Bipindi akom II lolodorf region, southwest

Yüklə 2.08 Mb.
Pdf просмотр
ölçüsü2.08 Mb.
1   2   3   4   5   6   7   8   9   ...   24
and small cacao plantations are found throughout the area. In general these plantations are not 
well maintained due to continuing low world market prices. The devaluation of the Franc CFA in 
1994 was an incentive for the production but still more than 50% of the cacao plantations remain 
abandoned. Recently, industrial size oil palm, pineapple and banana plantations have been 
created in the TCP area. Villagers who work and live in surrounding towns, appear to be the 
initiators of this development. 


The Tropenbos Cameroon Programme aims at the development of sustainable forms of land use 
in Southwest Cameroon. The specific objective of the first phase of the Lu1 project is to provide 
base line information on the biotic and abiotic environment of the TCP area. This information 
will be used for land evaluation and planning procedures and for more detailed ecological 
research in the area. Based on the size of the area and the need for detail for land evaluation, a 
landscape ecological survey at scale 1 : 100,000 (reconnaissance scale) was carried out. 
Zonneveld defines landscape (1995; after Schroevers, 1982) as `a complex of relationships 
systems, together forming ( . . . ) a recognizable part of the earth's surface, which is formed and 
maintained by the mutual action of abiotic and biotic forces as well as human action'. The 
landscape can thus be seen, as is done in the present study,  as a fully integrated entity that can 
and should be studied as a whole (e.g. Breimer et al., 1986; Hommel, 1987; Küchler & 
Zonneveld, 1988; Zonneveld, 1995).  
Tropenbos has developed a Common Methodology for Land Inventory and Land Evaluation to 
contribute to a systematic and interdisciplinary research approach and subsequently sound land 
use planning (Touber et al., 1989). This methodology focuses on the integrated description of the 
environment and was used for the land unit surveys conducted in the Tropenbos sites in Côte 
d'Ivoire (de Rouw et al., 1990; de Rouw, 1991) and Colombia (Duivenvoorden & Lips, 1993; 
1995). The holistic nature of the landscape approach requires a multidisciplinary team of 
An almost indispensable tool in all landscape oriented studies is the use of remote sensing 
materials. Depending on the scale of the study and the objects to be mapped, satellite images and 
aerial photographs can be used. For the present reconnaissance survey black & white aerial 
photographs have been used. These images reveal the identity of the landscape and their 
systematic interpretation enables the delineation of the different land units. A land unit is `a tract 
of land that is ecologically relatively homogeneous at the scale level concerned' (Zonneveld, 
1995). Aerial photo interpretation provides a basis for stratified sampling in the field.  
The aim of the fieldwork is to collect accurate and reliable materials to describe the photo 
interpretation units. In addition, the relevance of the boundaries of the photo interpretation units 
is studied. Ecological relevant data on soil, vegetation, geomorphology and land use, as well as 
geological, hydrological, zoological and other appropriate land attribute information are collected 
in selected sample sites. The land attributes are classified and a legend is compiled. The resulting 
landscape ecological map is an important tool for land evaluation. In the following sections, the 
methodological aspects of the present survey are treated in more detail.  
Topographical base maps at scale 1 : 50 000 were supplied by the CENADEFOR (1987) and are 
in fact enlargements of the 1 : 200 000 topographic maps of the Institute Géographie Nationale 
(CGN, 1976). The sheets Edéa NA-32-XXIII 1b, 2a and 2b and Nyabessan NA-32-XVII 3d, 4c 
and 4d were joint to make a single topographic map of the TCP area. The topographic map was  

updated as for the location of logging roads with the aid of a Global Positioning System 
Black & white aerial photographs of the TCP area were taken in 1963-66 at a scale of 
1 : 50 000 and in 1983-85 at a scale of 1 : 20 000. The 1963-66 series has insufficient 
contrast to allow for stereoscopic vision. The much better quality photographs of the 1983-
85 series have therefore been used for photo interpretation in the present survey. A list of the 
photographs is presented in Annex III. 
The interpretation of the aerial photographs resulted in the drafting of preliminary maps on 
landforms and vegetation at a scale 1 : 50 000 (Touber, 1993a). The maps have hierarchical 
legends and discern a total of 49 and 25 legends units, respectively. A five week mission to 
the TCP research area was carried out in 1993 and sites were selected for the efficient 
sampling of the most important land units in the area (Touber, 1993b). 
3.3.1 GENERAL 
Observation sites were selected on the basis of the photo interpretation maps. Fieldwork 
involved the description of the attributes of the units discerned by photo-interpretation. 
Fieldwork was not merely a check to verify the photo interpretation as it largely entailed the 
collection of new information. 
The land units sampled were considered the most important and widespread in the TCP area. 
The actual sample sites were selected on the basis of representativeness and accessibility. 
Transects, or transverses, of one to two kilometres with a general orientation perpendicular 
to the contour lines, have been laid out. These transects provided access to the land units to 
be described and enabled the detection of possible catenas or toposequential processes. 
Within the transect three to seven observation points were selected for detailed description. 
The locations of the observation points were determined by both the soil surveyor and the 
vegetation specialist. At each sampled locality, landform, soil and vegetation characteristics 
were described. In Figure 3.1 the distribution of observation points in the TCP research area 
is given 
During the field work the survey team consisted of a soil surveyor, a vegetation surveyor, a 
field botanist (part-time), a local tree spotter and a local soil survey assistant. The transects 
were laid out by a `compass man' and two line cutters recruited from the nearest village. The 
distance along the transects was measured with a 'Topofyl' and each hundred meter was 
(temporarily) marked with a pole. The survey team covered approximately one kilometre per 
day for normal sampling procedures. One or two days extra were needed to dig and describe 
soil pits. 
All transects and observation points have geographical coordinates and can be retraced on 
the base map and the aerial photographs. Photo interpretation and fieldwork were not strictly 
separated in time; efforts have been made to study the aerial photographs after each period 
of fieldwork and to adjust the preliminary legends and maps to the insights gained. The 
sampling procedures for landform, soil and vegetation aspects are discussed in the sections 
3.3.2 and 3.3.3. 


The described land and soil related attributes are based on Touber et al. (1989), FAO (1990) 
and Touber et al. (1993), and include: altitude, slope length, slope steepness, slope form, 
exposition, landform, stone and rock coverage, erosion and stability characteristics, drainage 
characteristics, groundwater level, biological activity and, humus form. Soil characteristics, 
described per horizon, are: colour, mottling, texture, structure, consistency, cutans, rooting 
characteristics and pores. Additionally, schematic sketches of the transects were made, 
showing the variation in slope percentage, slope direction, slope length, and exposition 
within each land unit.  
At least three auger hole observations per transect were made. The auger holes were made 
with a standard type Edelman auger and range in depth from 1.2  to 2.0 m. A total of 207 
soil and landform observations were made. In Figure 3.1 the distribution of observation 
points in the TCP area is presented.  
After a first survey of the transects, about 60 sites representing the different landforms and 
soil types of the TCP area were selected, soil pits were dug and soil characteristics were 
described in more detail. All soil pits have a minimum depth of 1.50 m. Samples of horizons 
were collected for chemical and physical analysis. 
Soil samples were chemically and physically analyzed at the IRA soil laboratory in Ekona. 
Duplicates of some 40 samples were analyzed as control at ISRIC, Wageningen. Chemical 
soil properties determined are organic carbon, total nitrogen, available and total 
phosphorous, pH H
O and KCl, exchangeable bases (Na, K, Mg and Ca), aluminum and 
hydrogen, and cation exchange capacity (CEC). Physical parameters determined are texture, 
water retention characteristics (pF) and bulk density. Additionally, the clay mineralogy of 
some twenty-five samples were analyzed by ISRIC. The methods used for chemical and 
physical analyses are described in Annex IV.  
All field data concerning the site and soil characteristics of the augerings and soil profile 
descriptions were compiled in the TROFOLIN Database (Touber et al., 1993). Data of 
chemical and physical analysis are stored in QUATTRO PRO. 
3.3.3 Vegetation 
The distribution of plant species in tropical forests is not random but reflects environmental 
conditions (Whitmore, 1984 and Zonneveld, 1995). Species which can successfully compete 
with one another within the limits of a particular combination of environmental features can 
be considered as a plant community. The mosaic of plant communities forms the vegetation 
in an area (Küchler & Zonneveld, 1988). Moreover, the pattern of vegetation types, defined 
as plant-communities, may be used to assess the distribution patterns of distinct plant 
In the present survey the vegetation was described on the basis of its structure and floristic 
composition, in the sense of the French-Swiss school (phytosociological approach; Braun-
Blanquet, 1964). The method implies a classification of vegetation types by tabular 
comparison of plot data, which leads to the identification of communities, characterized by 
phytosociological groups (Touber et al., 1989).   
The localities described in the vegetation survey lie within the most common and 
widespread photo interpretation units in the TCP research area (see 3.3.1). The stratification  

of sample points is based on the hypothesis that altitude (Fig. 2.1) and disturbance (both 
anthropogenic and natural) cause most of the variation in the vegetation in the TCP research area. 
The observation points are therefore evenly spread along these two environmental gradients 
(Table 3.1). Additionally, in relatively undisturbed forest vegetation efforts have been made to 
describe the vegetation along the toposequence, comparable to the study of catena's in the soil 
At each selected locality a detailed description of the vegetation, a `relevé', was made. As far as 
possible all growth forms, excluding mosses, ferns, epiphytes and small seedlings (due to 
difficulties in field identification), have been recorded in a plot of 100 m
. In addition, all trees 
with diameter at breast height (dbh) 
≥20 cm were recorded in the surrounding 1000 to 2500 m

Though not approaching the minimal area of tropical rain forest communities, this plot size is 
sufficiently large for classification purposes if all terrestrial growth forms of flowering plants and 
all size classes are included (Hommel, 1987; 1990; de Rouw, 1991).  
The vegetation within each sample plot was homogeneous with respect to its structure, and 
representative for the surrounding area (Kent & Coker, 1992; Hommel, 1995). The intention was 
to describe the characteristic vegetation type(s) within each photo interpretation unit and 
therefore a-typical situations were omitted from sampling. A total of 125 vegetation relevés has 
been described in the reconnaissance scale survey and is used for the classification of the 
vegetation in the TCP research area. Table 3.1 gives the distribution of the vegetation relevés 
over the environmental gradients altitude and disturbance. 
Table 3.1 Stratification of vegetation relevés along the environmental gradients altitude and disturbance 
40-180 m asl  180-340 m as  340-540 m asl 
> 540 m asl 
vegetation on  

(former) agricultural 
secondary forest  *** 

fields plantations 

logged-over forest 


dynamic forest on  
steep slopes 

valley bottoms 
primary forest 
summit areas 

Number of observations: - = not found; * = 1 relevé; ** = 2-5 relevés; ***=  6-10 relevés; ****= 11-15 relevés. 
The recording of plant species was carried out according to guidelines given by Touber et 
al.(1989), i.e. separate records per stratum and with cover/ abundance estimation in 14 classes 
(`ITC approach'). However, a complete recording of vegetation characteristics according to the 
TROFOLIN procedure (Touber et al., 1993) was considered too time-consuming for a 
reconnaissance survey (Hommel, 1995).  
Plant identification was done in the field with the help of a field botanist (part time) and a local 

 tree spotter. Plant material of unknown species and for verification purposes has been 
collected in `Quick Herbarium' style (Küchler & Zonneveld, 1988). The `Herbier National 
du Cameroun' (HNC), the Limbé Botanic Garden and the Herbarium Vadense were 
consulted for identification. Under the prevailing conditions the field identification is not to 
be regarded equal to the botanical identification. Each field `species' often consists a cluster 
of botanical species with a similar general appearance. Efforts have been made to limit the 
size of these groups as much as was deemed appropriate in light of reliability. In the 
analyses of the vegetation data these groups have been used as entity.  
Next to the detailed description of the sample points, the vegetation structure and land use 
along the transects was indicated on drawings. These drawings give insight in the mosaic of 
the vegetation in the different mapping units. In the second phase of the Lu1 project, will 
study these mosaics are studied in more detail. Also some 20 incomplete or so-called `quick 
relevés' were described. These incomplete descriptions do not support the classification of 
the vegetation, but help to investigate the relations between vegetation and abiotic factors. 
Moreover, they are of cartographic importance.  
All vegetation data have been stored in TURBOVEG, a software package for input, 
processing and presentation of phytosociological data (Hennekens, 1995). The output of this 
package is, in contrast to TROFOLIN (Touber et al., 1993), compatible with vegetation 
classification programmes like TWINSPAN (Hill, 1979a), SHAKE (Tongeren, unpubl.), 
DECORANA (Hill, 1979b) and CANOCO (ter Braak, 1988).  
Based on the variation observed within the TCP research area, preliminary classifications 
were drafted for landforms, soils and vegetation. To describe the landscape as a complex of 
these attributes, the relations between landforms, soils and vegetation types were studied by 
means of cross-tables. The aim of this exercise was to trace the parameters of the abiotic 
environment which may best explain the variation in vegetation types. Based on these 
analysis, the landforms, soils and vegetation classifications were slightly modified to attain 
an optimal `ecological' fit. 
The classification of landforms is based on differences in relief characteristics, i.e. slope 
length, slope steepness, relief intensity, and the number of interfluves. These characteristics 
are important in the light of land utilization. 
The classification of soils in the TCP research area is primarily based on soil drainage and 
soil texture. Soil depth and stoniness are other differentiating criteria but could not be used 
at this scale. Soil drainage and texture are found to have a correlation with vegetation in the 
TCP research area. As soil texture and soil drainage are indicators for moisture availability, 
they are two of the functional soil parameters for land evaluation as well. 
The classification of the vegetation of the area was done by means of `tabular comparison' of 
the relevé data using the computer programme TWINSPAN (Hill, 1979a). The programme 
performs a multi-variate analysis of vegetation data and produces a hierarchical 
classification of both sample points and species. Manual refinement of this classification 
included the reallotment of a limited number of borderline relevés. Also some 11 relevés  

were excluded from analysis because they proved to represent either intermediate situations on 
the highest level of classification, or represented ecologically aberrant situations that were not 
considered relevant to the reconnaissance survey.  
The cover values of the three structural layers, i.e. tree, shrub and herb layer, have been 
combined into one value for each species. Four coverage classes were used,  0-5%, > 5-25%, 
> 25-50% and > 50-100% cover, regardless of the abundance of the species. For classification 
of vegetation on a reconnaissance scale, this is considered sufficiently detailed.  
The classification by TWINSPAN is primarily based on the floristic composition of the 
sample plots. Next, a synecological interpretation of the vegetation types discerned has been 
carried out on the basis of the environmental data collected at the different plot sites. 
The final legend of the landscape ecological map is based on the classification systems for the 
individual attributes (landform, soil and vegetation), the study of correlations between these 
attributes and the annotated preliminary photo interpretation maps (Hommel, 1987; Küchler & 
Zonneveld, 1988). 
The landscape ecological map (1 : 100 000) is thus based on 256 landform and soil observations 
and 125 vegetation relevés, and covers a total surface of 167,350 ha. Consequently, the 
observation density for landform and soil attributes is one per 650 ha and one per 1150 ha for the 
vegetation attributes. These observation densities are theoretically just sufficient (Landon, 1991). 
The representation of the identified vegetation, soil and landform types on a map depends largely 
on mapping scale. In the present reconnaissance survey (scale 1 : 100 000) most of the units of 
the present landscape ecological map contain  mosaics of soil and vegetation types.  
Map compilation has been facilitated by the GIS package ArcInfo (ESRI, 1990; 1994). Digitized 
topographic base maps have been made compatible with the land attribute classification systems, 
enabling spatial analysis and the compilation of land attribute and evaluation maps (Bakkum, 


The geomorphology of Southwest Cameroon is described by Martin & Segalen (1966) and by 
Franqueville (1973) at a scale of 1 : 1 000 000. The western part of the TCP area belongs to the 
coastal lowland of late Tertiary-Quaternary age, whereas the eastern part belongs to the interior 
plateau of Eocene age. At the boundary of these two zones a transitional complex is found (Sega-
len, 1967). At this transition different planation levels are present. The planation levels at 200 to 
300 m and 600 to 800 m correspond with the erosion surfaces Africa II and I (Martin & Segalen, 
1966; Franqueville, 1973). The two erosion surfaces have a well pronounced relief. They are 
characterized by hills and dissected uplands with numerous small streams. The slopes of the hills 
and uplands have convex upper parts and concave lower parts. Laterite banks are of minor 
importance and the tropical clay soils are generally deep.We have subdivided the area into four 
altitude classes which party coincide with the mentioned erosion surfaces. The altitude classes are 
< 350 m, 350-500 m, 500-700 m and > 700 m.  
Important processes leading to the formation of today's landforms have been tectonic movements 
of parts of the Precambrian shield, e.g. block faulting along NE - SW lines, climatic changes and 
erosion processes related to rapid changes of the erosion basis. These processes resulted in 
different planation levels or erosion surfaces (Martin & Segalen, 1966; Buckle, 1978; Embleton 
& Thornes, 1979). The physiognomy of the landscape is still being `reformed'. The most 
important process is water erosion resulting in the dissection of the area. The intensity of this 
dissection is determined by the relative differences in erodibility of both soil cover and 
underlying rock, and by the amount of rainfall. Differences in erodibility of the rock are caused 
by differences in density of fractures and/or its mineral composition. 
Table 4.1 Relief characteristics of the different landforms 
Landform units 
Slope length 
Slope (%) 
Relief intensity 
No. of 
interfluves per 
Altitude range 

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