Vegetation zones in Niger
and Benin - present and past zonation
Alexander Wezel1, Brigitte Bohlinger2 and R. Böcker1
1Department of Landscape and
Plant Ecology, University of Hohenheim, 70593 Stuttgart, Germany,
Tel.: +49/711/4593628, Fax.: +49/711/4592831,
wezel@mail.uni-greifswald.de
2Department of Plant Production
and Agroecology in the Tropics and Subtropics, University of Hohenheim,
70593 Stuttgart, Germany
Keywords: fallow plant communities,
soil-adjusted vegetation index (SAVI), vegetation change, West Africa
1 Aims of the research
In the last decades, the vegetation of
West Africa was subject to strong changes. The steady increase of population
led to an extension of agricultural land. In subhumid or humid regions
forests were cleared (Ern 1979, Collins 1990), in semi-arid regions, tree
cover decreased (Anhuf et al. 1990, Breman and Kessler 1995) due to firewood
cutting, overgrazing and the droughts of the 70ies. Many former tree savannas
were changed to shrub/grass savannas or ended in bare land. Changes of
vegetation cover are mostly followed by micro and meso scale climatic changes.
Together with existing global changes, this will lead to shifts of vegetation
zones. The intention of this study is to provide maps for past and present
vegetation zonation in Niger and Benin as well as a characterisation of
the different vegetation zones. The vegetation zones in this study reflect
the actual state of vegetation and not the potential natural vegetation.
2.1 Vegetation zonation in the past
Different vegetation maps covering Niger
and Benin were selected from literature. The boundaries of vegetation zones
according to various authors were digitised within the contour of the respective
country. Some zonations were congruent to precipitation isohyets, others
used vegetation types for differentiation. This is a general problem, which
can not easily be solved (see also Schnell 1976). Altitudinal differences
only were of minor importance. For Niger, zonations from Mahamane (1990),
Peyre de Fabrèques (1980), Knapp (1973) and AÉTFAT (1959)
were used. Maps from Aubreville (1949), AÉTFAT (1959), Knapp (1973),
Adjakidji (1984) and Adjanohoun et al. (1989) were taken to show the different
zones in Benin. The zonation of White (1983) was disregarded because it
used a different isohyete orientation which might lead to visual misinterpretation.
The zonation of LeHouérou (1989) could not be transferred from the
original map. Although the vegetation zone classifications of the different
authors vary, they were found to be comparable in respect to temporal trend
analyses, but not to exact location of the zones.
2.2 Present vegetation zones in south-west
Niger
Four digital satellite images (Landsat
TM) from SW Niger towards the end of the vegetation period were used to
calculate the soil-adjusted vegetation index (SAVI) images: TM 192/50 (19.09.92),
TM 192/51 (04.10.92), TM 193/50 (25.09.92) and TM 193/51 (25.09.92). The
year 1992 was chosen because conditions of this year turned out to be average
for the last decade (Prince et al. 1995). The SAVI was calculated using
the channels 3 (red band) and 4 (near-infrared band), according to Huete
(1988):
,
where L is a constant soil adjustment factor
(L=0.5) to reduce the soil background effect. Although Huete (1988) found
the optimal adjustment factor to vary with vegetation density, he used
a constant (L=0.5), since this reduced soil noise considerably throughout
a wide range of vegetation densities.
Training areas were used to classify the
SAVI values into 7 classes: (1) water, (2) dunes or sand covered plateaus,
(3) laterite plateaus, (4) fields, (5) fallows, pastures or other areas
with low vegetation cover, (6) fallows, pastures or areas with medium vegetation
cover, (7) areas with dense vegetation cover (rice fields, forests, gallery
forests along rivers). Areas with dense vegetation (7) are typical for
vegetation types of the Sudanian Zone such as tree/shrub savannas with
Butyrospermum parkii, Combretum spp. and Anogeissus leiocarpus. Low to
medium degrees of vegetation cover (5, 6) are characteristic for savannas
of the Southern Sahel with sparse woody vegetation (Faidherbia albida,
Guiera senegalensis, Combretum spp.) and reduced cover of the herb layer.
This type of vegetation is often called 'steppe' (see also Table
2).
In addition to the SAVI images, results from vegetation surveys on fallow land in 1994-1996 were analysed for further information about a possible vegetation zonation (PROJECT FALLOW MANAGEMENT; Wezel 1998, Wezel and Boecker 1998). Thus, a map of the spatial distribution and occurrence of seven fallow plant communities in SW Niger was sketched to show northern and southern limits of their distribution. Here it has to be stressed, that this information was only used for the specification of a part of the vegetation zone boundary. For the whole area of SW Niger more vegetation surveys would have been needed.
Together, the SAVI images and the spatial
distribution of the fallow communities were used to mark off vegetation
zones for SW Niger. This zonation was then added into the already existing
map of vegetation zonations from former times. Obviously, SAVI images are
alone not suitable to distinguish different vegetation types, but it is
possible to monitor the actual degree of vegetation cover. This can be
a great advantage because vegetation classification and description of
different authors of the same region can be extremely variable.
The present vegetation zones of both countries Niger and Benin are shown in one view. The zonation was derived from results of Mahamane (1990) and own investigations in Niger and Adjanohoun et al. (1989) in Benin. More detailed, country-specific maps (map2: Niger , map3: Benin) were created for the different zonations and their changes over time according to various authors. Clear southward shifts of vegetation zones can be recognised in both countries.
3.1 Present vegetation zones in Niger
and Benin and their description
3.1.1 Niger
The Sahara presently covers the whole
northern part of Niger. The transition between the Northern and Southern
Sahel covers a strip about 200 km in the south (between 14 to 15°N).
The Northern Sudanian Zone is restricted to the south-west and small areas
of southern Niger.
In general, the physiognomy of the vegetation
zones changes from contracted vegetation in the Sahara
to tree, shrub or grass savannas in the Sahel (Table
1; for classification of savanna types see Table
2 and 3) with Mimosaceae and
Combretaceae tree and shrub species sparsely distributed. In the Northern
Sahel, grass savannas are mostly found
in depressions, sometimes on plateaux, whereas (thorn) shrub savannas predominate
on sandy soils. This zone is a pastoral zone, where persistent rainfed
agriculture is not possible due to low precipitation (200-400 mm). Combretum
thickets on laterite plateaux and grass or (thorn)
shrub
savannas on sandy terraces, dry valley floors or fixed dunes
are characteristic for the Southern Sahel (400-600 mm precipitation). A
particularity are ancient river valleys, so-called Dallols. In these valleys
a tree savanna with Faidherbia albida
and Hyphaene thebaica has its northernmost occurrence. Gallery forests
occur along rivers, if water supply is sufficient during at least few months
of the year.
3.1.2 Benin
Almost the whole northern part of Benin
is covered by the Southern Sudanian Zone. Central Benin is dominated by
the Northern and to a smaller degree by the Southern Guinea Zone. In the
south of Benin, the Guinea-Congolian Zone is found. A small band of coastal
vegetation exists along the Atlantic Ocean.
The prevailing vegetation types in the
Sudanian Zone are woodlands and savannas (Table
1). Along rivers, gallery forests can be found. In the Northern Sudanian
Zone, vegetation is dominated by Combretaceae and Mimosaceae woodland or
single trees with perennial grass layers of Andropogon gayanus (sandy soils),
Loudetia spp. (laterite, glacis) and Hyparrhenia spp. (moister sites) (Peyre
de Fabrègues 1980). In the Southern Sudanian Zone Isoberlinia trees
occur more frequently.
The vegetation in the Guinea Zones is
dominated by moist woodlands and savannas.
The separation between a northern and a southern part coincide with the
northern boundary of bimodal rainfall in southern Benin. The Northern Guinea
Zone is characterised by woodlands, tree and shrub savannas with abundant
Isoberlinia spp. and Butyrospermum parkii (Adjanohoun et al. 1989). In
the Southern Guinea Zone moister types of woodland and savannas with abundant
Daniella oliveri are found. In the Guinea-Congolian and Coastal
Zones a mosaic of forests and savannas exists. In the two zones most
of the original vegetation is replaced by secondary grasslands or savannas
due to human impact. This fact was emphasised by vegetation studies on
bush and grass fallows of Bohlinger (1998).
3.2 Comparison of present and past zonations
in Niger and Benin
Again it has to be stressed, that the
different vegetation zone classifications were analysed in respect to temporal
trends, but not to exact location of the zones.
3.2.1 Transition Sahara - Sahel
Beginning with the most northern boundary
between the Northern Sahel and the Sahara (map2:
Niger), no clear trends can be stated between 1959 and 1990. This might
be due to mapping resolution but also to different classification approaches
of this boundary. Under the extreme variable inter-annual rainfall distribution
of this region, vegetation can change enormously from year to year (see
also Tucker et al. 1991). Thus, a classification of this boundary strongly
depends on the rainfall patterns of the preceding years. Anyhow, a shift
of the Sahara - Northern Sahel boundary is not obvious. Peyre de Fabrègues'
zonation (Peyre de Fabrègues 1980) also attached a part of the Air
Mountains to the Northern Sahel, which explains the bulge to the north
in the centre of the map. Knapp's classification (Knapp 1973) was similar
to the other authors, but only in the western part of Niger. With regard
to the eastern part it seems that Knapp used a different approach, which
is not clear yet. All three zone boundaries of Knapp are delineated much
further north compared to other authors.
In contrast to the above boundary, a southward shift of the boundary between the Southern and Northern Sahel is indicated in the western part of Niger from 1973 to 1990. In the eastern part, Knapp's approach might lead to a misinterpretation. Unfortunately, AÉTFAT (1959) did not present this boundary.
3.2.2 Transition Sahel - Sudanian Zone
Clear changes are visible for the boundary
between the Sudanian Zone and the Southern Sahel. From 1959 to the present
situation, a shift to the south can be recognised. This trend might continue
in the future as the present 600 mm isohyete (1980-1994), which is often
used to sketch the Sahel-Sudanian boundary, is found even farther to the
south (map2: Niger) and
vegetation changes follow climatic changes with retardation (see also Breman
and Cissé 1977). In the last years, overall precipitation decreased
in Niger (Figure 1; Sivakumar et al.
1993). For the present boundary, the quantitative change of areas with
less to medium vegetation cover (white, red, yellow, light green, medium
green) to areas with high vegetation cover (dark green) of the SAVI images
was used to mark off the boundary between the Southern Sahel and the Northern
Sudanian Zone. On the border to Burkina Faso, this boundary was located
more to the north than that of Fontès and Guinko (1995) in Burkina
Faso. According to Fontès and Guinko (1995), a subzone (Sahelian-sudanian
transition zone) was also derived from the SAVI
images. This zone is characterised by areas with low to medium vegetation
cover (light to medium green). Additionally, the southern limits of appearance
of three communities (Chrozophora brocchiana-, Aristida sieberana-, Tribulus
terrestris - Limeum pterocarpum-community) were used (Figure
2). This communities were found on dry, sandy sites (Wezel and Boecker
1998) and have a characteristic species composition which occurred only
north of the subzone.
3.2.3 Transition Sudanian Zone - Guinea
Zone
In Benin, southward shifts of vegetation
zones can also be found (map3:
Benin). Aubreville (1949), AÉTFAT (1959) and Knapp (1973) mark
off the boundary between the Sudanian and Northern Guinea Zone farther
to the north, compared to Adjanohoun et al. (1989). LeHouérou (1989)
was the first to separate the Sudanian Zone into a Northern and Southern
Sudanian Zone. Annual precipitation of the years 1964-1993 decreased (Figure
3), compared to former means presented in Adjanohoun et al. (1989),
which might lead to further southward shifts of the Sudanian Zone.
3.2.4 Transition Northern Guinea Zone
- Southern Guinea Zone
No shift to the south of the boundary
between the Northern and Southern Guinea Zone can be stated (map3:
Benin). An exclave of the Southern Guinea Zone in north-west Benin
as indicated by Knapp (1973) is due to the classification of an area of
the Togolian mountains to the Southern Guinea Zone. Another exclave was
also located for the Guinea-Congolian Zone by Adjanohoun et al. (1989).
These areas can not be interpreted with respect to vegetation shifts.
3.2.5 Transition Guinea - Guinea-Congolian
Zone, Coastal Zone
In southern Benin, the boundaries between
the Guinea and Guinea-Congolian Zones of the various authors are very variable
and can not be interpreted with regard to south- or northward shifts. This
is also the case for the Coastal Zone of Benin. Anyhow, the climatic influence
of the Atlantic Ocean might be more decisive here.
Although the vegetation zone classifications
of the different authors varied, they could be analysed with respect to
temporal trends. In the last five decades, a southward shift of most vegetation
zones in Niger and Benin can be stated. This is the case for all boundaries
in regions with semi-arid and semi-humid climates. In contrast, boundaries
are stable in arid regions of Niger (Sahara-Sahel boundary) and in the
sub-humid region of southern Benin (Northern to Southern Guinea boundary,
Southern Guinea to Guinea-Congolian boundary). One reason for changing
vegetation zone boundaries is decreasing mean annual rainfall in central
and southern Niger and northern Benin. Secondly, increasing population
led to degradation of vegetation, especially
by clearing for agricultural land and cutting of trees for firewood use
and construction purposes.
Comparison of mean annual precipitation
before and after 1980 in Niger let expect a continued shifting of vegetation
zones to the south because vegetation changes follow climatic changes with
delay. More and more, human impact on vegetation will lead to vegetation
zones where former vegetation types will be replaced by secondary vegetation
types. Original vegetation will only be able to outlast in large protected
areas.
Acknowledgements
Many thanks to F.
Graef and U. Weller who provided the rainfall maps.
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AÉTFAT (Association pour l'Étude Taxonomique de la Flore d'Afrique Tropicale) 1959. Vegetation map of Africa. South of the Tropic of Cancer. University Press, Oxford, 24 pp.
Anhuf D., Grunert J., Koch E. 1990. Veränderungen der realen Bodenbedeckung im Sahel der Republik Niger (Regionen Tahoua und Niamey) zwischen 1955 und 1975. Erdkunde 44: 195-209.
Aubréville A. 1949. Climats, forêts et désertification de l'Afrique tropicale. - Paris, Vol. 1, 381 pp.