The influence of drainage and soil phosphorus on the vegetation of Douala-Edea
Forest Reserve, Cameroun*,**
D. McC. Newbery 1, J. S. Gartlan 2, D. B. McKey3 & P. G. Waterman4
~Department of Biological Science, University of Stirling, Stirling FK9 4LA, Scotland, U.K.***"
2Wisconsin Regional Primate Center, University of Wisconsin, Madison, Wisconsin 53715 U.S.A.****"
3Zoologisches Institut, Universitiit Basel, Rheinsprung 9, CH-4051 Basel, Switzerland; 4phytochemistry
Research Laboratory, Department of Pharmaceutical Chemistry, University of Strathclyde, Glasgow G1
IXW, Scotland, U.K.
Keywords: Africa, Cameroun, Douala-Edea, Drainage, Gradient, Phosphorus, Rain-forest
Abstract
All living trees (>_ 30 cm gbh) were enumerated in 104 80 × 80 m plots arranged along four transects in
the Douala-Edea Forest Reserve Cameroun, a system of low-lying ancient coastal sand dunes interspersed
by numerous streams and swamps. The extent of permanent and seasonal swamps was recorded for each plot.
Two hundred thirty taxa were recognized of which 63% were identified to species. Mean tree density was
376 ha -1, basal area 31.0 m 2 ha -1 and number of species per plot 39. The Olacaceae were the most abundant family in terms of basal area, but the Euphorbiaceae the most frequently represented. The most abundant species was Coula edulis (Olacaceae). Twenty-two plots had most of their area permanently or seasonally swamped. Percentage sand, silt and clay ranged between 32-100, 0-64, 0-21% respectively. The ranges for
other variables recorded were: pH (2.7-5.4), organic carbon (1.5-12.4%), available phosphorus (7-90 ppm)
and potassium (28-188 ppm), and nitrogen (ammonium 4-40 ppm, nitrate 1-12 ppm).
Classification of the plots on the basis of six soil variables provided three large distinct groups: swamp
plots and non-swamp plots, the latter divided into plots of low and high available soil phosphorus. Swamp
plots were distinguished by high abundances of Protomegabaria stapfiana and Librevillea klainei, though
correspondence ordination of plots in these groups showed P stapfiana associated with more clayey soils and
Librevillea klainei (and Gluema ivorensis) on the very sandy soils. Direct gradient analysis highlighted several
species associated with these lower phosphorus soils. Available soil phosphorus is not as low at Douala-Edea
as in parts of Korup, and the association of these Douala-Edea soils with the Caesalpinioideae is correspondingly weaker.
* Nomenclature follows Aubr6ville (1963-1983).
** The field work was supported by grant numbers
RR00167-14, RR00167-15 and RR0167-16 from the National
Institutes of Health for the operation of the Wisconsin Regional
Primate Research Center, and N.A,T.O. Scientific Affairs grant
number 1748 (to PGW and JSG). It was greatly facilitated by the
skill and dedication of Ferdinand Namata. R. M. Polhill and D.
W. Thomas assisted considerably in the identification of plant
species. Sue Gartlan collected and collated the meteorological
data, besides other field support. In the field phase J.S.G.,
P.G.W. and D.B.McK. were researches attached to the National
Vegetatio 65: 149-162, 1986
© Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands
Office of Scientific and Technical Research (ONAREST),
Yaound6. We thank M. D. Swaine for comments on earlier
drafts, R. Letouzey for checking species nomenclature, the
Computer Unit at Stirling University for facilities, M. Burnett
for the typing at Stirling, and the Department of Soil Science,
University of Wisconsin, for undertaking the soil chemical analyses.
*** Reprint requests to D.McC.N. at Stirling.
**** Publication No. 23-025 of the Wisconsin Regional Primate Center.
150
Introduction
In a previous paper (Gartlan et aL, 1985) we
reported the results of a multivariate analysis of the
vegetation of the Korup Forest Reserve (now Korup
National Park) in Cameroun.
We here report the results of a similar study
carried out in the Douala-Edea Forest Reserve. The
comparison of these two areas is of particular interest as both form part of the coastal forest belt of
Cameroun and show some overlap in tree species.
However, the history and edaphic environments of
these two forests are in marked contrast, the
Douala-Edea landscape probably being of comparatively recent origin and based on marine
deposits, whilst Korup seems to represent part of a
major West African refuge area (Gartlan, 1974)
and is based on weathered granite and quartzite
soils. For example, whilst Korup has had a long and
relatively stable history during recent periods of
glaciation, Douala-Edea is likely to have undergone
several cycles of drastic changes, ranging from dry
periods due to oceanic retreat during glaciation to
the present area of the Reserve being inundated
during interglacial periods.
Like Korup, the Douala-Edea Reserve contains a
large number of rare and endangered animal species, notably the black colobus monkey (Colobus
satanas Waterhouse) and the African Manatee
(Trichechus senegalensis Link.). The vegetation of
this Reserve has been the subject of a detailed and
continuing phytochemical survey which has revealed that it invests very heavily in a wide range of
secondary metabolites (Gartlan et al., 1980; Waterman, 1983). The impact of these compounds on
food selection and the feeding behaviour of the
black colobus monkey has also been investigated
(McKey et al., 1981). Although the Reserve has been
hunted to a limited extent recently by the local inhabitants, with evidence of old pigmy camps
around Lake Tissongo (R. Letouzey, pers. comm.),
there is no evidence of any large scale interference
with the forest.
The study area
The Douala-Edea Forest Reserve lies on the Atlantic coast of Cameroun from the south of the Nyong
River (3°13'N; 9°54'E) north to Souellaba Point
(3°49'N; 9°33'E) and extends inland to include a
total area of about 130 000 ha (Fig. 1 of Gartlan et
al., 1985). The Reserve is divided into two unequal
parts by the Sanaga River which, inland, forms the
northern boundary for the larger southern part in
which the study took place. About 1°70 of the Reserve is covered by open water, the principal area
being Lake Tissongo, the starting point for three of
the four transect lines used in this survey.
The entire Reserve lies below 80 m elevation and
rarely exceeds 50 m. The area around Lake Tissongo is about 15 to 20 m elevation and slopes gradually toward the coast, the altitude being about 10 m
in the area of transect A (Fig. 1 of Gartlan et al.,
1985). However, the topography of the Reserve is
not flat but is cut by many low-lying streams or
swamps through which the water drains north into
Lake Tissongo and the Sanaga or south into the
Nyong. Toward the coast, where the system of
coastal sand dunes is still extant, drainage channels
are very tortuous, often running along dune systems parallel to the sea for long distances. Drainage
is to a large extent dependent on the levels of the
Sanaga and Nyong and in periods of flood water
can actually flow back from the rivers into the Reserve, thereby inhibiting drainage and inundating
large areas.
The soils found in the study area are very high in
sand content which was anticipated as the Reserve
lies entirely upon marine sediments built up by
north flowing marine currents, a process that probably began during the Cretaceous and still continues today. Analysis of fossil microfauna to a
depth of about 2 700 m reveals a continuous marine origin (Gazel, Hourcq & Nickels, 1956). The
Precambrian rock that marks the 'true' edge of the
African continent lies as much as 60 km inland
from the present coast, well inland of the Reserve,
with the area of the Reserve built up entirely by marine deposits and preserved by tectonic uplift associated with volcanic activity along the Mount
Cameroun-Bioko line (Hori, 1977).
Meteorological data were collected at the base
camp by Lake Tissongo from March 1976 to
November 1978. Prevailing conditions throughout
the area of the Reserve appear to be very uniform
and these findings can be related to the whole study
area. The total annual rainfall is in the range of
3 000 to 4 000 mm per year. Heaviest rainfall was
between August and October, diminishing to the
151
very dry months of December and January. From
February onwards rainfall increases steadily with
June being the very wet. Thus the rainfall pattern
observed appears to be intermediate between the
unimodal distribution seen in Douala and in the
Korup Forest Reserve and the bimodal distribution
seen further south on the coast at Kribi. The mean
monthly temperature varied between a m a x i m u m
of 28.7 °C and a minimum of 24.6 °C with a mean
annual diurnal variation of only 8.8 °C.
Methods
Floristic data
Floristic analysis was based on 104 plots, each
80x 80 m, ranged along four transects labelled A,
B, C and D. The transect cutting and tree numbering was conducted between March 1975 and August
1976. Transects B, C and D were cut from Lake Tissongo roughly at right angles to prevailing physical
gradients. Each was intended to be 5 km in length
with a total of 34 plots spaced equidistantly along
the transects. In practice, transects C and D terminated at lesser distances, after 24 and 12 plots
respectively, as at those points they reached permanent Calamus swamp. Transect A was cut for a full
5 km from the Lomb6 C a m p site, SW of Lake Tissongo and starting inland about 7 km from the sea.
Thus the total area enumerated was 66.56 ha, taken
from within the area of the Reserve which spanned
20 km from west (plot A1) to east (plot C24) and
8 km from north (plot D12) to south (plot B34).
Within transect A plots were subdivided into
4 0 × 4 0 m subplots as in the Korup study (Gartlan
et al., 1985).
In each quadrat all living trees and lianes with a
girth (gbh) at breast height (1.3 m) of 30 cm or
more were enumerated. Each living individual was
identified or, where this was impossible, collected
and assigned a code number for future reference.
Edaphic variables
Soil samples for transect A were taken from a
single point within each subplot of a plot, the exact
position being decided randomly. Samples were
taken from the surface (0-10 cm) and at a depth of
45 cm and separately bulked for each plot. Only
the results from surface samples are reported here.
For transects B, C and D the samples taken were of
a composite nature collected from the top 10 cm of
soil after removal of surface detritus at points 20 m
N, E, S and W of the centre of the plot. All soil
samples were oven dried at 70 °C and stored in airtight polythene bags until their return to the
University of Wisconsin for analysis.
Soils were analysed for their mechanical fractions (percentage sand, silt and clay), pH, percentage organic carbon, available phosphorus and
potassium and for nitrate-nitrogen in the same way
as those in the Korup study (Gartlan et al., 1985).
Ammonium-nitrogen was measured by steam distillation.
Descriptive data were obtained for slope and
drainage (permanent and seasonal swamps and
streams) for each plot using the same criteria as in
Gartlan et al. (1985).
Floristics
A total of 24 997 trees and 501 lianes with a gbh
of 30 cm or more were measured in the course of
the enumeration. (Lianes are not considered further here.) From the trees a total of 198 taxa could
be recognized to at least the family level (Table 1),
leaving only 318 individuals (1.3%) of the total unaccounted for. Thirty-two (14%) taxa have not yet
been placed at any taxonomic level but all were rare,
being only represented by one or a few individuals.
The two families/subfamilies contributing the
greatest amount to the total basal area (Table 2)
were the Olacaceae with 22.3% and LeguminoTable 1. Level of taxonomic identification of trees (_> 30 cm
gbh) in 104 plots in the Douala-Edea Forest Reserve.
species
All taxa (=230)
Taxa based on
descending order
of total basal
areas of first
100 species
50 species
identification to
genus
only
family#
only
146
22
30
93
50
6
1
family
unknown
32
# Also distinguishes the three subfamilies of Leguminosae.
152
Table 2. Representation of the 12 most abundant families (or
subfamilies of Leguminosae) in 104 (80x80 m) plots in the
Douala-Edea Forest Reserve in terms of basal area. (1) Number
of species; (2) Basal area (m2 ha-~); (3) Number of stems
(ha-J).
Family (subfamily)
(1)
(2)
Olacaceae
Leguminosae (Caesalpinioideae)
Euphorbiaceae
Ochnaceae
Humiricaceae
Ixonanthaceae/Simaroubaceae
Ebenaceae
Guttiferae
Annonaceae
Anacardiaceae
Lauraceae
Rubiaceae
4
27
18
3
1
3
11
8
14
3
1
24
7.11
5.19
3.89
2.17
2.13
1.25
1.17
1.02
0.95
0.68
0.56
0.55
44.7
50.1
88.2
12.6
1.0
1.1
36.0
19.7
15.2
10.7
6.1
12.4
Other families (n = 33)
Unknown families
81
32
4.37
0.85
76.2
2.0
Table 3. Basal area and density of the 20 most common species, based on their total basal area in the enumeration, in the
Douala-Edea Forest Reserve sample of 104 80x 80 m plots. (1)
Family (subfamily); (2) Basal area (m2 ha-l); (3) Density
(ha- 1).
(3)
sae/Caesalpinioideae (16.3%). In terms of the
number of stems, the Euphorbiaceae (23.6%) were
the most frequent family. The contributions to the
Ochnaceae and Humiricaceae were almost entirely
due to the presence of significant numbers of the
large emergents Lophira alata and Sacoglottis
gabonensis respectively.
The mean density of trees per plot (with 95%
confidence limits) was 376+ 16 ha -1, the mean basal area 31.0+1.5 m 2 ha -1 and the mean number of
species per plot 39+2 (0.64 ha).
The most abundant species in the Douala-Edea
enumeration was Coula edulis (Olacaceae) which
formed 18.9% of the basal area and 6.7% of the
tree stems (Table 3). The other 19 most abundant
species in the enumeration in terms of basal area
representation are shown in Table 3 together with
their tree densities. They account for 69.7% of the
total basal area and 45.9% of the tree stems.
Only one commercially important timber tree
was recorded in this enumeration, Lophira alata.
Numerical analysis
Ordination of floristic data
Of the total number of taxa, 63% were identified
to species (Table 1). The unidentified species were
Coula edulis
Saeoglottis gabonensis
Lophira alata
Dichostemma glaucescens
Protomegabaria stapfiana
Cynometra hankei
Strombosia pustulata
Anthonotha macrophylla
Librevillea klainei
Trichoscypha patens
Erythrophleum ivorense
Odyendyea gabonensis
Klainedoxa gabonensis vat.
microphylla
Beilsebmiedia cf. gaboonensis
Mammea africana
Ctenolophon englerianus
Strombosiopsis tetrandra
Leptaulus daphnoides
Berlinia bracteosa
Anthocleista vogelii
Total
Remaining (210) species
(1)
(2)
(3)
Ola
Hum
Och
Eup
Eup
Leg
Ola
Leg
Leg
Ana
Leg
Ixo
5.87
2.13
1.99
1.32
1.29
1.16
0.83
0.82
0.78
0.67
0.66
0.61
25.0
1.0
3.8
42.3
21.4
2.7
17.4
17.9
1.2
I0.1
0.7
0.7
Eup
Lau
Gut
Cte
Ola
Ica
Leg
Log
0.57
0.56
0.44
0.41
0.40
0.39
0.36
0.35
0.4
6.1
3.3
1.7
1.6
12.5
0.7
1.9
21.6
9.4
172
204
Families: Ana, Anacardiaceae; Cte, Ctenolophonaceae; Eup,
Euphorbiaceae; Gut, Guttiferae; Hum, Humiricaceae; Ica,
Icacinaceae;
Ixo,
Ixonanthaceae/Simaroubaceae;
Lau,
Lauraceae; Leg, Leguminosae (Subfamily: Caesalpinioideae);
Log, Loganiaceae; Och, Ochnaceae; Ola, Olacaceae.
the least abundant and therefore will be of little importance in an ordination (as shown for Korup,
Gartlan et al., 1985).
Detrended correspondence analysis (Hill &
Gauch, 1980) of all plots and all species, using the
basal area of each species per plot, showed three
distinct outliers (B30, B31 and C19; see explanations below). These were removed and the data reordinated (Fig. 1). Transect A plots, which included
most of those in swamps, were dispersed to the
positive end of axis 1 and the majority of the plots
of transects B, C and D as a cluster of points at the
negative end of this axis. No clear division along
this gradient is evident that would enable the plots
to be reliably classified on the basis of their vegetation.
153
(a)
400
Transects
®
A4
300
+++;+++-'++++
s° @++++,
@
®
C21 BC22 5+1
c~
200
Cl -C17
C 2 0 ,D3
" -
®
®
DIO D~7
2 u+
IA2F~
B263~8D2 A19 A32
~ "-"
u~
A18 A34A~AlS
~DI~ A30
A24~'IA10
- • D9 A12 A2~7~A25
A1
;~+y A27
Am
100
@
A17 A33 A23 @
A21
A29
0
1~}0
0
400
r
(b)
2OIo
t A22
400
3010
AXIS1
5OO
Groups
3
I
300 l
¢N
.-m
×
|
2oo
3
3
2
43
23 3
5532
3
3
33333
1
1
p~s~3
5"555
5 3 535 55
5
2
1
21
2
21 1121 1
2
100
0
1
2
I
100
I
~o
~o
Axis
11
hOG
~;o
1
Fig. 1. Detrended correspondence ordination of all plots
(n = 101), excepting three outliers (B30, B31, C19) using the basal
area of trees (>-30 cm gbh) for all species showing the distribution of: (a) transects and plot numbers. Solid and broken circles
indicate presence of permanent and seasonal swamps respectively. (b) five soil classes of Fig. 4: 1,poorly-drained plots; 3, welldrained plots (low phosphorus); 5, well-drained plots (high
phosphorus), 2 and 4, smaller intermediary classes. * indicates
an outlying plot (n=4) from the soils classification.
Classification of soil data
correlations between sand and silt (r=0.708) and
sand and clay (r=0.520) are highly significant
(P<0.001) whilst the correlation between silt and
clay was insignificant ( r = - 0 . 0 9 3 , P>0.05). Percentage sand was therefore removed and the resulting six variables were used to classify 101 plots no soil data were collected for plots B30, B31 and
C19 - by the 'within-group sums of squares' technique of Orl6ci (1967).
Information on drainage (presence of swamps
and streams) was not included in the classification
because of its presence/absence nature. Sixteen
plots (15°70) of the total of 104 were recorded as permanently swamped, six (607o) seasonally swamped
and 15 plots (14%) had permanent streams present.
Apart from two outlying plots (A1 and A12) and
a small group of two plots, the other 97 plots fall
into five soil classes (Fig. 2). The first consists entirely of transect A plots and is distinguished from
the other classes by its high soil organic carbon percentage (Table 4). The third soil class has transect
B plots whilst the fifth class is a mixture of plots
from transects B, C and D. Plots in classes three
and five have soils with low and high available soil
phosphorus concentrations respectively (Table 4).
The other two, smaller, classes (two and four,
Fig. 2) are intermediate in soil character to the larger classes, and in the interest of detecting the major
differences in the vegetation on different soils
(highlighted by the main three classes), these
smaller soil classes will not be considered further.
120
110
loo
90
~ 80
g
7o
~
E
6o
I
4o
3o
The seven variables, excepting nitrogen (for
which there were data only for 67 plots), were each
normalized by suitable transformation (usually
logarithmic) and then standardized. Since percentage sand, silt and clay are linearly interdependent,
one of these variables must be omitted in any multivariate analysis or replaced by orthogonal components (Austin, Ashton & Greig-Smith, 1972). The
Number of
plots
~1
1
1
2
6
2"1
2
1
4,
13
27
2
30
Group
Number of
p l o t s in
transect
1
(L)
A 1
10
1
-
-
1
8 CD -
3
-
20
1
1
10
6
1
-
13
-
-
5
7
-
Fig. 2. Agglomerative classification of i01 plots in DoualaEdea Forest Reserve based on six soil attributes (plots B30, B31,
C19 excluded).
154
Table 4. Ranges and medians of nine edaphic variables and the frequency of swamps and streams for plots in five groups from the
agglomerative classification of soils in 101 Douala-Edea Forest Reserve plots. The three largest soil groups are designated letters for
subsequent reference. (Four plots in three small soil classes are omitted; see Fig. 2).
Soil group
number of plots
Sand (%)
Silt (%)
Clay(%)
pH
Organic carbon (%)
Available
phosphorus (ppm)
potassium(ppm)
Nitrogen
ammonium (ppm)
nitrate (ppm)
Drainage (%) plots:
Swamps:
permanent
seasonal
Streams-permanent
1
2
3
J
21
13
K
27
-100
84
0 - 28
10
0 - 11
3
2.73.9
3.4
1 0 . 0 - 12.5
12.5
6
78
84
63
78
7
8
7
9
30
nd
nd
-30
15
43 - 8 8
58
-95
92
0 - 5
2
2 -11
6
3 . 8 - 4.8
4.1
1 . 5 - 3.2
2.3
5
L
30
71
- 55
24
78 - 1 8 8
133
-97
88
2 -19
6
1 - 6
4
3 . 0 - 4.0
3.5
4 . 0 - 8.0
5.5
4
-31
9
28 - 5 0
38
18
-31
22
5 . 3 - 8.3
6.3
33
10
29
-80
74
6 -16
10
12 - 2 1
15
3 . 7 - 4.0
3.8
2 . 7 - 5.2
3.4
-27
15
27 - 4 0
38
4
-28
8
1.0
7.8
4.0
15
8
15
-93
89
0 - 9
2
5 -13
10
3 . 6 - 4.7
4.1
1 . 7 - 4.4
2.5
-90
53
30 - 6 3
44
6
-34
24
6.0-11.5
8.9
-40
15
2 . 5 - 9.8
4.4
0
17
17
0
3
0
17
0
13
5
nd = not determined
Association between vegetation and soils
Axis 1 of the floristic ordination (Fig. 1) is associated with increasing percentage soil organic
carbon, percentage silt and the concentration of
available soil potassium, and with decreasing percentage clay and the pH value (Table 5). Axis 2 is
associated with increasing percentage of sand but
decreasing percentage of organic carbon, percentage of silt and the soil concentrations of available
potassium and phosphorus, and of nitrogen (Table 5). A similar, but clearer pattern is shown by the
distribution of the soil classes on the ordination
(Fig. lb).
It is convenient from here on to relabel the soil
classes one, three and five, referred to above, by plot
groups J, K and L (Fig. 2, Table 4).
The vegetation on group J plots is considerably
Table 5. Coefficients of correlation (Spearman's r~) between
the plot scores from an ordination of all plots in the DoualaEdea Forest Reserve (excepting three outliers) and nine edaphic
variables. (n = 101, except for nitrogen data where n = 67).
Range
Axis
Eigenvalue
Sand (%)
Silt (%)
Clay (%)
pH
Organic carbon (%)
Available
phosphorus (ppm)
potassium (ppm)
Nitrogen
ammonium (ppm)
nitrate (ppm)
1
2
0.64
0.36
32 - 100
0 - 64
0 - 21
2.75.4
1 . 5 - 12.5
-0.06
0.52***
-0.59***
-0.60***
0.74***
0.46***
-0.53***
0.01
0.48***
-0.50***
7
28
-0,32**
0.56***
-0.33**
-0.41"**
4
1
- 90
- 188
-
40
12
* P-<0.05; ** P-<0.01; *** P_<0.001.
0.11
0.21
-0.30*
-0.43***
155
different from that on soil groups K and L plots
(Table 6) and these latter two groups differ much
less from one another. A direct floristic comparison between groups used the procedure developed
Table 6. Mean basal areas per plot (m2 ha 1) of tree species
within soil group J and soil groups K and L combined in
Douala-Edea Forest Reserve. Species listed have at least
0.1 m 2 ha- 1in one of the groups. (a) basal group in group J >
threefold basal area in groups K and L; (b) basal area in groups
K and L > threefold basal area in group J; (c) _< threefold
difference between groups but basal area in both at least
0.1 m 2 ha-L In (c), species are ranked in order of decreasing
mean basal area over all plots in groups J, K and L. Only the
first ten species, out of lists of 22, 20 and 17 species for (a), (b)
and (c) respectively, are shown.
Family
(subfamily)
Group
Eup
Leg
Eup
Leg
Sap
Gut
Leg
Log
Corn
Ebe
4.12
3.07
2.00
1.69
1.25
1.22
1.15
0.79
0.75
0.70
number of plots
(a)
Protomegabaria stapfiana
Librevillea klainei
Klainedoxa gabonensis
Erythrophleum ivorense
Gluema ivorensis
Mammea africana
Berlinia bracteosa
Anthocleista vogelii
Strephonema pseudocola
Diospyros dendo
J
21
K+ L
57
in Newbery & Proctor (1984) the criteria for which
are given in Table 6. Extensive lists of species
resulted, and only a portion of them are shown in
Table 6. (The full lists are available on request from
D.McC.N.). A species ordination (Fig. 3), based on
all 101 plots and all species, indicates that species
in the lower right part of this figure correspond to
those species characteristic of soil group J plots
and species of the upper left correspond to those
species characteristic of soil group K and L plots
(Table 6), and reciprocating the arrangement of
plots in Figure la. (In Fig. 3 the species scores are
corrected so that the lowest values on each axis are
just positive for the purposes of illustration.)
The five most common species in soil group J
plots which are also low or absent in soil groups K
and L plots are Librevillea klainei, Klainedoxa
gabonensis var. microphylla, Erythrophleum
500
0.14
0.00
0.07
0.08
0.00
0.13
0.00
0.20
0.00
+ 0.00
Get]
Mq
Ha
Sta
0.61
0.00
0.37
0.22
0.17
0.00
0.00
0.00
0.00
0.00
8.49
1.65
1.57
1.10
0.80
0.43
0.35
0.33
0.30
0.27
(c)
Sacoglottis gabonensis
Lophira alata
Anthonotha macrophylla
Odyendya gabonensis
Trichoscypha patens
Ctenolophon englerianus
Strombosiopsis tetrandra
Leptaulus daphnoides
Pausinystalia johimbe
Diospyros gracilescens
Hum
Och
Leg
Ixo
Ana
Cte
Ola
Ica
Rub
Ebe
2.82
2.85
0.64
0.32
0.64
0.73
0.58
0.22
0.26
0.24
1.97
1.45
0.87
0.76
0.58
0.44
0.35
0.45
0.25
0.23
Families: abbreviations follow those in Table 3. In addition:
Corn, Combretaceae; Ebe, Ebenaceae; Med, Medusandraceae;
Rub, Rubiaceae; Sap, Sapotaceae.
Gi
Eo
aa
8i
~ci
cECced
Sga
Ste
La Av
Dgl
Ld
(b)
Ola
Leg
Eup
Ola
Lau
Eup
Ebe
Leg
Med
Sap
Sma
Og
Bga
c~
G~PU
Coula edulis
Cynometra hankei
Dichostemma glaucescens
Strombosia pustulata
Beilschmiedia cf. gaboonensis
Martretia quadricornis
Diospyros cinnabarina
Hymenostegia afzelii
Soyauxia talbotii
Englerophytum oubanguiense
Cs
MI
Tp
xqPJ
AmDg r
Psf Kg
Go
Dd
Gm
BMa Sps
I
0
I
IbEi
Axis 1
I
ilk
600
Fig. 3. Species ordinations for all plots (excluding B30, B31,
C19). Abbreviations for species: Am, Anthonotha macrophylla;
Av, Anthocleista vogelii; Ba, Berlinia auriculata; Bb, Berlinia
bracteosa; Bga, Beilschmiedia cf gaboonensis; Bi, Baikiaea insignis; Ced, Coula edulis; Cen, Ctenolophon englerianus; Ch,
Cynometra hankei; Cs, Casearia stipitata; Dci, Diospyros cinnabarina; Dd, Diospyros dendo; Dgl, Dichostemma glaucescens; Dgr, Diospyros gracileseens; Ec, Enantia chlorantha; El,
Erythrophleum ivorense; Eo, Englerophytum oubanguiense;
Gcr, Grewia coriaeea; Gi, Gluema ivorensis; Gm, Garcinia
mannii; Go, Garcinia ovalifolia; Ha, Hymenostegia afzefii; Kg,
Klainedoxa gabonensis var. microphylla; La, Lophira alata, Ld,
Leptaulus daphnoides; Lk, Librevillea klainei; Ma, Mammea
africana; M1, Mareyopsis longifolia; Mq, Martretia quadricornis; Og, Odyendyea gabonensis; Pj, Pausinystalia johimbe;
Psf, Protomegabaria stapfiana; Sga, Sacoglottis gabonensis;
Sma, Strephonema mannii; Sps, Strephonema pseudocola;
Spu, Strombosia pustulata; Sta, Soyauxia talbotii; Ste, Strombosiopsis tetrandra; Tp, Trichoscypha patens; Xq, Xylopia
quintasii.
156
ivorense, Gluema ivorense and Mammea africana.
Similarly, the five most common species in soil
groups K and L combined, which also are lowest or
absent from soil group J plots, are Coula edulis,
Cynometra hankei, Dichostemma glaucescens,
Strombosia pustulata and Beilschmiedia cf.
gaboonensis. The species which distinguished soil
groups K and L were Odyendyea gabonensis,
Ctenolophon englerianus and Hymenostegia afzelii
in soil group K plots and Martretia quadricornis
and Grewia coriacea in soil group L plots. The
numbers of species in categories (a), (b) and (c) in
an equivalent table (not shown) to Table 6 were 24,
6 and 23 respectively.
The analyses have defined two different forest
types on very different soils (Figs. 1, 2): swampy,
poorly-drained, high organic matter soils (group J)
versus well drained, lower organic matter sandyloams (groups K and L). Plots in soil groups K and
L are more species rich, have a greater density of
trees (___30 cm gbh) and a higher proportion of
those trees in the 10-20 cm dbh class (though correspondingly fewer very large trees greater than 1 m
in dbh) than plots in group J (Table 7).
Outlying plots
Numerical analysis has highlighted five unusual
plots. Of the three plots excluded at the ordination
stage, C19 is distinctive in its dominance by Martretia quadricornis (4.58 m 2 ha -1) and Pachypodanthidum barteri (1.09) with all other species in this plot
having very low (< 0.01 m 2 ha -i) basal areas. Plots
Table 7. Comparison of mean (_+ 95 % confidence limits) tree
density, abundance, species richness and dbh frequency distributions between plots in soil group J with plots in soil groups
K and L in Douala-Edea Forest Reserve.
Group
number of plots
N u m b e r of species (plot -I )
Basal area (m 2 ha -j)
Density (ha 1)
% of tree in dbh class (cm)
1 0 - 20
21 - 100
> 100
J
21
31 _+ 2
34.5+ 5.0
291 _+20
42.7
54.6
2.7
K+L
57
41 + 2
29.8_+ 1.4
406 _+ 18
62.6
36.4
1.0
B30 and B31 were very similar with high basal areas
of Englerophytum oubanguiense (2.19), Acioa
chevalieri (1.49), Anthostema aubryanum (1.54)
and Cola cf. nitida (1.42) with other species of basal areas less than 0.7 m 2 ha -1. The vegetation of
plot C19 is very similar to that lining the edge of
Lake Tissongo and was inundated by moving lake
water. Plots B30 and B31 were seasonallyinundated river beds with a deep fine-textured
muddy peat layer.
The two most outlying plots (A1 and A12) in the
soil classification (Fig. 1) were not so markedly
different floristically from the main soil groups'
vegetation despite their unusual soil characteristics.
Floristically A1 is similar to the soil group J plots
(Table 6), whereas A12 is more intermediate between soil groups J and L.
Ordination of groups of plots and their correlation
with environmental variables
Plots of soil groups J, K and L were floristically
ordinated separately; three outlying plots (B9, B32,
B33) of soil group K and two outlying plots (C22,
D3) of soil group L were omitted. Soil groups
J, K and L were distinct in the relative importance of different environmental variables
which are associated with their principal fioristic
gradients. For soil group J plots, percentage sand
and pH were significantly positively correlated,
whilst percentage silt and clay were each significantly negatively correlated with axis 1 (Table 8).
Available soil potassium was highly negatively correlated with axis 2. Apart from the weak correlation for percentage sand, no variables were highly
significantly correlated with axis 1 for the soil
group K plots ordination, suggesting that some
other variables that we have not measured may be
more responsible for this gradient. On axis 2, however, available phosphorus and potassium were significantly positively correlated, as to a lesser degree
were percentage clay, pH and nitrate-nitrogen concentration. Soil group L plots did not show the
same pattern of strong correlations as soil group K,
with percentage clay negatively ( P < 0.05) correlated with axis 1 and ammonium nitrogen negatively
correlated (P<0.01) with axis 2.
An ordination of the 57 plots in soil group K and
L combined was unsatisfactory.
157
Table 8. Coefficients of correlation (Spearman's rs) for plot scores of the first two axes of the floristic ordinations of three groups of
plots and soil variables in the Douala-Edea Forest Reserve.
Group
J
Axis
Eigenvalue
number of plots
Sand (07o)
Silt (%)
Clay (%)
pH
Organic carbon (%)
Available
phosphorus (ppm)
potassium (ppm)
Nitrogen
ammonium (ppm)
nitrate (ppm)
1
0.63
K
2
0.35
1
0.42
21
L
2
0.24
1
0.29
24
2
0.20
28
0.58**
- 0.48*
- 0.80***
0.50*
- 0.02
0.02
- 0.03
0.26
0.08
- 0.26
0.36*
0.08
0.33
0.18
0.13
- 0.23
0.31
- 0.27
- 0.73***
0.02
0.26
0.67***
0.57**
- 0.39
0.14
0.12
0.17
0.07@
0.01@
0.33@
0.48*@
- 0.11
-0.38
- 0.52**
-0.15
nd
nd
nd
nd
-0.35
0.10
0.43*
0.33
0.52*
0.31
0.01
-0.43*
0.00
-0.11
0.27
-0.21
-0.21
0.28
-0.29
nd = n o t determined
@ n=23
* P<-0.05; ** P<-0.01; *** P<-0.001.
Table 9. Ranges and medians of nine edaphic variables for plots in two subgroups of soil group J, corresponding to low and high
s0il clay content, and two subgroups of soil group K, corresponding to low and high available soil phosphorus concentrations, in the
Douala-Edea Forest Reserve.
Group
J
K
subgroup
clay (%)
phosphorus (ppm)
n of plots
Sand (070)
Silt (070)
Clay(070)
0
pH
Organic carbon (070)
Available
phosphorus (ppm)
potassium (ppm)
Nitrogen
ammonium (ppm)
nitrate (ppm)
a
- 1
b
>_6
9
7
- 100
95
0 - 28
5
0 1
0
2.73.9
3.5
1 0 . 0 - 12.5
12.5
- 84
79
8 - 20
10
6 - 11
6
2.73.6
3.0
10.0- 12.5
12.5
a
b
<-9
11
>_ 16
9
71
74
80
-94
92
1 - 6
3
2 -14
6
3 . 9 - 4.3
4.0
1 . 5 - 3.2
2.1
84
7
8
7
16
- 50
24
95 - 188
140
- 29
20
78 - 1 4 3
85
nd
nd
nd
nd
27
3
-
9
8
-40
38
-
9
6
1.0- 6.0
3.5
n d = not determined
-95
88
0 - 5
2
5 -11
9
3 . 9 - 4.8
4.3
1 . 7 - 3.2
3.0
-31
20
33 - 5 0
40
6
-25
13
1.5- 5.5
4.5
158
Table 10. Mean basal areas per plot (m 2 ha -~) and plot frequency of tree species in two soil subgroups of plots with low
and high percentage clay content in group J plots, Douala-Edea
Forest Reserve. Species listed have a mean basal area of at least
0.1 m 2 ha ~, and a frequency of occurrence in at least four of
the plots in one of the subgroups. (a) basal area in subgroup a
> threefold basal area in subgroup b, (b) basal area in subgroup b > threefold basal area in subgroup a, (c) _< threefold
difference in basal areas between subgroup means, both subgroups with basal areas > 0.l m 2 ha -~. Only the first 10 species out of lists of 16, 8, 16 species for (a), (b) and (c) respectively are shown.
Family
(Subfamily)
Subgroup
a
_< 1
9
clay (%)
number of plots
b
>6
7
b.a.
f
b.a.
f
Leg
Sap
Cte
Corn
Ebe
Leg
6.30
2.91
1.67
1.06
1.02
0.47
7
4
5
5
9
6
+0.00
0.00
0.00
0.00
0.20
0.07
1
0
0
0
7
3
Lau
Eup
Leg
Sam
0.37
0.36
0.25
0.21
8
5
5
4
0.03
+ 0.00
0.00
0.00
2
7
0
0
(a)
Librevillea klainei
Gluema ivorensis
Ctenolophon englerianus
Strephonema mannii
Diospyros dendo
Baikiaea insignis
Beilschmiedia cf.
gaboonensis
Drypetes ivorensis
Baphia hylophila
Casearia stipitata
Eup
O.74
4
6.70
7
Och
Log
Ana
Com
1.00
0.44
0.13
0.34
6
7
4
1
4.52
1.33
1.10
1.09
7
7
7
5
Eup
Leg
Mel
Och
0.00
0.19
0.00
0.02
0
3
0
3
1.00
0.90
0.40
0.20
4
6
5
6
Hum
4.36
5
1.94
2
Eup
Gut
Ola
Leg
Leg
Gut
Eup
Gut
2.03
0.70
0.46
0.71
0.55
0.45
0.55
0.29
5
5
3
4
8
9
5
8
1.47
0.94
1.18
0.82
0.41
0.40
0.22
0.48
3
7
5
4
7
7
4
7
Eup
0.19
4
0.39
6
(c)
Sacoglottis gabonensis
Klainedoxa gabonensis
var. microphylla
Mammea africana
Coula edulis
Erythophleum ivorense
Berlinia auriculata
Garcinia mannii
Uapaea staudtii
Garcinia ovalifolia
maprounea
rnembranacea
In soil group J plots percentage clay correlates
most highly with the first floristic gradient (axis 1).
Two soil subgroups of plots can be defined as Ja,
with a soil clay content of 1% or less, and Jb with
a clay content of 6% or more. Apart from the silt
content (which is also higher on the high clay plots,
Table 9) these subgroups are otherwise similar. The
five commonest species which have higher basal
areas on the very sandy and virtually clay-free soils
are Librevillea klainei, Gluema ivorensis, Ctenolophon englerianus, Strephonema mannii and Diospyros dendo (Table 10a). These species also appear at the positive end of axis 1 of the species
ordination of soil group J plots. Of species with
much greater basal area on the finer textured soils
(Table 10b), the five most common are Pro-
tomegabaria stapfiana, Lophira alata, Anthocleista
vogeHi, Trichoscypha patens and Strephonema
pseudocola. Several common species appear to be
indifferent to the sand or clay contents, e.g.
Sacoglottis gabonensis, Klainedoxa gabonensis var.
microphylla and Coula edulis (Table 10c).
Comparison of the vegetation on soils of high and
low phosphorus levels
(b)
Protomegabaria
stapfiana
Lophira alata
Anthocleista vogel#
Trichoscypha patens
Strephonema pseudocola
Dichostemma
glaucescens
Anthonotha macrophylla
Trichillia zenkeri
Ouratea affinis sens. lat.
Comparison of the vegetation on soils of differing
clay content
Families: abbreviations follow those in Table 3. In addition:
Com, Combretaceae; Ebe, Ebenaceae; Mel, Meliaceae; Sam,
Samydaceae; Sap, Sapotaceae. Baphia: Leg (subfamily Papilionoideae).
Available soil phosphorus concentration on
axis 2 (although not the principal indirect gradient)
is the variable most highly correlated for soil group
K plots and these plots can be allotted to two soil
subgroups Ka, with 9 ppm or less phosphorus, and
Kb, with 16 ppm of phosphorus or more. Table 9
shows a two and a half fold difference between the
median phosphorus concentrations of these soil
subgroups: organic matter and soil nitrogen concentration are also higher in the high phosphorus
subgroup. The important floristic differences between soil subgroups Ka and Kb are shown in Table 11. The five commonest species which show
greater basal areas on low phosphorus soils are
Hymenostegia afzelii, Guibourtia demeusei,
Strombosiopsis tetrandra, Casearia stipitata and
Dialium pachyphyllum. Of the five commonest
species which have higher basal areas on the relatively higher phosphorus soils, only four meet the
criteria for inclusion in Table 11; Anthonotha mac-
rophylla,
Strombosia
pustulata,
Diospyros
159
Table ll. Mean basal areas per plot (m z ha -l) and plot frequency of tree species in two soil subgroups of plots with low
and high available soil phosphorus concentrations in group K
plots, Douala-Edea Forest Reserve. Species listed have a m e a n
basal area of at least 0.1 m 2 ha ~, and a frequency of occurrence in at least four of the plots in one of the subgroups. (a)
basal area in subgroup a > threefold basal area in subgroup b,
(b) basal area in subgroup b > threefold basal area in subgroup
a, (c) _< threefold difference in basal areas between subgroup
means, both subgroups with basal area > 0.1 m 2 ha -~. Only
the first 5 species (of a list of 15) are shown in (c).
Family
(Subfamily)
Subgroup
a
_<9
11
phosphorus (ppm)
number of plots
b
_> 16
9
b.a.
f
b.a.
f
Leg
Leg
Ola
Sam
Leg
Leg
1.79
0.74
0.64
0.44
0.36
0.27
7
5
8
7
6
9
0.00
0.00
0.16
0.00
0.11
0.04
0
1
3
0
2
3
Eup
0.23
6
0.04
4
Anthonotha macrophylla Leg
Strombosia pustulata
Ola
Diospyros gracilescens
Ebe
Ouratea affinis sens. lat.
Och
0.16
0.24
0.17
0.05
10
8
7
6
2.08
1.05
0.60
0.27
9
8
5
3
Ola
Hum
Och
5.82
2.24
2.40
11
5
11
8.90
1.86
0.82
9
2
7
Eup
Ixo
1.09
2.11
10
8
2.07
0.86
9
2
(a)
Hyrnenostegia afzelii
Guibourtia demeusei
Strombosiopsis tetrandra
Casearia stipitata
Dialium paehyphyllum
Berlinia auriculata
Maprounea
membranacea
(b)
(c)
Coula edulis
Sacoglottis gabonensis
Lophira alata
Dichostemma
glaucescens
Odyendyea gabonensis
the influence of restricted drainage is so marked on
the vegetation, the 19 permanent swamp and
stream plots were excluded. Of the 100 common
species in the whole enumeration, four were completely restricted to the excluded swampy plots. For
the remaining 96 species, the mean basal area was
calculated for those plots in each class and these
values regressed on the linear and quadratic terms
of a polynomial function of phosphorus concentration. Species with a significantly good fit
(P_< 0.05) are shown in Table 12. In each significant
case the form of the response was ascribed to one
of six basic models. These are shown in Fig. 5 of
the Korup paper (Gartlan et al., 1985). Models I
and VI have high basal area at the lower and upper
extremes of the gradient respectively, model III is a
Gaussian-shaped response, model IV a U-shaped
response and models II and V represent gradually
decreasing and increasing basal area response to
the gradient respectively.
The frequency distribution of plots with respect
to increasing concentrations of available soil potassium was highly positively skewed and therefore unsuitable for direct gradient analysis. The range of
potassium in the 85 non-swamp plots was
27-163 ppm and 66 plots had concentrations between 27 and 60 ppm.
Thirty-three species showed a significant response to soil phosphorus concentration, with 7
species having a model III form. Model I responses
were the most frequent (17 species) whilst species
with models II, IV and VI were relatively infrequent (Table 12). Modal phosphorus concentrations for model III species mostly lay between 21
and 50 ppm.
Families: abbreviations follow those in Table 3. In addition:
Ebe, Ebenaceae; Sam, Samydaceae. Subfamily: as Table 3.
Comparisons between Douala-Edea and Korup
gracilescens and Ouratea affinis. The difference in
floral composition between these subgroups is
therefore weak.
Direct gradient analysis
All plots in the Douala-Edea Reserve can be
placed into eight increasing classes of available soil
phosphorus concentration for direct gradient analysis, in a similar manner to that performed for the
Korup Reserve species (Gartlan et al., 1985). Since
Floristics
In origin, history and environment the Korup
and Douala-Edea Forest Reserves are distinct.
Korup is dominated by Oubanguia alata and
Scyphocephalium mannii whereas at Douala-Edea
the one main dominant, in our enumeration, is
Coula edulis (Table 3). Species richness is greater at
Korup, with 411 taxa in total in the enumeration
(274 to the specific level; Gartlan et al., 1985), than
160
Table 12. Species with significant response curves for mean
basal area per plot on available soil phosphorus concentration
in Douala-Edea Forest Reserve, classified according to six basic
models. Of the 96 species tested only those with an analysis of
variance of regression F-ratio significant at P < 0 . 0 5 are listed
with the coefficient of determination, R%. The modal concentrations are recorded for model III species.
Model
Species
Family
(Subfamily)
Rz
P
I
Afzelia sp.
Anthostema aubryanum
Baphia hylophila
Casaeria stipitata
Cryptosepalum
pellegrinianum
Ctenolophon
englerianus
Gluema ivorensis
Guibourtia demeusei
Hymenostegia afzelii
Klaineanthus gaboniae
Martretia quadricornis
Odyendyea gabonensis
Pachypodanthium
barteri
Santiria trimera
Strychnos mimfiensis
Symphonia globifera
Toubaouate
brevipaniculata
Anthonotha
lamprophylla
Baikiaea insignis
Maprounea
membranacea
Antidesma vogelianurn
Buchholzia coriacea
Diospyros iturensis
Klainedoxa gabonensis
var. microphylla
Mammea africana
Mareyopsis longifolia
Pausinystalia johimbe
Soyauxiatalbotii
Afzelia bella
Coula edulis
Garcinia conrauana
Heinsia crinita
Balaniteswilsoniana
Leg 2
Eup
Leg
Sam
53
53
53
85
*
*
*
***
Leg ~
53
*
Cte
Sap
Leg2
Leg 2
Eup
Eup
Ixo
59
73
86
56
82
82
91
*
**
***
*
**
**
***
Ann
Bur
Log
Gut
76
55
65
53
**
*
*
*
Leg I
53
*
Leg I
Leg 2
63
56
*
*
Eup
Eup
Cap
Ebe
91
71
76
96
***
*
**
***
Ixo
Gut
Eup
Rub
Med
Leg 2
Ste
Gut
Rub
Zyg
57
65
72
71
87
56
89
70
68
53
*
*
**
*
***
*
***
*
*
*
II
III
IV
V
VI
Mode
(ppm)
at Douala-Edea where 230 taxa in total were recorded (198 to the specific level). This difference may in
part be due to the area of enumeration at DoualaEdea being c. 25% less than that at Korup. DoualaEdea and Korup differ little in the mean basal area
of all trees ( > 3 0 cm gbh), 31 and 28 m 2 ha -1
respectively. The mean density of stems at Korup
was similar to, whilst that at Douala-Edea much
less than, the mean of 487 ha -1 for all forest types
in Ghana studied by Hall & Swaine (1981).
Three of the five families/subfamilies contributing the most stems at Korup fall within the same
category at
Douala-Edea,
the
Leguminosae/Caesalpinioideae,
Euphorbiaceae
and
Olacaceae. However, of the remaining two families
that have the highest frequencies at Korup, the
Sterculiaceae contributes less than 1% at DoualaEdea and the Scytopetalaceae, the most frequent
family at Korup, appears to be completely absent
from Douala-Edea.
Responses to phosphorus
31-40
51-60
41-50
21 - 30
21-30
31 - 4 0
41-50
Families: Abbreviations follow those in Table 3. In addition:
Ann, Annonaceae; Bur, Burseraceae; Cap, Capparidaceae;
Ebe, Ebenaceae; Med, Medusandraceae; Rub, Rubiaceae; Sam,
Samydaceae; Sap, Sapindaceae; Ste, Sterculiaceae; Zyg,
Zygophyllaceae.
Superscripts to Leg (Caesalpinioideae) are the Tribes: 1, Amherstieae; 2, Detarieae.
* P-<O.05; ** P<O.OI; *** P-<O.O01.
Thirty-five species are common to both Korup
and Douala-Edea enumerations. Of these, 13 occurred in the first 100 most common species in both
Forest Reserves and their responses to soil phosphorus gradients were analysed. Bearing in mind
that the ranges of phosphorus are different for
Korup and Douala-Edea (2-29 and 7-90 ppm
respectively in well-drained plots), 6 of 13 species
had a significant response in one or both of the
Reserves to phosphorus: Hymenostegia afzelii,
model VI at Korup, model I at Douala-Edea;
Strephonema pseudocola, model I at Korup, not
significant at Douala-Edea; Lophira alata and
Strombosiopsis tetrandra, both model II at Korup,
not significant at Douala-Edea; Klainedoxa
gabonensis, model VI at Douala-Edea, not significant at Korup; Coula edulis, model V at DoualaEdea, not significant at Korup. There is no agreement in the response to soil phosphorus for the 6
species compared in the two Reserves.
Of the 22 species characteristic of soil group J
swamp plots at Douala-Edea and the 12 characteristic of swamp plots at Korup (Gartlan et aL,
1985), only three species are in common. These are
Mammea africana, Mitragyna stipulosa, and Sym-
phonia globulifera.
161
Discussion
Effect of drainage on forest type
In contrast to Korup (Gartlan et al., 1985) where
the plots were arranged along a gradual gradient, at
Douala-Edea the forest is clearly divisible into two
types. The swamp forest association is dominated
by species (Table 6) which are rare or absent in the
well-drained plots of soil groups K and L. The
swamp plots of soil group J show more variation in
their flora and soil characteristics than the tightly
clustered plots of soil groups K and L. Of the five
most abundant species at Douala-Edea (Table 3),
three are clearly characteristic of either forest type.
At Douala-Edea, Letouzey (1975) recognised
three forest formations: the littoral forest, riverine
forest and high forest of the interior. Our sampling
along transect A avoided the true littoral forest and
the ends of transects B, C and D connected with the
riverine forest. For the large part, however, the present enumeration sampled the interior forest, especially around Lake Tissongo. Letouzey (1968) classifies the interior forest as that being typical of
low-altitude, coastal rain forest in the Cameroun,
characterised by high abundances of Sacoglottis
gabonensis and Lophira alata, and within it recognizes two types: 'for~t sur sol sec d'une part et forat
mar6cageuse ou p6riodiquement inond6e sur sol
humide d'autre part' (Letouzey, 1975, p. 532). Our
results show Sacoglottis gabonensis and Lophira
alata to be approximately equal in abundance in
soil group J and soil groups K and L (Table 6), and
Coula edulis, which Letouzey mentions as being especially abundant
around
Lake Tissongo,
dominates our enumeration. On the well-drained
plots of soil groups K and L we found, like Letouzey, important species such as Cynometra hankei
and Strombosia pustulata but the agreement is
poor for the swamp plots. Only some of the species
typical of the wetter soils referred to by Letouzey
(1975) occur more commonly in soil group J plots,
notably Uapaca staudtii, Mitragyna stipulosa and
Klainedoxa gabonensis. The dominance of soil
group J plots by Protomegabaria stapfiana seems
to make transect A a special case, though this species is in the Euphorbiaceae which have general associations with wetter soils in the Camerouns
(Letouzey, 1968). There are two likely reasons to explain these differences: first, that the periodically
inundated vegetation is floristically quite variable
at Douala-Edea and Letouzey collected more to the
west of the Reserve; and second, that whilst most
swamp plots have high levels of soil organic matter
(an important criterion in the classification of the
soils on which the floristic groups are based) not all
high organic plots may have been so regularly inundated in recent years. However, inspection of the
ordination of all plots and species (Fig. 3) and noting those plots permanently swamped does not
strengthen the comparison with Letouzey's subjective findings.
Variation within poorly drained forest
Not all the plots of soil group J with their
characteristically high concentrations of organic
matter have permanent or seasonal swamps at present but may have done so in the recent past. The
most extreme conditions are found in very sandy
soils with permanent streams flowing through the
plot and these show an effect in a decrease in available soil potassium concentration (Table 8). In
these poorly-drained plots the vegetation does not
appear related to soil phosphorus.
Associations between the vegetation and available
soil phosphorus in well-drained plots
The dominant vegetation is similar in groups K
and L.
No other soil variables separated soil group K
and L plots to the same extent as phosphorus and
yet for the main ordination phosphorus was only
weakly (though significantly) correlated with axes I
and II. This presumably is because drainage
dominates the main floristic gradient, and phosphorus and drainage are poorly interrelated.
Floristic ordination of the lower phosphorus
plots (soil group K) demonstrated that the very
lowest phosphorus concentrations in well drained
plots were associated with a distinguishable vegetation on the second axis. But the weak correlation
with sand on the first axis is not supported by comparisons with soil group J plots.
Direct gradient analysis using all well drained
plots without permanent streams showed a predominance of model I responses by species to increasing soil phosphorus concentration. These included several species which have high abundances
162
in soil group K plots, but not soil group L plots.
The association between Hymenostegia afzelii
(I), Guibourtia demeusei (I), Casearia stipitata (I)
and Maprounea membranacea on the low phosphorus soils is further highlighted by a comparison
of the selected low and high phosphorus plots of
group K (Table 11). In addition, whilst these low
phosphorus associated species are almost entirely
absent from the high phosphorus plots, the common species associated with high phosphorus are
present to some extent in nearly all the low phosphorus plots.
At Korup, Gartlan et al., (1985) showed a strong
association of species in the tribes Amherstieae and
Detarieae, of the subfamily Caesalpinioideae (family Leguminosae) with soils of low phosphorus
concentration. This result is also shown, though
less strongly, at Douala-Edea with 8 species of
model I and II response in this subfamily. The
number of species occurring in each of the five
tribes (Polhill & Raven, 1980) are: Amherstieae, 14
(of the genera: Anthonotha, Berlinia, Cryptosepalum, Didelotia, Gilbertiodendron, Librevillea and
Toubaouate): Caesalpinieae, 1 (of the genus
Erythrophleum); Cassieae, 1 (of the genus Dial#
urn); Cercidea, none, Detarieae, 12 (of the genera:
Afzelia, Baikiaea, Cynometra, Hymenostegia,
Guibourtia and Leonardoxa). The genera Baphiopsis and Swartzia, placed in a sixth tribe, Swartzieae,
by Aubr6ville (1963-1983) have been recently made
part of the subfamily Papilioniodeae by Polhill &
Raven (1980). Malloch, Pirozynski & Raven (1980)
have found that species of the two tribes, Amherstieae and Detarieae are commonly infected with
ectotrophic mycorrhizae and this in part provides a
possible explanation for the floristic gradient associated with available soil phosphorus. Whilst the
concentrations of available soil phosphorus at
Douala-Edea (7-90 ppm) are generally higher than
those at Korup (Gartlan et aL, 1985; 2-29 ppm),
the principal effect of drainage at Douala-Edea is
likely to mask any smaller, more subtle effects of
phosphorus. The most important conclusion from
the analysis of the Douala-Edea enumeration is
that available soil phosphorus levels appear to be
too high to select strongly for species in the Casalpinioideae. At Korup (Gartlan et al., 1985) the critical threshold value was 5 ppm phosphorus and the
results from Douala-Edea support this finding.
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Accepted 5.9.1985.