The nutrient contents of the soils in the TCP research area are comparable with data from
other studies on humid tropical forest soils (Anderson & Spencer, 1991; Gillman et al.,
The nutrients from the soil are taken up by the crops grown. For instance, maize takes up an
average of 16 kg N, 1.5 kg P and 13 kg K per ton harvested grain. With groundnuts an average of
33 kg N, 2 kg P and 17 kg K is removed per 1000 kg of product. With 10 tons of cassava tubers
120 kg N, 7 kg P and 80 kg of K disappears from the field.
The nitrogen is mainly contained in the organic matter fraction of the soil. Assuming a 2%
mineralization rate and an availability of 50%, gives for the Ebom soils a potential supply of 89
kg N per ha per year. This means that in the first year after clearing, N will not be a limiting
factor. Only if the soil is left bare and organic matter decomposes rapidly, N may become in
limited supply. The phosphorous stock, however, will be depleted fast with cultivation and
already after one year deficiencies may occur, especially in Nyangong soils. The potassium stock
will last a few years but with continuous cultivation would become depleted within 4 to 5 year,
depending on the cropping intensity.
The soils in the TCP area have low (chemical) fertility levels. Especially the Nyangong and the
Ebom soils are very acid. The cation exchange capacities, which determine the capacities of the
soils to retain and subsequently release nutrients, are low to moderate but will drop rapidly with
declining organic matter levels. This is because the contribution of the clay fraction to the CEC is
very limited as the dominating clay mineral is kaolinite.
A large part of the nutrients is stored in the biomass of the vegetation. Through recycling in a
stable environment, these nutrient stocks remain in tact. Removal of the vegetation severely
interrupts the nutrient cycle and trough leaching and erosion part of the nutrients may disappear
out of the system.
5.5 SOIL-LANDFORM RELATIONS AND SOIL GENESIS
5.5.1 Soil and landform relations
The TCP research area can be divided into three broad areas: (i) the mountainous and hilly
eastern region with deep to very deep, well drained, yellowish brown to strong brown clay soils
(Nyangong soils); (ii) the rolling and hilly uplands with scattered hills in the transitional or
central and northern regions covered with deep to very deep, well drained, brownish yellow to
strong brown clay soils (Ebom soils) and (iii) the dissected erosional plains and rolling and hilly
uplands in the western lowlands with moderately deep to deep, moderately well to well drained,
yellowish brown sandy clay loams to sandy clay soils with sand to sandy loam topsoils
(Ebimimbang soils). A fourth category are the valley bottoms with moderately deep to very deep,
poorly to very poorly drained soils formed in stratified unconsolidated recent alluvium showing
alternating layers of sand, loam and clay (Valley Bottom soils). These Valley Bottom soils are
found all over the TCP research area, the majority of which could not be mapped individually.
The major part of rocks in the TCP research area do not differ much from each other. In the
eastern part, fine textured gneisses dominate and diorites and/or granites rich in pyroxene are
present as well, whereas in the western lowlands, coarse grained gneisses dominate and
migmatites occur locally. These relatively uniform mixtures of metamorphic rocks (mainly
gneisses) and igneous rocks are not decisive in differentiating landforms and soils in the area.
The relatively small variations in mineralogy and density of fracturing of the parent material,
result in only small differences in erodibility and soil fertility. Normally, lithological differences
largely contribute to the formation of today's landscape (Embleton & Thornes, 1979).
The combination of block faulting and the process of denudation (mechanical and chemical
erosion) have resulted in planation or erosion levels of different age, which might be the
explanation for the differences encountered in the landforms and soils in the TCP research
area. The eastern region of the TCP region has been lifted upwards in relation to the western
lowlands along NE-SW orientated faults. The denudation, which can be called (valley floor)
pedimentation (Zonneveld, 1981; Embrechts & Dapper, 1987), has resulted in almost flat
erosional plains with inselbergs in the western lowlands and in a very dissected landscape in
the mountainous eastern region. This dissected landscape has been developed originally
from an old erosional plain. The progressive erosion (slope retreat) from West to East has
resulted in very small differences in altitude (low relief intensity) between the western part
of the TCP research area and the Atlantic Ocean, i.e. the erosion basis. It is concluded that
the landscape in the western part of the TCP research area is younger than the eastern one.
The soil forming processes, which are mentioned in section 5.5.2., may have been causing
spatial variations. A downward trend from West to East in rainfall and temperature may
have caused differences in intensity and nature of the soil forming processes. This trend,
however, is locally disturbed by the topography. Lower amounts of rain are observed around
Bipindi (Waterloo et al., 1997) and can be an additional explanation for richer and less deep
soils in these western lowlands.
Additionally, the age of the landscapes within the TCP research area varies and as a result,
the soil forming processes differ in their periods of activity. Hydrolysis in the western
lowlands is not in as advanced state as in the eastern region. Moreover, the importance of
soil forming processes such as plinthite formation and clay illuviation decreases from West
to East. Besides the variation in rainfall and temperature and the different ages of the
landscapes in the research area, the spatial distribution of the soil forming processes is also
influenced by topographical position. All these factors result in differences in soil drainage,
texture and soil depth.
The soils are relatively uniform in colour but differ in texture, depth or drainage. The
variation in texture seems to be related to differences in the age of the landscape. Perhaps
small, during the survey not noticed, differences in texture and mineralogical composition of
parent material could also be important. Younger areas like the western lowlands have less
weathered soils, resulting in coarser textured soils (Ebimimbang soils), whereas the deeply
weathered soils in the eastern region have a predominant clayey texture. The less weathered
soils are richer in nutrients, have higher pHs and clear clay cutans (evidence of active clay
movement to the subsoils) and are less deep than the clayey textured Nyangong soils formed
in an older landscape.
Drainage differences are especially related to topographical positions. The valleys have
poorly to very poorly drained soils, whereas on the other topographical positions,
moderately well to well drained soils occur.
The variability in soil depth may be large at small distances for which relative resistance to
weathering of the parent rock and the degree of fracturing may be an explanation.
Further research (e.g. clay mineralogy, geomorphological processes, geology) is needed to
elaborate these theories. Also, research on the variation of soil depth with topographic position
(catena) will be very useful for a better understanding of processes which are active in the TCP
5.5.2 Soil genesis
The following soil forming processes have been contributing to soil formation in the TCP
research area: formation A-horizon, hydrolysis, ferralitization, kaolinitization, plinthite and
laterite formation, eluvation and illuvation of clay and oxidation/reduction processes.
Accumulation of litter which is decomposed by soil flora and fauna in the mineral topsoils
results in the formation of an A-horizon. Mineralization of organic matter releases nutrients
which can be taken up by the surrounding vegetation. The process of decomposition,
mineralization and uptake by the vegetation is relatively quick, therefore A-horizons are thin.
Low pH levels, however, will retard the decomposition and organic matter may accumulate
(Mohr et al., 1972). The chemical fertility of the tropical soil is strongly related to the presence
of organic matter in the topsoil because of its storage and release capacity of nutrients. In the
TCP area there are significant differences in organic carbon contents between the different
topsoils (section 5.4.2.).
The main soil forming process in the TCP area has been hydrolysis (cations in the primary
silicate structures of minerals are exchanged against H
-ions). The hydrogen ion weakens the
mineral structure, facilitating the dissolution of Si and Al from the clay lattices. Ferralitization or
desilication is hydrolysis in an advanced stage. A combination of slow release and subsequent
leaching of cations and silica keeps the concentration in the soil solution low. If the soil
temperature is high and percolation is intense, ultimately all weatherable primary minerals will be
removed from the soil mass. Less soluble compounds such as iron and aluminum oxides and
hydroxides, as well as coarse quartz grains, remain behind (Driessen & Dudal, 1989; Mohr et al.,
1972). A low pH, low concentrations of dissolved weathering products in the soil solution (low
EC - values) and geomorphic stability over prolonged periods of time are conditions which
accelerate the process of ferralitization (Driessen & Dudal, 1989). All these conditions are
present in the TCP area. Due to the presence of gneiss (acid rock) with few easily weatherable
minerals and much quartz, ferralitization proceeds much slower. Although much silica disappears
through leaching (desilication), silica contents remain higher than in soils formed on basic
material. This silica combines with aluminum to the 1:1 clay mineral kaolinite, which is called
the kaolinitization process. Gibbsite is normally absent. It is however, formed under freely
drained conditions and from richer rocks. The dominant minerals in the soils in the TCP research
area are kaolinite, goethite (FEO(OH)) and gibbsite (Al(OH)
). The colour of the soils, orange to
yellowish brown, is determined mainly by the presence of goethite (section 5.4.5.). Hematite
), which gives the soil a bright-red colour, is not observed in the TCP research area.
Plinthite is an iron-rich, humus-poor mixture of clay and quartz. It is formed by the (relative
and/or absolute) accumulation of sesquioxides (i.e. removal of silica and bases by ferralitization
and/or enrichment from outside) and the segregation of iron mottles (alternating reduction and
oxidation). In the TCP research area plinthite is regularly found on the lower slopes between 40
and 500 meters altitude (uplands and dissected erosional plains). Within the Ebimimbang soils, a
subtype can be distinguished with plinthite in the upper 125 cm. Laterite formation is the
hardening of plinthite to laterite. The main processes are the crystallization of amorphous iron
compounds to aggregates of iron oxide minerals and the dehydration of goethite to hematite and
of gibbsite to boehmite (Aleva, 1994; Driessen & Dudal, 1989). Laterite gravels are present in
limited amounts in the TCP research area. The laterite gravels are remnants of old eroded surfaces.
Ferruginated rock fragments are more common.
Clay eluviation/illuviation is the redistribution of clay in the profile, resulting in an increase in clay
content with depth. Mobilized clay is transported downward and deeper in the profile where it is
immobilized (Driessen & Dudal, 1989). Cutans of clay on the structural elements in the subsoil are
evidence for recent illuviation. Biological activity may destruct these clay cutans. These cutans are
mainly found in the Ebimimbang soils in the western lowlands in the TCP research area (40-350 m
asl). All soils in the TCP research area have a clay increase with depth, but cutans are rarely found
in the eastern part. Clay movement is probably related to the past when the soil was still less
weathered and soil pH was somewhat higher than at present (van Kekem et al., 1997).
In the oxidation phase, the presence of oxygen leads to the transformation of soluble ferrous
compounds to ferric compounds. These precipitate on soil particles, giving the soil its reddish
colour. The reverse occurs during the reduction phase. The lack of oxygen causes dissolution of
ferric compounds, giving the soil the colour of non-ferrous minerals, forming its matrix (grey,
olive or blue matrix colours). This soil forming process of oxidation-reduction is associated with
the fluctuation of the groundwater table. In the zone with alternating oxidizing and reducing
conditions, mottles are often formed (Driessen & Dudal, 1989). This soil forming process is
important in the Valley Bottom soils.
6.1 LITERATURE REVIEW
Flora and vegetation of Cameroon are relatively well known. The most relevant studies for the
present vegetation inventory are Letouzey's `Étude Phytogéographique du Cameroun' (1968) and
his `Carte Phytogéographique du Cameroun au 1 : 500 000' (1985).
The TCP research area is part of the Guineo-Congolian domain of dense humid evergreen forests
(see 2.7) and for the greater part belongs to the Biafran Atlantic district (low and medium
altitude). Only along the fringes elements of the low and medium altitude Littoral Atlantic district
may be found. Individual small summit areas belong to the submontane zone of the Guineo-
Congolian domain (Letouzey, 1968, 1985).
In Table 6.1, the original french denominations of the districts are presented, together with the in
the TCP area represented formations. In Figure 6.1 the area relevant to the present survey of
Letouzey's (1985) phytogeographical map is reproduced. In the following a brief description of
the different vegetational zones is given.
Table 6.1 Forest types of the TCP research area and its surrounding; 1 : 500,000 phytogeographical map
Étage submontagnard (800-2 200 m)
Forêts submontagnardes 800-2 200 m (n
Étage de basse et moyenne altitude (0 - 800 m)
District atlantique biafréen
Forêts atlantiques biafréennes à Caesalpiniaceae (n
Forêts atlantiques biafréennes à Caesalpiniaceae encore abondantes, avec Saccoglottis
gabonensis et autres indices littoraux (n
Forêts mixtes, toujours vertes atlantiques et semi-caducifoliées, avec prédominance
d'éléments de forêts toujours vertes atlantiques (n
Faciès de dégradation prononcée des forêts toujours vertes (n
234 = n
District atlantique littoral
Forêts atlantiques littorales à Caesalpiniaceae relativement rares, avec Saccoglottis
Faciès de dégradation prononcée des forêts toujours vertes (n
251 = n
The majority of the TCP research area lies within the Atlantic Biafran forests zone rich in
228 in Letouzey, 1985). The vegetation can be characterized as
evergreen tropical moist forest with many species of the Caesalpiniaceae family. Of the total
of 130-140 species of shrubs and trees of this family recorded in Cameroon, more than half
appear to be concentrated in the Biafran Atlantic forests. Many Caesalpiniaceae species are
gregariously distributed. The range in altitude of this forest type is (100) 200-500 m asl.
and the vegetation resembles `cloud forests' with many epiphytes and a canopy that reaches only
20-30 m. Another striking characteristic is the near complete absence of members of the
Caesalpiniaceae family, apart from typical mountain species like Monopetalanthus spp.,
Plagiosiphon sp., Hymenostegia sp. and Anthonotha cf. cladantha.
Along the main access routes and in the vicinity of the larger villages strongly degraded remnants
of evergreen forests (n
234 and 251) are found. In general the degraded vegetation is
characterized by the absence of a tree layer, except for a few residual forest trees, and the
abundance of pioneer species such as Haumania danckelmanniana, Harungana
madagascariensis, Megaphrynium macrostachyum, Xylopia aethiopica, and Musanga
cecropioides. According to Letouzey's map, the degraded areas are not differentiated by
phytogeographical zone, i.e. littoral Atlantic district and Biafran Atlantic district.
6.2 BOTANICAL DIVERSITY
A total surface of approximately 20 ha was inventoried during the present vegetation survey. A
total of 490 taxa have been identified to the species level, be longing to 76 families. The most
species rich families are: Euphorbiaceae (47 species), Caesalpiniaceae (43 species), Rubiaceae
(29 species) and Annonaceae (18 species). All identified species are listed in Annex Va.
Identification in the field proved to distinguish some 530 `species', including several (small)
groups of botanical species. The basis for the vegetation classification is the field identification.
The number of plant species encountered will surely increase with time when collecting is
continued and the skill of the collectors grows. It is therefore difficult to indicate the plant
diversity of the TCP area on the basis of these results. A large and species rich area like ours,
needs intensive collection to properly reveal species number, rarity and endemics. The second
phase of the Lu1 project will give some indications in this respect, while the forthcoming Ecol1
project is will be properly equipped to address the question of plant diversity.
6.3 Vegetation classification
The vegetation of the TCP research area can be divided into seven distinct plant communities.
Information gathered during the reconnaissance vegetation survey clearly indicates that within
each of the plant communities two or three variants can be distinguished.
A dendrogram representing the hierarchical vegetation classification is presented in Figure 6.2.
The hierarchy is based on similarities in species composition as given by the TWINSPAN
analysis. A division at a high level coincides with major differences in floristic composition. A
division at a lower level coincides with more subtle differences in species composition.
Not surprisingly, a distinction is made on the highest level between the floristic composition of
forest and shrub land. Of more interest is the distinction at the second level between the forest at
low altitudes and those on high altitudes. The flora of mountain forests (> 700 m above sea level)
proves to deviate strongly from that of lowland forests, apparently regardless of soil type and
former disturbances. In Table 6.2 an overview of the vegetation types is presented. Each
community is named after a characteristic combination of occurring genera.
boundaries of the TCP area, similar ecological requirements and thus indicative value. A total of
20 sociological species groups have been identified for the TCP research area. The composition
of these groups is presented in Annex Vb. Table 6.3 is a concise `summary table' presenting the
most important differentiating species for the vegetation of the TCP research area.
Table 6.3 Concise summary table of plant communities in the TCP research area listing the most important
differentiating species, clustered in `sociological species groups'
Drypetes `group 1'
II I +
r . +
Monop. `group 1'
II I II + .
Raphia `species 1'
+ + . . .
Geophila `species 1'
. r . + .