The study was carried out in a gathered forest management unit (FMU, 10-030 and 10-031) of PALLISCO Company in the East Region of Cameroon. The gathered FMU has a total surface area of 118,052 ha (FMU 10-030 = 76,850 ha and FMU 10-031 = 41,202 ha), located between latitudes 3˚05'N and 3˚30'N and longitudes 14˚00'E and 14˚30'E. The climate is the Equatorial Guinea sub-type with two seasons; the main wet season (September to November) and main dry season (December to February), and two minor seasons designated as short wet (March to May) and short dry (June to August). Mean annual temperature is 23.1˚C and mean annual rainfall is 1566 mm [22] . Altitude varies between 600 and 760 m above sea level. The area mostly has semi-deciduous forests, with a vegetation canopy dominated by Meliaceae, Sterculiaceae and Ulmaceae families [17] . The soils are mainly ferralitic (Ferralsols) [23] and are developed from various parent materials such as micaschists, gneisses and granites [24] . A dense hydrographic network exists with many streams while areas of low altitudes (<600 m) contain marshes and raffia swamps (Figure 1). Two sites located in the northeastern part of the FMU, about 8 km away from Makalaya (a forest camp in the gathered FMU), were chosen for the study, given that these sites had undisturbed (natural) forest. The two sites, adjacent to one another and separated by a distance of about 1000 m were chosen based on the abundance of P. elata stems in each site (Authors, field observation). From field observations, the site of high P. elata stems had a distinct reddish yellow soil colour (5YR 6/8) compared to the site of low P. elata stems which had a dark reddish colour (10R 3/6).

Data collection and soil sampling were done in October 2014. In each site, a rectangular plot of 10 ha (200 × 500 m) was demarcated and within each, square sub plots of 0.25 ha (50 × 50 m) were established, following procedures outlined by Picard and Gourlet-Fleury [25] . In all, thirty eight sampling units of 50 × 50 m were established inside the P. elata stand while sixteen sampling units were delineated outside the P. elata stand. In each sub plot, diameter at breast height

(dbh) was measured and individual trees counted to obtain the number of stems per sub plot. Only trees with dbh > 30 cm were considered because dbh > 30 cm for the species constitute the minimum fertile and effective fruiting diameters [26] , capable of reproducing. Within each sampling unit, soil samples were randomly collected at two depths: 0 - 20 cm and 20 - 40 cm, and bulked to obtain composite samples for each of the depths. These depths were chosen based on the fact that nutrient cycling within tropical forests occurs primarily within the upper layer of the soil, following decomposition of plant litter [14] . Soil samples were collected and stored in polythene bags.

Fresh soil samples from the field were air-dried at room temperature, crushed and sieved through a 2 mm sieve. The <2 mm soil fraction was analyzed for both physical and chemical properties. Particle size analysis was done following the hydrometer method [27] . Soil pH was determined electrometrically with a 1:2.5 soil:H₂O and 1:2.5 soil:KCl ratio. Organic carbon (OC) was determined by the Walkley and Black wet combustion method as described by Pauwels et al. [28] . Exchangeable bases (Ca²⁺, Mg²⁺, Na⁺, K⁺) were determined following the Schollenberger method using a 1 M ammonium acetate solution buffered at pH = 7. The concentrations of Na⁺ and K⁺ in the extract were determined by flame photometry while Ca²⁺ and Mg²⁺ were determined by complexometry using a 0.002 M Na₂-EDTA solution. Cation exchange capacity (CEC) was determined as a direct continuation of the Schollenberger’s method using a 1N KCl saturation solution. Exchangeable acidity (Al³⁺ + H⁺) was determined following procedures outlined by Dipak and Abhijit [29] using a 1N KCl solution for soil leaching. Effective CEC (ECEC) and base saturation (BS) were determined by the summation method [28] .

Descriptive statistics were performed for soil physical and chemical properties in and out of the P. elata forest stand. Mean values of soil properties between the two sites were compared using the Student’s t-test. The Shapiro-Wilk test [30] was used to test for normality of soil properties within each site. Based on the distributions obtained in the P. elata stand, optimal soil conditions for P. elata distribution were established for surface and subsurface soil properties using normally distributed properties. Optimal soil conditions within 0.25 ha sub plots with ≥ 5 stems, 3 - 4 stems, 1 - 2 stems and sparse (no stem) were respectively considered highly suitable (S1), moderately suitable (S2), marginally suitable (S3) and not suitable (N) classes following the FAO land suitability classification [31] . The aforementioned was refined with soil data available in the literature for the Congo Basin. Correlation analysis was carried out to identify soil properties correlated with tree stems per sub plot. Principal component analysis was performed on tree and soil data in the P. elata stand to identify factors responsible for the variation in tree population and soil properties. Statistical analyses were facilitated with SPSS software for windows (Version 19).

The correlation between P. elata stems and clay content gives evidence that the distribution of P. elata is a function of soil texture. Clay is generally considered

Notes: ^a: Extraction method, principal component analysis; Rotation method: Varimax with Kaiser normalization. Component 1 is the soil acidity/saturation factor; Component 2 is base status factor; Component 3 is the topsoil-pH/tree factor; Component 4 is the texture factor.

Notes: S1: Highly suitable (≥5 stems/0.25 ha); S2: Moderately suitable (3 - 4 stems/0.25 ha); S3: Marginally suitable (1 - 2 stems/0.25 ha); N: Not suitable (no stems). Source: Modified from Boyemba [34] .

as the active part of soil because it plays a role in the supply and availability of nutrient elements [35] . The impact of edaphic heterogeneity on species assembly has been reported in rainforests of the Congo Basin, where differences in soil texture (clayey versus sandy soils) considerably influence biodiversity and habitat preference [2] [36] [37] . According to Silver et al. [38] , soil texture is primarily responsible for nutrient availability in lowland tropical forests. Also, the observed effect of silt in this study is in line with observations made by Iwara et al. [9] , where silt content was observed to have a strong influence on the distribution of some particular tropical lowland forest species such as Anthonota macrophylla. Although the soils in our study area are dominated by low activity clays such as kaolinite and Fe and Al oxides [24] , it is suggested that clay content in this case plays an important role in the availability of acidic elements such as Al³⁺ in the soil solution. This observation is in accordance with the relationship between P. elata stems and soil pH, indicating that a high number of P. elata stems may not be favoured at particular pH values (pH between 4.0 and 5.0). It has been observed that Al³⁺ in acidic soils has a distinct negative effect on survival of some forest tree species [39] through Al toxicity. Increasing aluminum levels in the soil solution have been reported to decrease uptake and translocation of calcium, magnesium, and potassium [40] . The primary target of Al toxicity is the root apex and aluminium affects a host of different cellular functions. Exposure to Al causes stunting of the primary root and inhibition of lateral root formation in some plants. This phenomenon is still to be verified for P. elata. The resulting restricted root system is impaired in nutrient and water uptake, making the plant more susceptible to drought stress [40] [41] . However, Boyemba [34] observed that high concentrations of Al³⁺ inhibited proper growth and survival of P. elata in a humid tropical forest in the Democratic Republic of Congo. With respect to availability and uptake of Nitrogen, it has been reported that P. elata, being a Fabaceae, is a nitrogen-fixing plant thanks to its symbiotic relationship with some bacteria [42] . The supply of Nitrogen has been reported to exert strong control over the composition, diversity and productivity of many ecosystems [43] . This is because nitrogen metabolism is one of the most important factors often limiting plant growth in natural ecosystems [44] . Nitrogen-fixing trees such as P. elata have tree sources of inorganic N (nitrate, ammonium and atmospheric nitrogen) fixed by symbiotic bacteria, even though we did not determine nitrogen in this study. However, Nitrogen supply can be greatly reduced at high aluminium concentrations through the inhibition of specific enzymes responsible for nitrogen assimilation [45] . Soil pH also has an indirect influence on organic matter decomposition and nutrient availability by affecting soil microorganisms [32] . The relationship observed between P. elata stems and exchangeable Ca²⁺ is striking. Although calcium is one of the most abundant mineral elements in soil, this is not the case in tropical soils. Calcium has several distinct functions in higher plants [46] , but Ca²⁺ concentration in this case seems to affect P. elata stems negatively. It has been observed that tree communities are greatly influenced by exchangeable bases (Ca²⁺ and Mg²⁺) [9] , [39] . Also, Iwara et al. [9] observed that low base saturation is suitable for establishment of particular tree species in humid tropical forests. The negative correlation between P. elata stems and soil pH suggests that high pH conditions (pH > 5.0) reduce tree population, and so, P. elata is suggested to have a well defined acidity range necessary for its survival. John et al. [4] identified soil pH as the strongest factor that influenced the distribution of tree species in three tropical forests.

Organic matter plays an important role in binding soil cations and in ameliorating soil structure, thus providing a favorable condition for plant growth. Correlation analysis indicates that a high number of P. elata stems is favoured by low amounts of organic matter. This correlation could be explained by the classical relationship between clay and organic matter [47] , given that there was a positive correlation between P. elata stems and clay content. The low organic matter content is certainly due to high rates of organic matter decomposition as influenced by the quality of litter type, which favours nutrient cycling [14] with the consequence of making nutrients available for immediate uptake by plants. Additionally, soil organic matter contains a large number of exchange sites (high surface area and hence high cation exchange capacity) that increase the capacity of the soil to adsorb these nutrients and prevent them from leaching below the rooting zone [48] . It has been reported that present P. elata patches in the study area considered in southeastern Cameroon are related to anthropogenic disturbances (most likely resulting from shifting cultivation that occurred ca. two centuries ago) [16] . However, other studies reveal that not all patches of African light-demanding tall trees (such as P. elata) are caused by human-induced disturbances [49] . In our study sites, no signs of anthropogenic disturbances (e.g. presence of charcoal or pottery sherds) were observed in the soil layers, contrary to the observations made by Bourland et al. [16] about 30 Km away from our study site, where they found a link between anthropogenic disturbances and P. elata population. Among other factors, edaphic properties have been reported to significantly affect the distribution of many tree species within humid rainforests and semi-deciduous forests of the Congo Basin [2] [37] [42] . In a study of within-plot relationships between tree species occurrences and hydrological soil constraints in a lowland rainforest of French Guiana, it was reported that soil hydrological conditions (particularly soil drainage), were the main structuring factors of the local multispecies spatial pattern observed [50] . However, our study did not take into account groundwater availability (which could probably influence the spatial distribution of P. elata) due to the complexity in quantifying groundwater availability and also because the species has been reported to tolerate a wide range of water regimes ranging from well drained soils to seasonally waterlogged ones [7] .

In the literature, the controversy that exists with respect to the influence of soil properties on the distribution of forest tree species has been argued either through experimentation or field exploration. For the case of P. elata, the survival rate within forest environments has been attributed to factors such as light availability [15] [17] , influence of pests [20] , herbivory [51] and edaphic heterogeneity [2] [34] [42] . The findings in the present study derive from field observation, wherein the distribution of P. elata is certainly influenced by soil properties among others. Therefore, the edaphic properties identified in this study could serve as a starting point for studies seeking to establish edaphic requirements for the survival of P. elata. The values of pH, CEC, BS and exchangeable bases observed in this study permit us to suggest that slightly acidic soils within a pH range from 4.10 - 4.30 in surface soils (0 - 20 cm) and 4.60 - 4.90 for sub-surface soils (20 - 40 cm), could be considered as baseline information that can guide or stimulate further research aiming at establishing pedological requirements of P. elata. In this study, our values for exchangeable acidity do not exceed 4.67 cmolc∙kg⁻¹ within 40 cm soil depth, and so, further studies are highly recommended in order to complement these findings in other P. elata natural forest stands within the Congo Basin.