A study of the typology of soils and floristic diversity was carried out in the forest of the “Cité Scientifique” of Brazzaville. The study aimed to assess the influence of soil cover on the distribution of taxa. The study covers 18 plots of 625 m 2 each, distributed in different soil profiles. The floristic inventory concerns the measurement of diameter of trees with dbh ≥ 10 cm and the counting of herbaceous and shrubs of dbh < 10 cm. The floristic composition reveals a total of 129 plant species distributed in 48 families dominated by Fabaceae and a preponderance of pioneer taxa. The Jaccard coefficients of similarity reveal a low degree of homology of woody plots, whereas for the flora as a whole, the floristic composition is heterogeneous. The Shannon biological diversity index of ligneous varies from 0.6 to 2, against a maximum diversity of 0.69 to 3. The Pielou index of ligneous (0.5 to 0.9) does not reveal any dominance. Soil cover consists of ferrallitic and podzol soils. Their texture is sandy, weakly structured, fragmentary polyhedral throughout the profile, but lumpy and/or nuciform at the surface and sub angular in depth. The porosity decreases proportionally with the depth and thickness of the horizons. The floristic analysis presents an ecosystem with a low degree of diversification for a high floristic richness. The preponderance of anthropization markers is a proof of its very advanced state of disturbance. The study shows that soil cover does not influence the distribution of timber. Indeed, the establishment and development of pioneer species are less prone to soil properties.
Forest ecosystems cover between 2.4 and 6 billion hectares worldwide and those in the Congo Basin account for about 227 million ha [
Notwithstanding the assumed roles and functions, the relational complexity, the correlation between soils and forests is not sufficiently mentioned in the studies. Soils and forests are intimately linked and mutually exert a reciprocal influence. Together, they impact the immediate environment. The positive effects of this relationship help to preserve health and ensure a healthy environment; improve livelihoods in rural areas, and climate change through carbon storage [
Despite the vital functions inherent in human survival, Congolese soils and forests are undergoing irreversible degradation and significant land regression due to anthropogenic actions (poorly adapted agricultural practices, clear-cutting of trees, fires recurring bush, urbanization, the establishment of infrastructures, etc.). This situation, which is very worrying, now affects some forest islets found in the south of Brazzaville city. These islets are a result of the degradation of the original forest of “Patte d’Oie”, a particular and unique forest ecosystem previously found in Brazzaville. Indeed, several species are threatened with extinction within these ecosystems, or even themselves. This vision is more than plausible with the establishment of artificial forest facies based on allochthonous species.
Although the Republic of Congo has adopted adequate regulations, notably the law n˚ 003/91 of 23 April 1991 on environmental protection and law n˚ 16-2000 of 20/11/2000 on the Forest Code, the future of the forest islet of the “Cité Scientifique” of Brazzaville remains worrying with the increase of anthropogenic pressure (collection of wood energy and phytotherapeutic products, recurrent bush fires, deforestation leading to soil degradation, snail collection, etc.).
Recurrent bush fires and excessive deforestation lead to soil degradation that ipso facto affects the development and resilience of the forest ecosystem. This vision is more than plausible with the establishment of artificial forest facies based on non-native species as shown by the work of Kimpouni et al. [
Despite the threats to the urban islets of natural forests and the proven services to the community, these ecosystems have still not received the attention they deserve, especially after the disappearance in Brazzaville of the forests of Tsiémé, Glacière Chad Ravine and Corniche, without any floristic trace [
The objective of this study is to improve the knowledge of the specific and ecosystemic diversity of the Congo, in particular of the mesophilic and tropophilic forests, through a typical mesh of soil and floristic distribution in the forest islet of the “Cité scientifique”.
The “Patte d’Oie” forest reserve of Brazzaville, represents a particular element of the urban landscape. Its initial area fixed by the decree of the Governor-General of French Equatorial Africa (AEF), n˚ 3037 of 13 August 1938, was 240 hectares of peri-urban natural forest. Due to the extent of the anthropogenic activities, it was reduced to 118 hectares by Order n˚ 2789/SF of 4 December 1951. In 13 years, the reserve is imputed 51% of the initial area. With a view to its conservation, reforestation carried out in 1995 made it possible to increase this area slightly from 118 hectares to 136.52 hectares.
In order to consolidate what has been gained, the State has decided to classify a large portion of the “Patte d’Oie” forest by Decree n˚ 2009-149 of 8 May 2009, classifying the area of the Forest Reserve of the “Patte d’Oie”. The area of the classified domain is 94.55 hectares, with less than 10 ha of natural forest. The natural forest resulting from the degradation of the forest of the “Patte d’Oie” is presented in three islets, among which that of the Scientific City of Brazzaville or the “Cité scientifique” of Brazzaville [
The experimental site is the forest of the “Cité Scientifique de Brazzaville” ex-Orstom forest. Located in the south-east of Brazzaville (4˚16'S and 15˚14'E), its area is about 24.3 hectares. This forest grows on a plateau on the psammitic soil and whose altitude oscillates between 279 and 323 m (
Designated Low-Congolese, the climate is characterized by moderate rainfall of about 1300 mm/year (
The soils of Brazzaville have developed on a variety of materials originating essentially from three major sedimentary series [
The city of Brazzaville, located on the right riverbank of the Congo River, is dependent on rivers of different sizes [
The vegetation of Brazzaville department is none other than that of the “plateaux Teké” described by Makany [
The ecological data took place from March to April 2016. During this inventory, the herbal sheets was constituted and deposited at the national herbarium (IEC) in Brazzaville. Species identification was done in situ for the most common and ex situ in the IRSEN laboratory compared to the IEC herbariums and diagnoses of Pauwels [
To ensure a perfect mesh between the floristic distribution and the soil types, the study area is plotted in 25 × 25 m plots (either 625 m2). Taking into account the high degree of anthropization of the site [
This inventory consists mainly of the census and enumeration of all the floristic diversity. All trees with diameter at breast hight (dbh) greater than or equal to 10 cm were identified and their diameter measured. The shrubs of dbh less than or equal to 10 cm, the creepers and herbaceous plants were identified and counted. About herbaceous plants, only presence was noted.
In addition to the floristic inventory, a more or less exhaustive mapping of the spatial distribution of the flora was carried out. It was based on the dominant species.
The floristic inventory data will be treated through the Jaccard similarity coefficient, the Shannon biodiversity indices and the equilibrium index or the regularity index of Pielou.
1) Jaccard similarity coefficient
This coefficient makes it possible to evaluate the similarity between two plant communities [
Jaccard ( S % ) = 100 × [ C / ( A + B − C ) ] (1)
With A = number of species in the first plot, B = number of species in the 2nd plot, C = number of species common to both plots.
2) Shannon and Weaver Diversity Index
This index makes it possible to better appreciate the floristic diversity taking into account the number of species and their abundance [
H ′ = − ∑ i = 1 S p i ln p i (2)
I = species, pi = ni/N, where ni = the number of individuals of the species and/or family and N = total number of individuals, of all species combined, Ln = natural logarithm.
3) Maximum diversity
The maximum floristic diversity provides information on the diversity that a plant community can achieve, in the case of a homogeneous distribution of the individuals constituting the flora [
H ′ max = L n ( S ) (3)
With S = total number of species; L n ( S ) = the maximum specific diversity of Shannon [
4) The index of regularity or Pielou Index
The regularity or Pielou index (equitability) is the ratio of the Shannon diversity index to the maximum diversity index [
J = H ′ / H max (4)
The soil profile data was obtained in May and July 2016. In order to appreciate the spatial heterogeneity of the flora and of the different soil types, a soil pit placed in the centre of the plot was described. As for the floristic inventory, 18 soil pits have been described (
For mapping of soil cover and floristic distribution, each plot is georeferenced. The database includes geographic coordinates, soil type and dominant plant communities. To obtain raster maps, the special association approach used is the interpolation by Inverse Distance Weighting (IDW) method on QGIS software [
The total inventory shows a total of 129 plant species belonging to 48 families. The most important families are Fabaceae (21 species, 43.75%) followed by Rubiaceae (11 species, 22.91%), Connaraceae (6 species, 12.5%), Lamiaceae and Phyllanthaceae (5 species, 10.41%). The remainder of the families making up the majority have a contribution of about 1 to 4 species.
The floristic composition of trees with dbh ≥ 10 cm shows 501 individuals, the vast majority of which are pioneer species (
The distribution of trees in different diameter sizes shows a very important regenerative cohort supporting sufficient natural regeneration. However, a detailed analysis of this population reveals a low rate of individuals characteristic of forest ecosystems. In the case of this forest islet of the “Cité scientifique”, the markers are Millettia eetveldeana, Bosqueiopsis gilletii, Strychnos variabilis, Dracaena arboreus, Antiaris toxicaria and Syzygium brazzavillense, each of which gives an erratic curve with very few individuals in the first diameter classes.
The Shannon Index ranges from 0.60 to 2.22 with an average of 1.62 ± 0.15. However, the maximum diversity index oscillates from 0.69 to 3 for an average of 2.02 ± 0.11. As for fairness, it is between 0.50 and 0.96, which is an average of 0.80 ± 0.19.
The coefficient of similarity of Jaccard holding dbh tree counts ≥ 10 cm, denotes 6 records for 3 pairs including: 2 and 18, 4 and 7, 5 and 8 whose value is between 50% and 58.3%. For the rest of the combinations, the values are between 0 and 45.5%. However, based on total floristic diversity (woody and herbaceous), values range from 6.90% to 37.70% for all combinations. These values being less than 50% between plots, the floristic composition of each is an entity in its own right.
According to the zones of dominance observed and confirmed by the floristic inventory and the indices of biological diversity, 17 sub-facies can be identified (
The soils of the “Cité Scientifique” have the typical characteristics of Arenosols and Podzols. Generally deep with a variation in color ranging from very dark brown to yellowish brown, a sandy texture, a lumpy structure on the surface and polyhedral in depth. They are well drained and have a high porosity, especially in the first 20 centimeters. The litter with a thickness of 3 to 9 cm, favoured the formation of a deep humiferous horizon, up to 120 cm of depth. These morphological characteristics distinguish three types of soil: Ferralic Arenosols, Albic Arenosols, and Podzols (
This type covers almost 55% of the area of the “Cité Scientifique” of Brazzaville (
0 - 5 cm (O horizon): moder litter type, dominate by organic material consisting of undecomposed and partially decomposed litter, such as leaves and twigs, and whole decomposed litter (humus). Many biological activities (termite, ants, channels and other insect activities). Abrupt boundary.
5 - 30 cm (A11 horizon): dark brown (10 YR 3/3). Sandy texture weak to moderate structure with very fine to fine subangular blocky. Friable material. High porosity and common roots. Common biological activity (termite and ant channels). Clear boundary.
30 - 90 cm (A12 horizon): brown (10 YR 5/3). Sandy texture. Weak to moderate structure with very fine to medium subangular blocky. Friable material. High porosity and few roots. Common biological activity (termite and ant channels and nests). Clear boundary.
90 - 110 cm (B1 horizon): yellowish brown (10 YR 5/6). Sandy texture. Weak to moderate structure with fine to coarse subangular blocky. Medium porosity and few roots. Few biological activity (insect channels). Gradual boundary.
110 - 200 cm (B2 horizon): brownish yellow (10 YR 6/6). Sandy texture. Weak to moderate structure with medium to coarse subangular blocky. Medium porosity and few roots.
This type of soil has the same morphological characteristics as the typical Ferralic Arenosols. They are differentiated at the level of the A horizon which has traces in the form of bleached sandy beaches called leached. These soils are characterized by high permeability and extreme poverty in minerals.
44.44% of the area of the Scientific City of Brazzaville is classified as leached ferrallitic (
0 - 5 cm (O horizon): moder litter type, dominate by organic material consisting of undecomposed and partially decomposed litter, such as leaves and twigs, and whole decomposed litter (humus). Many biological activity (termite, ants, channels and other insect activities). Clear boundary.
5 - 50 cm (Ae horizon): dark greyish brown (10YR 4/2). Sandy texture. Weak structure with very fine to medium friable subangular blocky. Few biological activity (termites, insect channels). Common porosity and roots. Clear boundary.
51 - 120 cm (B1 horizon): yellowish brown (10YR 5/4). Sandy texture. Weak to moderate structure with fine to coarse subangular blocky. Few biological activity (insect channels). Common porosity and very few roots. Gradual boundary.
120 - 200 cm (B2 horizon): brownish yellow (10YR 6/8). Sandy texture. Weak to moderate structure with fine to coarse subangular blocky. Few biological activity (insect channels).
They cover 5.56% of the area of the Scientific City of Brazzaville and are located on the slope of a valley. The brown fibrous litter is of the mor type. The whole profile presents: a generally particulate structure, a sandy texture and high porosity, and a diffuse transition between horizons. The depleted humic horizon is dark gray (10YR 4/1). Within the E horizon there is a succession of intersecting bands of accumulation of organic matter (horizon Bh). All soils types are a granular structure in the topsoil and blocky subangular structure in the deep horizons. The soil profile description:
0 - 6 cm (O horizon): mor litter type, dominate by undecomposed very fine roots and whole decomposed litter (humus). Few biological activity (insect channels). Abrupt boundary.
6 - 50/80 cm (A11 horizon): dark grey (10YR 4/1). Sandy texture. Weak structure with very fine to medium subangular blocky. Few biological activity (insect channels). Common roots and porosity. Gradual and irregular boundary.
50/80 - 110/120 cm (A12 horizon): grey (10YR 6/1). Sandy texture. Weak structure with very fine to medium subangular blocky. None biological activity. Gradual and irregular boundary.
110/120 - 200 (E horizon): white (10YR 8/1). Sandy texture. Few roots and common porosity.
Analysis of the distribution of taxa in relation to soil cover does not reveal a close relationship between the type of soil and any species, which are intimately subordinated (
The forest of the “Cité scientifique” develops on ferrallitic and podzolic soils from Batéké sand [
The floristic distribution observed, on each of the soil facies marking the soil cover, would be the corollaries associated with the anthropic action on this woody ecosystem. The anthropogenic level of effects is progressive from the outside to the interior of the ecosystem; thus, woody flora growing on leached and podzolic ferrallitic soil is more affected. The combined actions of climatic elements (in particular precipitation) and anthropic degradation (bush fire, clear-cut or non-clear cutting of trees, slash-and-burn crops) would be the main source that would feed the soil pedological regression process of the “Cité scientifique”. Indeed, according to a scale of the level of degradation associated with the presence of climatic markers, natural or subspontaneous, one distinguishes typical ferrallitic soils (ferralic arenosols), leached ferrallitics (albic arenosols) and podzols.
Typical ferrallitic soils form the soil cover of the least disturbed forest facies.
The floristic diversity is based on key taxa like Millettia laurentii, Bosqueiopsis gilletii, Syzygium brazzavillense, Strychnos variabilis, Dracaena arborea, Millettia eetveldeana, Delonix regia, Markhamia tomentosa, which have modest densities. A well-stocked undergrowth in Palisota ambigua, Costus afer;
The leached ferrallitic soils are home to a pioneering woody flora (heliophilous) that is clearly dominated by Caloncoba welwitschii. Chromolaena odorata is the characteristic of undergrowth with Commelina diffusa, Panicum maximum;
Podzolic soils are those with the most significant effects of anthropization. On these soils graminaceous species such as Digitaria horizontalis bioindicator of skeletal and acid soils develop. The woody ones are in very low density and most often markers of the secondarisation of the habitat.
The floristic composition of the 18 plots does not reveal any clear distinction in the spatial distribution of taxa. Despite the fact that some families are present and/or dominant only in a few plots, the distribution of species is almost the same in our study area. A detailed analysis of the shrub flora demonstrates that it is over 75% dominated by pioneer taxa, indicative of anthropogenic degradation [
Notwithstanding the secondary nature of this ecosystem, it combines a low degree of floristic diversification with a high specific richness. These parameters affect both the tree scale and the level of the overall floristic diversity (shrub, trees and herbaceous plants). According to data from the Pielou index, a homogeneous spatial distribution of taxa is observed globally within the forest of the “Cité Scientifique” [
These soils are considered to have undergone the pedogenetic phenomenon of ferrallitisation without any other secondary process, physical or chemical, intervening in such a way as to significantly modify the normal evolution. The only differentiation is due to the greater or lesser penetration of organic matter [
The most important factors in the structure of Ferralic Arenosols are the roles played by iron and organic matter. Indeed, the free iron (Fe2+), being very active, forms stable aggregates with other minerals such as kaolinite or with organic matter forming humo-ferric complexes; and the humification of the organic matter releases the polysaccharides which allow a high cohesion of the soil structure. Thus, in Ferralic Arenosols, if iron is essential in the structure of deep horizons, it shares this role with organic matter in surface horizons [
The described podzol is located on a low slope glacis. This topographic position is favourable to the hydromorphic conditions necessary for the genesis of podzols known as “terraces” [
According to [
Water availability is an important factor for soils under forest, and its circulation has physical characteristics of the substrate [
Ferralic Arenosols, under rain forests, have a very low capacity in exchangeable bases. Their physical properties differ fundamentally depending on the nature of the parent rock and the flora has a very high specific diversity [
In terms of soil-plant relationships, the differentiation of ecosystems and forest types depends on the distribution of roots, the chemical characteristics of the litter, the accumulation of organic matter [
The plant can restore only what it has taken from the ground. Its intervention in the improvement of the mineral status of a soil will be done by the displacement of the mineral reserves and their conservation in the organic matter of the soil. Trees modify the characteristics of the soil in a direction that is often beneficial in the longer term, allowing them to restore degraded or depleted soil to a certain degree of fertility [
Sabatier and Prevost [
The soil and the forest are in unstable equilibrium; they do not resist against the destructive action of man, which is at the origin of their regressive evolution [
Plots (400 m2) | Trees of dbh ≥ 10 cm | Floristic markers | Undergrowth and dominance | Soil types | Soil profile description | ||||
---|---|---|---|---|---|---|---|---|---|
Number | Species number | Species | % | TM | % | ||||
1 | 82 | 8 | Caloncoba welwitschii | 70.73 | shrub | 24 | Albic arenosols | O-A1e-AB-B2 O: 7 cm; A1e: 37/56 cm | |
liana | 44 | ||||||||
herb | 32 | ||||||||
2 | 19 | 4 | Caloncoba welwitschii | 73.68 | shrub | 46 | O-A1-A1e-B1h-B1-B2; O: 2 cm; O-A1e: 50 cm | ||
liana | 35 | ||||||||
herb | 19 | ||||||||
3 | 12 | 8 | Caloncoba welwitschii | 25 | shrub | 39 | O-A1e-AB-B2; O: 5 cm; O-A1: 40 cm | ||
liana | 32 | ||||||||
Millettia eetveldeana | 25 | ||||||||
herb | 29 | ||||||||
4 | 73 | 10 | Markhamia tomentosa | 32.88 | shrub | 27 | Ferralic arenosols | O-A1-AB-B2; O: 5 cm; O-A1: 42 cm | |
Caloncoba welwitschii | 26 | liana | 67 | ||||||
herb | 6 | ||||||||
5 | 30 | 11 | Caloncoba welwitschii | 26.33 | Shrub | 46 | Albic arenosols | O-A1e-B1-B2; O: 4 cm; O-A1: 29 cm | |
liana | 33 | ||||||||
Senna angustifolia | 20 | ||||||||
herb | 21 | ||||||||
6 | 11 | 3 | Millettia laurentii | 81.82 | shrub | 55 | Ferralic arenosols | O-A11P-A12-AB-B2; O: 5 cm; O-A12: 46 cm | |
liana | 27 | ||||||||
herb | 18 | ||||||||
7 | 16 | 9 | Bosqueiopsis gilletii | 18.75 | shrub | 35 | Albic arenosols | O-A11e-AB-B2; O: 4 cm; O-A11: 40 cm | |
Chaetocarpus africanus | 18.75 | liana | 48 | ||||||
herb | 17 | ||||||||
8 | 65 | 16 | Markhamia tomentosa | 30.77 | shrub | 32 | Ferralic arenosols | O-A11-AB-B1-B2; O: 4 cm; O-A11: 36 cm | |
Millettia eetveldeana | 18.46 | liana | 47 | ||||||
herb | 21 | ||||||||
9 | 13 | 5 | Paramacrolobium coeruleum | 30.77 | shrub | 44 | Albic arenosols | O-A1e-AB-B1-B2; O: 6 cm; O-A1: 25 cm | |
Senna angustifolia | 23.08 | liana | 28 | ||||||
Caloncoba welwitschii | 23.08 | herb | 28 | ||||||
10 | 24 | 11 | Syzygium brazzavillense | 33.33 | shrub | 34 | Ferralic arenosols | O-A11-AB-B1-B2; O: 4 cm; O-A11: 33 cm | |
Anthocleista vogelei | 16.67 | liana | 43 | ||||||
herb | 23 | ||||||||
11 | 75 | 16 | Caloncoba welwitschii | 24 | shrub | 42 | Albic arenosols | O-A11e-AB-B2; O: 3 cm; O-A11: 34 cm | |
Markhamia tomentosa | 24 | liana | 26 | ||||||
herb | 32 |
12 | 3 | 2 | Acacia mangium | 66.67 | shrub | 4 | Ferralic arenosols | O-A11-AB-B1-B2; O: 8 cm; O-A11: 37 cm |
---|---|---|---|---|---|---|---|---|
Hymenocardia ulmoides | 33.33 | liana | 74 | |||||
herb | 22 | |||||||
13 | 13 | 6 | Delonix regia | 53.85 | shrub | 21 | O-A1-A1e-AB-B2; O: 1 cm; O-A1e: 52 cm | |
liana | 29 | |||||||
herb | 50 | |||||||
14 | 15 | 9 | Anthocleista vogelei | 20 | shrub | 34 | Albic arenosols | O-A11-AB-B1-B2; O: 9 cm; O-A1: 46 cm |
Caloncoba welwitschii | 20 | liana | 19 | |||||
herb | 47 | |||||||
15 | 10 | 7 | Delonix regia | 30 | shrub | 38 | Ferralic arenosols | O-A11-A12e-Eh O: 6/3 cm; O-A12: 120 cm |
Caloncoba welwitschii | 20 | liana | 31 | |||||
herb | 31 | |||||||
16 | 20 | 6 | Senna angustifolia | 20 | shrub | 46 | O-A1-AB-B1-B2 O: 1 cm; O-A1: 42 cm | |
liana | 45 | |||||||
herb | 9 | |||||||
17 | 29 | 12 | Markhamia tomentosa | 27.59 | shrub | 38 | Podzol | O-A11-A12-B1-B2 O: 1 cm; O-A11: 32 cm |
liana | 34 | |||||||
herb | 28 | |||||||
18 | 11 | 5 | Caloncoba welwitschii | 54.55 | shrub | 36 | Ferralic arenosols | O-A1-AB-B1-B2 O: 6 cm; O-A1: 17 cm |
liana | 64 |
modify it, following a sequence: the occupation of the ground and the disturbance of the landscape; and vegetation dynamics [
At the present stage of the development of the forestry ecosystem of the “Cité Scientifique” and the pioneering character of its flora, the pedological cover less influences this distribution. Dominated by pioneer species, these taxa are less demanding for the physico-chemical properties of the substratum, a factor determining their spatialization [
The urban forest ecosystem of the “Cité scientifique” of Brazzaville has a very advanced state of anthropization. It is characterized by a floristic composition clearly dominated by the so-called heliophilic pioneer species, symbols of a beginning of the reconstitution process. The low rate of species typical of this mesophilic and tropophilous forest compared to Millettia laurentii is evidence of its tendency to become its characteristic. This observation is more plausible insofar as the variation in the magnitude of the degree of anthropogenic impact within this ecosystem creates pockets dominated by Millettia laurentii for the trees of the upper floor.
Soil cover shows two main types of soils including Arenosols and Podzols that undergo the pedogenetic phenomenon of ferrallitisation. By coupling the taxon distribution map and the soil cover, this mesh does not discriminate a single group of plants that are subordinate to a given soil type. The fact that the floristic cohort is dominated by the pioneer species that witness a degradation of the environment, no influence of the soil is noted on its distribution. Their settlement and development are less dependent on the physicochemical properties of the soil. That is why there is an almost homogeneous floristic distribution in this urban forest ecosystem.
Nzila, J.D., Kimpouni, V., Watha-Ndoudy, N., Nanitelamio M.M., Mouhamed Salisou, Y. and Nguila-Ntsoko, D.P. (2018) Soils Typology and Floristic Diversity of the Forest of the “Cité Scientifique” of Brazzaville, Congo. Open Journal of Ecology, 8, 286-304. https://doi.org/10.4236/oje.2018.84018