Bipindi akom II lolodorf region, southwest

Yüklə 5.01 Kb.
Pdf просмотр
ölçüsü5.01 Kb.
1   2   3   4   5   6   7   8   9   ...   24
Soil type 
Total N 
Available P 

Nyangong soil (n=6) 

Ebom soil  (n=4) 
Ebimimbang soil (n=6) 
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. 
5.4.7 Conclusions  
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. 
5.5.1 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 
research area. 
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. 

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 
(Letouzey, 1985) 
Région Guinéo-Congolaise   
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 
gabonensis (n

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 
Caesalpiniaceae (n
 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. 
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' 

Maranthes - 
Podococcus - 
- Polyalthia 
Diospyros - 
Carapa -
Drypetes `group 1' 

II I + 

. . 
Anisophyllea polyneura 

r . + 

Maranthes glabra 
. . 

Scorodophloeus zenkeri 
II + 

. + 

Monop. `group 1' 
II I  II + . 
Raphia `species 1' 
+ + .  .  . 
Geophila `species 1' 
. r . + . 

Поделитесь с Вашими друзьями:
1   2   3   4   5   6   7   8   9   ...   24

Verilənlər bazası müəlliflik hüququ ilə müdafiə olunur © 2019
rəhbərliyinə müraciət

    Ana səhifə