Soil Properties Influencing the Abundance and Distribution of Isoberlina Woodlands in Nigeria
Rufus M. Ubom
The influence of soil factors affecting the abundance and distribution of Isoberlinia woodlands in savanna ecosystem was studied. Fourteen woodland plots (ca 62 ha) were sampled systematically for frequency, density, basal area, crown cover and height of woody species ≥3 m high. Three soil pits were excavated in each woodland and soil physical, morphological and chemical properties described and analyzed, for each soil horizon. Results show that the soils are sandy loam to clay loam with dark grayish brown (10 YR 3/2) A-horizons over dark brown (7.5 YR 4/4) to yellowish red (5 YR) 4/6) subsurface horizons; moderate medium subangular blocky peds of friable consistence and contain large amounts of iron concretions. The pH is low with high mobile H+; low capacity for cation absorption and low mineral nutrients. Matrix of PCA scores of soil variables used to establish relationships between vegetation and soil parameters give very high significant values indicating the joint structural patterns between these two systems. Canonical correspondence analysis between the soil parameters (PCA) scores and species data elucidates the distribution and grouping patterns of Isoberlinia and its associes. Thus, variation in species composition in the woodlands reflects microsite differences in soil properties and their ecological amplitudes.
Isoberlinia Craib and Stapf is a tree of the family Fabaceae and subfamily
Caesalpinioidae. It is used as fuelwood, timber, planks; for shaft lining; as
hut poles and the wood ash as soap (Wickens, 1980). It occurs in clusters forming
woodlands within the Nigerian Guinea Savanna. These woodlands serve as refuge
for (animal) wildlife. The woodlands are sporadically located at discrete positions
within the Guinea Savanna, that is, they exhibit zonation in spatial context
related to a complex environmental factors (Ubom and Isichei, 1998). The woodlands
are associated with quartzite ridges, steep eroding slopes with shallow soils,
stony grounds not suitable for farming, laterite platenux and more extensive
on higher grounds. Thus, Isoberlinia has the capacity to adapt to certain
soil conditions and its woodlands are not found in all areas of Guinea Savanna
and never present in any other savanna type in Nigeria. The influence of soil
factors in the composition and distribution of savanna vegetation in Nigeria
has been reported (Child, 1974, Muoghalu and Isichei, 1991). Others have discussed
relationships between soil characteristics and distribution of plant communities
in the Guinea savanna (Menaut et al., 1985; Cole, 1986; Sharma, 1988;
Abdulhameed et al., 2004). Evidence shows that vegetation associations
within the Guinea Savanna and tree species reflect differences in soil texture,
structure and mineral contents (Ubom, 1998; Ubom and Isichei, 1998, Abdulhameed,
2005). The aim of this research is to elucidate the complex interrelationships
between Isoberlinia woodlands and soils based on measurements of vegetation
and soil variables at same woodland sites. The hypothesis is that the occurrence
and distribution of Isoberlinia woodlands are influenced by soil characteristics.
The basic approach is quantitative involving the use of multivariate approaches.
The premise is that vegetation is multivariate viewed in terms of gradients
while the soil consists of independently varying properties. Having elucidated
the important soil properties that influence the woodland components, the soil
system may be viewed as multivariate and a joint structure of interrelationships
sought between vegetation and soil systems. The procedures involve the use of
Principal Component Analysis (PCA), Canonical Correlation Analysis (COR) and
Canonical Correspondence Analysis (CCA). PCA is used to interpret the vegetation
and soil structure within their covariance matrices. The appropriateness of
this in ecological research is achieved when data are standardized; it requires
minimum assumptions and is computationally unambiguous (Greigh-Smith, 1983).
COR is used to extract the main directions of vegetation and soil variation
when both systems are considered jointly. COR assumes linearity of input variables
(Ter Braak and Prentice, 1988), its interpretation is relied upon if the original
data matrices are replaced by matrices of eigenvectors. CCA extracts axes of
known soil variables by imposing linearity of the variables and relating these
to vegetation variation directly. That is CCA focuses on grouping patterns of
vegetation composition variation that can be explained by soil variables.
Materials and Methods
The study area is Kainji Lake National Park located between latitudes 9°45
and 100°23 and longitudes
30°40 and 50°47E
in Northwestern Nigeria (Fig. 1).
The area is in the Northern Guinea Savanna Zone (Keay, 1959). It covers 5340.82 km2 with two non-contiguous sectors. Borgu and Zugurma. The climate of the area has been described by Afolayan (1982) as follows: mean annual rainfall of 100-1200 mm with a peak in July-August. Rain does not occur between November and March. Annual evapotranspiration ranges from 1400 to 1560 mm with rainfall/evapotranspiration ratio ranging from 0.64 to 0.89.
Mean annual relative humidity is 53%. The park is underlain by rocks of he basement complex and Nupe sandstones. An important geomorphological feature is the occurrence of plinthite or ironstone on or close to the surface in most parts (Child, 1974). The topography is thus gently undulating. The Borgu sector is drained by Oil River and its tributaries while the Zugurma sector has few channels and is poorly drained. This study was conducted from October 2002 to October 2005 for two seasons of each year.
|| Kainji lake national park: sampled isoberling woodland plots
Vegetation/soil Sampling and Analyses
Fourteen woodland plots (totaling 62 ha) were used for this study (Fig.
1). In each woodland plot, five belt transects were laid and in each transect,
vegetation and soil were systematically sampled (Cochran, 1963) in two hundred
and fifty 10x10 m quadrats regularly spaced at 50 m intervals. In each quadrat,
frequency and density of all woody species (≥3 m high), basal area as cross
sectional area of individuals, crown cover as crown horizontal area were determined;
height using Haga altimater was measured. Three soil pits were excavated in
each woodland and a total of forty-two of them were excavated. Some soil physical
and morphological properties were described in the field. These include colour
(using Munsel Soil Colour Charts, 1973 edition), texture, consistence, horizon
boundary, concretion and structure according to USDA (1981) Soil Survey Manual.
Rooting depth was also determined in each pit. Soil samples were collected from
each horizon of each pedon, air-dried and description for colour was again made
on them. The soil samples were ground and analyzed for particle size (hydrometer
method), pH, exchangeable acidity (Al+3, H+) and cations
(Black et al., 1965), organic carbon (Walkey-Black method), available
phosphorus (Bray No. 1 method) and total nitrogen (Kjeldahl method) (Juo, 1979).
The hypothesis was that the distribution of Isoberlinia woodlands
was influenced by variation in soil properties. Testing the hypothesis required
the use of ordination techniques to analyze the floristic and soil gradients.
The first step is indirect gradient analysis, which was used to summarize the
main variation in the species and soil data through Principal Component Analysis
(PCA). The matrix of PCA scores (varimax rotation) of soil variations was used
to establish patterns and joint structure of interrelationships between vegetation
and soil parameters using canonical correlation analysis (Gittins, 1985). This
analysis utilized the original vegetation data (Aweto, 1981). Secondly, matrix
of PCA scores of species data were used to elucidate the distribution and grouping
patterns (in relation to soil variables) of 22 species that had importance value
>5% from all the woodlands. Importance value was derived as the sum of relative
frequency, relative density and relative dominance of a species (Babour et
al., 1987). This relationship was explored using the canonical correspondence
analysis technique (Ter Braak, 1986).
The components of the woodlands and their attributes are given in Table
1. Species with a frequency of less than 10% is not included in the Table
1. Of the 26 woody species recorded, 22 have girth size >5 m in twelve
families. The most common species with 50% occurrence include Isoberlinia
sp., Burkea africana, Daniellia, oliveri, Detarium
microcarpum, Monotes kerstingii and Vitellaria paradoxa. Table
1 shows that the vegetation is dominated by species of the family Fabaceae.
There are variations in the vegetation attributes of the woody species. Isoberlinia
sp has the highest values of al the attributes except height. Daniellia
oliveri is the tallest plant and Gardenia ternifolia the shortest.
Acacia hockii has the smallest crown size, girth size and density. The
influence of the variations of the species attributes is exemplified in their
importance values. Accordingly the first six important woody species (in descending
order) in the woodlands are Isoberlinia sp., Burkea africana, Viterllaria
paradox, Monotes kerstingii, Daniellia oliveri and Detarium microcarpum.
Soil Physical Properties
The soils of the woodlands have been described as ferruginous tropical soils
according to the Soil Map of Africa (D Hoore, 1963). Depending on the
structure, texture and topographical position the soils are classified as Paleustalfs,
Grossarenic and Typic ustipsamments.
|| Mean woody species attributes of Isoberlinia woodlands
Analysis of the soils reveals that in their physical properties (Table
2) sand is the most variable and silt the least. Sand proportions decrease
while those of silt increase with soil depth, clay content is low but more variable
than silt and increases with soil depth.
The morphological properties of selected pedons studied in the Isoberlinia
woodlands are presented in Table 3. The soils are fairly
shallow (<150 cm deep), poorly drained with sandy loam A-horizons. The soils
have variable colours depicting the drainage status of each of the woodlands.
Generally, they have very dark grayish brown (10 YR 3/2) A-horizons over dark
brown (7.5 YR 4/4) through dark red (2.5 YR 3/6) to yellowish red (5 YR 4/6)
subsurface horizons and moderate medium subangular blocky peds of very friable
to friable consistence. The soils contain large amounts of iron concretions.
The horizon boundaries are mostly gradual (to subtle). The rooting depth varies
from 30 to 85 cm.
The chemical properties of the soils (Table 4) show that
pH values are low, indicating the acidic conditions of the woodland soils. The
pH (CaCl2) values decrease as soil depth increases. Thus, A-horizons
are less acidic than B-horizons and in some instances tend toward neutrality.
High mobility of H+ ions contribute to the acidic nature of soils.
The woodland soils also have low capacity for cation absorption and this may
be a reflection of low levels of clay which is kaolinitic in nature.
||Mean (+95% confidence interval) physical characteristics of
representatives soils of Isoberlinia woodlands in northwestern Nigeria
|*c represents concretions and or nodules
|| *Morphology of selected pedons in representative Isoberlinia
woodlands in Nigeria
|*Coded according to soil survey staff (1981). Colours are
Munsell notations. Codes are as follows: d = dry, m moist, w = wet, Texture:
1 = loam, ls = loamy sand, cl = clay loam, sl = sandy loam, scl = sandy
clay loam. Structure: 1 weak, 2 = moderate, 3 = strong, vf = very fine,
f = fine, m = medium, o = coarse, gr = granular, Sbk = subangular blocky.
Consistence-dry: so = soft, sh = slightly hard, h = hard Moist: vfr = very
friable, fr = friable, fi = firm, vfi = very firm Wet: po = non plastic,
ps = slightly plastic, p = plastic, so = non sticky, ss = slightly sticky,
s = sticky, Boundary: c = clear, g = gradual, s = smooth, w = wavy, a =
abrupt. Pedon identification numbers
|| Ranges, means and confidence intervals of soil variables
in Isoberlinia woodlands, northwestern Nigeria
|| Canonical factor structure of soil-vegetation interrelationship
of Isoberlinia woodlands in Nigeria
Thus, the levels of exchangeable cations and micronutrients are low with almost
similar variation. The soils also exhibit low levels of total nitrogen, organic
carbon and available phosphorus; the later being probably due to its fixation
by iron and aluminium because of acidic soil conditions.
Canonical Correlation Analysis
This produced three significant canonical correlations (Table
5). The significance of the correlations represents the patterns of interrelationships
between the vegetation and soil characteristics. On the first canonical correlation
vegetation attributes of density, crown cover and basal area have high negative
weights. The soil attributes of nutrient status and pH have high negative weights
while organic carbon, substrate texture and Fe have high positive weights. This
correlation indicates a reciprocal relationship between vegetation and soil
attributes. All the soil properties are significant in determining vegetation
properties. The second correlation indicates a strong positive relationship
between the soil nutrient status and woody plant height and basal area, confirming
the importance of nutrients in growth of Isoberlinia and its associes.
The joint structure on the third correlation indicates a strong positive relationship
between soil pH and crown cover as influenced by nutrient status. In all, the
most highly weighted soil attributes that influence species distribution and
abundance in the woodlands are pH and soil texture.
Canonical Correspondence Analysis (CCA)
Figure 2 shows that the soil properties relate well to the
first two species axes of CCA. The first axis is defined by absorption capacity
of woodland soils, which differentiates into ECEC probably controlled by Ca+2
silt and iron. The second axis is defined principally by soil pH which differentiates
H+ and Al+3; soil texture (sand and clay) as well as organic
carbon and nutrient status differentiating into N, P, K, Mg, Mn and Fe components.
Figure 2 accounts for 68% of the variance and shows that the
weighed averages of the species and soil properties result in similar rankings.
Species whose distributions are controlled or affected by say, H+
and Al+3 lie to the top right hand corner of the diagram. Similarly,
those species restricted to silt and Na are found on the left hand corner of
the diagram. The scores of most species lie outside the centre of the diagram.
Only species influenced or restricted by iron and manganese are found around
the centre of the diagram. In all, the eigenvalues (not included here) of the
soil properties show that the gradients are long, thus indicating that the soil
properties have control over the distribution of Isoberlinia sp. and
||Detrended correspondence analysis (DCA) of soil-species variable
of isoberlinia woodlands. The diagram represents 68.01% (Axis I, 35.91%,
Axis II, 32.10%) of variance accounted for by the first 4 ordination axes.
Species nomenciature as in Table 1
Variations occur in the distribution of the woodland (species) components which relate the species responses to complex soil variables. Differences in species attributes, which culminate in differences in species importance values, are indications of these responses. Isoberlinia sp. is the most dominant and numerous (with highest importance value) giving a monospecific picture of the woodlands. However, due to less capability of Isoberlinia to regenerate, other competitive species such as Burkea africana, Vitellaria paradoxa, Monotes kerstingii, Terminalia schimperiana, D. microcarpum and Crossopteryx febrifuga establish in the woodlands in gaps created by dead or fallen individuals of Isoberlinia species. This culminates in the variation of species density and other measuration data obtained from the woodlands, giving the woodlands the variable structural patterns and physiognomy. Variation in species composition may also be attributable to microsite differences in soil properties and wide ecological amplitudes possessed by some species such as Burkea africana and Detarium microcarpum (Ubom, 1998); their distribution to competition for environmental factors (Kernel, 1986). The trees, through less dense, have canopy that depicts a continuous upper storey. The understorey vegetation is sparse and of low taxonomic diversity probably as a result of shading by almost closed canopy formed by upper storey species. This suggests that the understorey species are heliosciophytes and that at this instance they are unable to compete with upper storey species.
Surface soils are sandy loam and subsoils sandy clay loam in texture resulting in their poor structural stability, exchange capacity and water-holding capacity. The increase of clay with soil depth is an indication of argilluviation in the soils but is not strong enough to result in argillic horizon (Raji et al., 1996). The soils are nutrient-poor due to their porosity and unabiilty to retain nutrients and organic carbon. Low soil nutrients is attributed to low litter and low mineralization rate, low clay content and moisture in relation to length of wet season (Jones, 1973). Low cation exchange capacity suggests that the soils have low sink for cations and other nutrients and from observation soils of upper slopes have lower nutrient contents than soils of lower slopes.
The variable colour of the woodland soils, the granular surface and subangular blocky sub-surface peds together with very friabe to friable consistence and large amounts of iron concretions are indications of structural of the soils. These suggest that the drainage potential of the woodland soils is related to periodic and seasonal alternating wetting and drying (Siderius, 1974). The woodlands exhibit spatial distribution and show niche relations to these soil attributes (Table 3), suggesting these soil properties could serve as indicators for the woodlands establishment. Their presence on ironstone outcrops where there is low water table suggests that the species components are drought-tolerant. Ironstone outcrops and quartzite ridges offer secondary sites availability to the woodlands as they are drier than slopes, valleys and higher grounds which are the primary sites because they provide more soil moisture to woodlands species. Establishment of woodlands on sites may not only be limited by soil moisture but may imply dependence also on nutrients availability and substrate texture (Ubom, 2005) and competition (Ukpong, 1997). Evidence from observation shows that soils of the upper slopes of ironstone outcrops and quartzite ridges have lower concentrations of nutrients than soils of the lower slopes and consequently woodlands in lower slopes are more luxuriant than those at upper slopes. There is no evidence to show that competitive effects of Isoberlinia with other species are important in woodland establishment since Isoberlinia does not decrease in abundance or incidence in the woodlands. The presence of other species in the woodlands is in gaps if they are canopy species (e.g., Daniellia oliveri) heliosciophytes if understorey species (e.g., Acacia hockii, Entada africana). Hence, temporal variation in species availability may be determined by soil moisture (though not measured in this study) and spatial variation associated with soil properties gradients (Boeken and Shackak, 1998).
The presence of woodlands and their species components on soils with high iron concretions suggests restricted establishment of the woodlands in these specific sites where the species can perform well (Ubom, 1998). Siderius (1974) makes an assertion that Isoberlinia woodlands are located on soils underlain by soft plinthite at 64 cm depth and hard plinthite at about 130cm depth where soils are yellow, red and sandy clay loam in texture. Observations reveal that tall woodlands occupy yellow red sandy clay loam soils underlain by soft plinthite with weak structure while short woodlands are associated with soils underlain by fairly hard plinthite.
The efficacy of Canonical Correlation (COR) and Canonical Correspondence Analysis
(CCA) techniques as indirect gradient analysis tools (Ter Braak, 1986) has distinguished
patterns of vegetation and species variations in the woodlands. COR shows that
the Isoberlinia woodlands is influenced or encouraged by soil texture
(silt, clay fractions) and nutrient status. Silt and clay fractions probably
enhance growth of woodland species by improving soil water-holding capacity,
porosity, cation exchange capacity and play a cementing role between mineral
particles. The distribution of the woodlands correlates with organic carbon,
soil reaction (pH) and iron contents of the soil. Thus, the vegetation attributes
of the woodlands may be determined by these soil parameters, as the latter determine
the soil conditions of the woodlands resulting in woodland variation. But Huntley
and Birks (1979) assert that vegetation variation is not only related to soil
conditions but to aspects of biotic environment such as grazing and burning.
Field observations reveal that burning affects woodlands vegetation variation
as it removes Isoberlinia saplings and creates gaps for other species
(e.g. Burkea africana, Daniethlia oliveri) to establish. The CCA indicates
different relationships between species composition of the woodlands and soil
properties. This discrepancy suggests that soil properties constitute complex
environmental gradients to which the species respond (Ter Braak, 1986). Accordingly,
Isoberlinia, Pericopsis, Pseudocedrella and Terminalia
are best developed on silt, iron and calcium rich soil sites within the woodlands
(Fig. 2). Vitellaria paradoxa is a better indicator
of Mg2+ rich soils while Acacia gourmaensis and Annnona
senegalensis are associated with Al3+, P and clay rich soils.
Spatial segregation of Lannea acida and L. schimperi suggests
niche separation between species. The former is probably sensitive to soil reaction
(pH) while the latter is sensitive to phosphorus (Ubom, 1998). Lannea acida
occurs among short woodlands and L. schimperi occurs among tall woodlands
(Ubom and Isichei, 1998). The pattern of segregation of Gardenia ternifolia,
Pterocarpus erinaceus and Terminalia avicennioides suggests that
their distribution may be monotonic probably due to their insensitivity to changing
conditions of the soil properties.
This study shows that woody plant species in Isoberlinia woodlands possess qualitative and quantitative response patterns to major axes of habitat hyperspace (Whitney and Moelier, 1982; Ubom, 1998) as influenced by soil properties. Such soil properties include texture, nutrient status, pH, consistence, structure and soil inclusions such as concretions. Their quality and quantity determine the type of vegetation components found within the woodlands and hence, influence their abundance and distribution. Isoberlinia sp. can therefore be described as habitat-specialist (Maliakal-Witt et al., 2005) as its woodlands are restricted to a narrower range of microhabitats within the Guinea Savanna. As a specialist Isoberlinia abundance and distribution are due to high resource but selective requirements in terms of suitability of sites. Since suitable sites are harder to find, its distribution and population are less common within the Guinea Savanna. Hence conservation and management approaches to forestall the disappearance of Isoberlinia and its woodlands and the utilization of the species are recommended.
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