indicators of grassland dynamics during the
late Holocene in intertropical Africa
Anne Alexandre, J.-D Meunier, Anne-Marie Lezine, A. Vincens, D. Schwartz
To cite this version:
Anne Alexandre, J.-D Meunier, Anne-Marie Lezine, A. Vincens, D. Schwartz. Phytoliths : indicators
of grassland dynamics during the late Holocene in intertropical Africa. Palaeogeography, Palaeocli-
matology, Palaeoecology, Elsevier, 1997, 136, pp.213 - 229. �10.1016/S0031-0182(97)00089-8�. �hal-
01909698�
ELSEVIER
Palaeogeography, Palaeoclimatology, Palaeoecology 136 ( 1997 ) 213-229
P tE0
Phytoliths: indicators of grassland dynamics during the late
Holocene in intertropical Africa
A. Alexandre a J.-D. Meunier a,,, A.-M. L6zine u, A. Vincens c, D. Schwartz a
a GOoscienees de l'Environnement, URA 132-CNRS, CEREGE, Universitd Aix-Marseille III, BP 80,
13545 Aix en Provence cedex 4, France
b Paldontologie et Stratigraphie, URA 1761-CNRS, Universitk Pierre et Marie Curie, Bofte 106. 4place Jussieu,
75252 Paris', cedex 5, France
¢ GOologie du Quaternaire CNRS, CEREGE, BP 80, 13545 Aix en Provence cedex 4, France
d DS, ORSTOM, CEREG (URA 95 CNRS), 3 rue de l'Argonne, 67083 Strasbourg, eedex, Franee
Received 25 November 1996; received in revised form 8 April 1997; accepted 14 April 1997
Abstract
The reconstruction of African tropical grassland history during the late Holocene can be carried out using phytolith
analysis. Fossil phytolith assemblages from Lake Guiers, in the Sahelian region of Senegal, and from Lake Sinnda,
in the Guineo-Congolian region of Congo were investigated. The results are interpreted on the basis of modern
phytolith assemblages from the same regions and compared to pollen data previously obtained. Tall or short grass
associations are discriminated by their phytolith index lph(%)=saddle/(cross + dumbel + saddle), while the density of
shrubs and trees is indicated by relative proportions of the dicotyledon phytoliths.
The phytolith data emphasize that, in the Guineo-Congolian region around Lake Sinnda, the driest phase of the
late Holocene occurred between 4000 and 1200 yr B.P., commencing with the opening of the dense forest and its
replacement by a short grass savanna. From ca. 1000 yr B.P., wetter climatic conditions developed, as represented by
the setting up of a tall grass savanna woodland. The modern shrub and tall grass savanna was developed ca. 700 yr
B.P. In the Sahelian region around Lake Guiers, the driest phase occurred after about 2000 yr B.P. and has not been
followed by moister conditions. A tall grass savanna woodland was gradually replaced by a shrub and short grass
savanna which still occurs. A short period of development of swampy vegetation, which can be related to a lake level
change, interrupted the semi-arid adaptation of the vegetation, between about 2000 yr B.P. and the present. © 1997
Elsevier Science B.V.
Keywords."
phytoliths; tracers; paleoenvironment; Holocene; savannas
1. Introduction
Past vegetation reconstructions in intertropical
Africa have mostly been based on pollen, charcoal,
* Corresponding author. E-mail: meunier@cerege.fr
0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
PII
$0031-0182(97)00089-8
macroflora remains and carbon isotope data
recorded in lacustrine and marine sediments and
in soils. During the late Holocene, equatorial and
North tropical Africa experienced large amplitude
changes in the vegetation distribution, linked in
the Northern Hemisphere tropics to variations in
the Atlantic monsoon activity (Kutzbach and
214
4..4lt,.xandrc ct a/. l)aAwo,,'cograpltl '. I~ahwoclimalo/ogl . Palaeoeco/o~v 136 ~ 1997~ 2/3 229
Street-Perrott, 1985). The climate experienced
drier phases ca. 7500 yr B.P. and from ca. 200t) yr
B.P. at Saharan and Sahelian latitudes (Servant-
Vildary, 1978; Hillaire-Marcel et al., 1983: Ritchie
et al., 1985: Ritchie and Haynes, 1987: Ldzine,
1988, 1989; Lezine and Casanova, 1989: Petit-
Marie. 1991; Ballouche and Neumann, 1995), and
ca. 4000-3000yr B.P. at equatorial latitudes
( Talbot et al., 1984; Talbot and Kelts, 1986; Maley,
1992; Schwartz, 1992: Elenga et al., 1994; Vincens
et al., submitted). These drier phases resulted in
reduction of forests and a large extension of grass-
lands. However, the lack of precise pollen identifi-
cation of species o1" subfamilies of Poaceae which
constitute the main part of tropical grasslands
strongly limits the reconstruction of tropical vege-
tation history. In particular, arid and humid grass
associations cannot be discriminated through
pollen and carbon isotope data.
Phytoliths are micrometric hydrated opaI-A par-
ticles that precipitate in cells and/or between cells
of living plant tissues. They are abundant in
Poaceae where silicon ranges t¥om less than I% to
more than 5% of the dry weight (Johnston et al..
1966: Jones and Handreck, 1967) and, unlike
pollen, are well preserved under oxidizing condi-
tions. Phytolith assemblage analysis can be used
to identify past Poaceae subfamilies associa-
tions, as shown by numerous studies carried out
in America (Fredlund, 1985; Kurmann, 1985:
Bartolome and Klukkert, 1986: Piperno, 1988:
Fredlund and Tieszen, 1994: Fisher et al., 1995:
Piperno and Becker, 1996). ~3C of organic carbon
included in phytoliths have been analysed to distin-
guish Ca from C3 vegetations ( Kelly et al., 1991 }.
Phytolith assemblages recovered from marine sedi-
ments off west equatorial and tropical Africa. have
been used as indexes of the continental aridity
through the climatic cycles (Diester-Haas et al.,
1973: Parmenter and Folger, 1974; Jansen et al.,
1989). Although Palmer (1976) and Runge (1995)
have considered the potential of phytoliths ['or
African vegetation reconstruction, no paleo-
environmental study from phytolith assemblages
have been made in intertropical Africa.
Here, we present phytolith data from lacustrine
sediments recovered in the Sahelian and
Guineo-Congolian phytogeographical regions
(White. [983) of West and Central Africa, in
Senegal (Lake Guiers) and Congo (Lake Sinnda).
The aim of this study is to determine the Holocene
grassland history. Our results are interpreted on
the basis of modern phytolith assemblages from
the Sahelian, Sudanese and Guineo Congolian
regions, and discussed according to the fossil
pollen data previously obtained from the same
sites (Ldzine. 1988: Lezine and Edorh, 1991:
Vincens et al., 1994).
2. Environmental setting
Lake Guiers (1615 N, 15 50 W) is located in
the Sahelian region of Senegal (Fig. I ). It was ted
by the Ferlo River during the Holocene, up to ca.
2000 yr B.P., and is now supplied by the Senegal
River. The mean annual rainfall is 355 mm and
the dry season is 8 months long. The lake is today
surrounded by a discontinuous grassland mainly
represented by the Poaeae subfamilies
Chloridoideae (e.g.
Schoene~4diu gracilis)
and
Arundinoideae (e.g.
Aristida funiculata
and
.4ristida mutabilis)
and a minor proportion of
Panicoideae (e.g.
C2,nchrus biflorus).
Trees and
shrubs (mainly
Acacia)
are scattered (Trochain,
[940). This grassland type is defined as a shrub
steppe in the Yangambi nomenclature (C.S.A.,
1956). the only system that describes with accuracy
the different grasslands through continuity of the
herbaceous cover, height of grasses and density of
the ligneous elements.
However, the use of the term steppe remains
controversial because it is better applied for cold
grassland formations (Trochain, 1980: Menaut,
1983: White, 1983; Riou. 1995). Following the
North American nomenclature, which distingu-
ishes between short grass and tall grass prairies
tTwiss, 1987: Fredlund and Tieszen, 1994), we
propose to use the terms tall grass savanna and
short grass savanna in place of savanna and steppe,
respectively. Other patterns and terms of the
Yangambi classification are followed. Thus, the
vegetation surrounding the Lake Guiers is defined
here as a shrub and short grass savanna.
Lake Sinnda (3 50S, 12 48E), is located in the
Guineo-Congolian region of Congo, in the Niari
A. Alexandre et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997)213 229
215
I I I I I
10 o 0 o 10 o 20 ° 20 °-
~~ XVI
I 0o
A. Main phymehoria after White (1983)
I: Guineo-congolian regional centre of endemism
XI: Guinea-congolia/sudania regional transition zone
III: Sudanian regional centre of endemism
XVI:
Sahel regional transition zone
B. Senegal C. Congo
xvI ~ Rainforest
III ~ Savanna
XI
Lakes cores • Soil sampled •
Fig. 1. Environment of the sites studied. (A) Main phytochoria after White (1983). (B) Senegal. (C) Congo.
Valley, surrounded by the forested Mayombe (to
the southwest) and Chaillu (to the northeast)
ranges (Fig. 1 ). The mean annual rainfall is about
1100 mm and the dry season is 5 months long.
The whole Niari Valley is covered by continuous
grasslands or shrub and tall grass savannas, domi-
nated by the Panicoideae grass subfamily (e.g.
Hyparrhenia or Andropogon) (Koechlin, 1961;
Descoing, 1969). A local ring of forest occurs
along the northern and southern banks of the lake.
3. Material and methods
3.1. Material
Sixteen samples of the upper 6.50 m of the core
$2, taken from the southern part of the Lake
Guiers (Senegal) (Lezine, 1988) and 17 samples
of the core SN2 of 3.80 m length, taken from the
western part of the Lake Sinnda (Congo) (Vincens
et al., 1994) were analysed. The samples were
2 3 cm thick. The sampling ranges were narrowed
at layers where some vegetation changes had been
evidenced by pollen analysis. A sedimentary gap
occurs in the core from Lake Sinnda between 0.82
and 0.77 m depth.
In order to interpret the fossil phytolith assem-
blages, modern materials were sampled on the soil
surface or in the humic layer developed under four
distinct plant associations. Soil samples are pre-
ferred to lacustrine surface sediments because they
do not include a fluvial contribution. Soil samples
thus permit an easier phytolith assemblage inter-
pretation. The plant associations sampled were:
( 1 ) The Sahelian shrub and short grass savanna
216 .4 Ale.vandr{, ~,I aL Paha'ogeograp/o', I)alat:ocl#natology. Palaeoecoh~gy 136 ! 1997) 213 229
of Senegal, The grass community, dominated by
the Chloridoideae subfamily, is similar to the
botanical environment of Lake Guiers. Two
samples of soil surface were collected at 15 20
N-13~'20W(sample83/47)andat 1510N 1540
W (sample 83/13) (Fig. 1).
(2) The Sudanian tree tall grass savanna of
Senegal. The grass community is dominated by
the Panicoideae (e,g. Andropogon gayanus), and
Arundinoideae subfamilies (e.g. Aristida hmgiflora)
( Trochain, 1940). The Chloridoideae one is absent.
Two samples of soil surface were collected at 14:10
N-15'20W(sample 85/2) and at 14'05 N 1305
W (sample 83/80) ( Fig. 1 ).
(3) The Guineo-Congolian tall grass savanna
of the Niari Valley in Congo. One sample of the
soil humic layer was collected at 4 01 S 13 07 E
(sample LDA1) (Fig. 1).
(4) The Guineo-Congolian forest of Congo.
West of the Niari Valley, the Mayombe range is
covered by a mixed moist semi-evergreen rain
forest (White, 1983). Under the continuous stand
of trees, some monocotyledons are present includ-
ing Palmae and C3 grasses belonging to the
Panicoideae or to the Bambusoideae subfamilies
(e.g. Commelinidium mavumbense, Puelia ciliata,
Guaduella marant(/blia and Oplismenus hirtellus,
Koechlin, 1962). One sample of humic horizon was
collected at Dimonika, 4 S 1230 E (sample Ft
( Fig. 1 ).
3.2. Phvtolith exlraction amt counting
Between 20 and 2g of dry soil or sediments
were analysed. Phytoliths were extracted from the
2 50 jam fraction after granulometric separation,
organic matter oxidation using H20 2, and densi-
metric separation in a ZnBr2 solution with a
density of 2.3 (Kelly, 1990; Fredlund and Tieszen.
1994). They were mounted on microscope slides
in Canada Basalm. The extracted material includes
phytoliths and diatoms. More than 200 phytoliths
with a diameter greater than 5 jam were counted
under the optical microscope, at × 600 magnifica-
tion. They were classified into the following mor-
photype groups: ( 1 ) saddle; ( 2 ) dumbel and cross:
(3) elongated, point and fan-shaped; (4) circular
crenate; (5) circular rugose; (6) unclassified; and
( 7 ) cork cell-like morphotype ( Fig. 2). Phytoliths
were further examined under a Scanning Electron
Microscope (SEM).
3.3. Phvtolith taxonomy and assemblage analysis
Twiss et al. (1969) and Twiss (1992) proposed
a morphological classification of phytoliths that is
related to grass taxonomy: the morphology of
phytoliths precipitated in short cells of the grass
leave epidermis can be traced to three of the five
grass subfamilies occurring in the world. These
three subfamilies are the Festucoideae, Panicoideae
and Chloridoideae. The two remaining subfamilies
are the Bambusoideae and Arundinoideae
(Watson et al., 1985). Grass genera belonging to
a same grass subfamily often present the same
photosynthetic pathway and suitability to a given
environment: the C3 Festucoideae subfamily
requires cold, temperate or high elevation environ-
ments: the Panicoideae subfamily, composed of
mixed C4 C3 tall grasses occurs in warm and wet
environments: the Chloridoideae subfamily, com-
posed of C4 short grasses, occurs in warm and dry
environments.
The saddle morphotype ( Fig. 2a) is produced in
large quantity by the Chloridoideae, and by some
taxa of the Bambusoideae (C 3 genera) and
Arundinoideae (mostly C3 genera); The dumbel
and cross morphotypes (Fig. 2b,c) are produced
in large amounts but are not exclusive to the
Panicoideae. They are also produced in smaller
quantities by the Chloridoideae, Arundinoideae,
Bambusoideae, and by Stipa; The dumbel, cross
and saddle morphotypes are sometimes difficult to
distinguish because of their orientation on the slide
i Fig. 2d). The rectangular and circular morpho-
types are produced in large amounts by the
Festucoideae.
Both Arundinoideae and Bambusoideae sub-
families do not produce any distinctive morpho-
type but some dumbel, cross and small amounts
of saddle morphotypes.
Phytoliths from the epidermal long specialized
and bulliform cells of grass leaves are produced
by the whole Poaceae family. They are elongated
and smooth or spiny (Fig. 2f), point and fan-
shaped (Fig. 2f) (Twiss et al., 1969; Twiss, 1992).