The peri-urban forest of the Loukaya River is located south-west of Brazzaville between 4˚21'21.6" - 4˚21'36" south latitude and between 15˚6'21.6" - 15˚6'32.4" east longitude (Figure 1). Evolving in a densely populated area, the Loukaya Forest is bounded to the north by the village Makana 1, to the northeast by Nganga-Lingolo about 4.90 km from the tollgate, to the east by the village Djoumouna, to the south by the village Bissinza, to the southwest by the village Bouanissa, and to the west by the village Makana 2.

The climate of the locality is Bas-Congolese of the Sudano-Guinean type [25] [26] whose specificities are: an average annual temperature of about 25˚C, a low annual thermal amplitude of 5˚C to 6˚C (Figure 2); average annual rainfall of 1200 to 1400 mm [27]. The rains that punctuate the alternation of the seasons begin very weakly at the end of September and settle from October to May, with a very marked slowdown from January to February. The hottest and wettest months are usually March, April and November. On the other hand, the months of June to September are the driest, while July and August are the coolest [26] [28]. Relative humidity, always above 70%, is vital for the flora, especially in the dry season. The annual hygrometric amplitude is low and the daily average varies from 33% in the rainy season to 46% in the dry season [27] [28]. Evaporation varies in the opposite direction of atmospheric humidity (Figure 2) and presents a relative minimum in June and an absolute maximum in August and September [27]. In relation to the seasons, the relative minimum occurs at the turn of the rainy and dry seasons, while the absolute maximum is observed in the last phase of the dry season. In sum, evaporation is moderately low and stable in the rainy season follows the opposite trend of rainfall in January - February; and rises sharply in the dry season.

According to Denis [25], the geological substratum is composed of sedimentary formations of Cenozoic (Tertiary) age, of sandstone nature that form the

Inkisi sandstone series. According to the authors, this bedrock either is from the Upper Kalahari or is the result of in situ alteration of the schistose sandstone system. This system is composed of two superimposed sets, namely 1) the upper layers known as sandy siltstones, which constitute a large set of ochre-coloured aeolian sandy siltstones that occupy the upper parts of the plateaux. These highly mobile silts have given rise to major reworking, both in situ and by dragging into the valleys; 2) the lower layers known as polymorphic sandstones, which are represented by soft, yellow, white or pink sandstones, with fine and regular grain, without marked stratification.

The soils belong to the class of highly desaturated ferrallitic yellow psamite soils on Batékés sandstones of plateau slopes and hills, as well as hydromorphic soils [25]. From the physical and chemical point of view, highly desaturated ferrallitic soils are acidic soils (pH < 5.5), with a low sum of exchangeable bases (1 meq/100g) and a low saturation rate (less than 20%) of the absorbent complex.

The vegetation in the study area is a forest-savanna mosaic [29] [30]. Forests are established on various topographical positions, hilltops, mainly where there are sandy overburden, slopes, valleys, and often swampy shallows. The forest in the valley bottoms takes the form of narrow galleries and is generally much degraded. In addition to these formations, larger massifs, located along the Congo River, generate ombrophilous Gilbertiodendron dewevrei or more often mesophilic semi-caducifolia formations.

The plant material consists of the trees of d^1.30 ≥ 10 cm of the selected species and their regenerative cohort of diameter between 2 ≤ d^0.20 < 10 cm, at 20 cm from the ground. The selected individuals in the regenerative cohort are at least 30 cm in height because below this value the survival of individuals is very random [31]. The selection of regeneration species is based on the fact that they are characteristic of the dense tropical rainforests of Central Africa. The taxonomic ordination adopted is APG IV [32] and the nomenclature followed is that of Lebrun and Stork [33]. The parameters monitored are types of diaspora and affinity to light.

The types of diasporas recognized follow the classification of [34] taken up by [10] [16] [23] [24] [35] [36]:

­ Ballochores (Ballo): diaspores expelled by the plant itself;

­ Barochores (Baro): non-fleshy, heavy diasporas;

­ Pterochores (Ptero): diasporas with aliform appendages;

­ Sarcochores (Sarco): diasporas totally or partially fleshy.

The type of affinity to light described by Lebrun [35] [37] and taken up by [36] reveals the groups of:

­ heliophytes (Heliophytes), plants that enjoy full sunlight. In this group, one finds mainly plants from well-lit grassy groups, trees and lianas from the upper tree dome;

­ hemi-heliophytes (Hemi-Helio), plants of the intermediate strata of the forests, trees of the dome which regenerate in undergrowth and can live for a certain time in diffuse light. These plants receive on average 50% to 75% of the total illumination [38];

­ hemi-sciaphytes (Hemi-Scia), plants of the forest undergrowth that still receive 5 to 50% of relative illumination [38];

­ sciaphytes (Scia), plants of the lower and humigenous strata of forests in general. The illumination received is limited to 1% to 5% of the total light [38].

The research system is based on three adjacent surveys of 1 ha, each unit consisting of 25 plots of 400 m² (Figure 3).

For dense tropical rainforests, several methods are used to study natural regeneration. For this study, the full floristic inventory coupling the count of all individuals of selected species from the regenerative cohort (2 ≤ d^0.20 < 10 cm) to trees of d^1.30 ≥ 10 cm is carried out over the entire study area [10] [39]. This method, which provides a better understanding of the notion of natural regeneration, has the advantage of making data reliable and modelling.

The inventory shows 23 species corresponding to 21 genera grouped in 13 families (Table 1 and Table 2). As for the number of individuals counted, it should be noted that it amounts to 1255 of which 31.71% are trees of d^1.30 ≥ 10 cm. The regenerative cohort 2 ≤ d^0.20 < 10 cm makes up 1/3 of the count. The mean number of individuals per species is 37.26 ± 6.05 for a range of 2 to 137 plants per species (Table 2 and Figure 4). With more than 100 plants, Pentaclethra eetveldeana and Petersianthus macrocarpus account for 28.35% of the population. These taxa are followed by Carapa procera and Pentaclethra macrophylla with a number of plants ranging from 60 to 100, i.e. 19.03% of the total. Finally, with at least 20 plants, Syzygium brazzavillense, Synsepalum dulcificum, Maranthes glabra, Pycnanthus angolensis, Millettia laurentii, Bosqueiopsis gilletii, Millettia eetveldeana, Ongokea gore and Anisophyllea meniaudi form the third group, accounting for 43.17% of the population. The least represented group of species, less than 20 plants, covers 9.45% of the inventory and consists of Xylopia acutiflora, Dialium pachyphyllum, Allablanckia floribunda, Tetraberlinia sp., Symphonia globulifera, Pterocarpus soyauxii, Albizia ferruginea, Celtis mildbraedii, Anonidium mannii and Paramacrolobium coeruleum.

A positive correlation was noted between the height and diameter of individuals monitored in the Loukaya peri-urban forest, regardless of the inventory level (Figure 7). The Spearman’s coefficient (“rho”) being, in all cases, higher than the Pearson’s coefficient (“r”), reflects a monotonous and non-linear relationship. This observation reveals that the diameter of individuals increases concomitantly with height, but not proportionally. This phenomenon could be a consequence of accidents occurring during their growth stages. These include 1) anthropogenic actions and natural accidents such as windfalls; 2) the diversity of growth rates of the species considered, as well as their distribution pattern within the study area.

Sarcochores are preponderant with 60.87% of the species, corresponding to a proportion of 50.53% of individuals (Figure 8(a) and Figure 8(b)). Ballochores

come in the second position with 26.09% of the species and 35.82% of the individuals. This observation made at the scale of the study is also valid when considering the surveys, where sarcochores remain dominant ahead of horehounds.

In relation to light affinity, 82.61% of the species surveyed are heliophilic, i.e. 89.39% of individuals (Figure 9(a)). The hemi-heliophilic plants have a specific proportion of 13.04%, while hemi-sciaphiles are ahead of them in terms of individuals with 6.18% of the individuals recorded (Figure 9(b)).

Like all the intra-urban and peri-urban forests in Brazzaville (Patte d’Oie and Djoumouna Forests) whose phytodiversity data are almost under control, the Loukaya woodland formation is no exception. Mesophilic and tropophilic in type, the Loukaya forest is a very diverse and concomitantly tree-poor formation [10] [16] [51] [52] [53] [54] [55]. This state is the tangible result of anthropogenic effects that severely impact all levels of biodiversity (α, β, γ). These woody formations, although they are part of the dense tropical rainforests, without being compared to the neotropical and palaeotropical forest ecosystems of South-East Asia, are very atypical in terms of specific and floristic richness [52]. These structural parameters would be dependent on pedoclimatic determinism, in general. These indices establish the link of their belonging to the physiographic area of the Cataractes district [10] [16] [51] [52] [53] [54] [56].

The passage from one diameter class to another induces a decrease in density following the conquest of space, which is not extensible [20] [57] [58]. Thus, high mortality of juvenile individuals (2 ≤ d^0.20 < 10 cm) is observed under the influence of intrinsic and extrinsic factors (descent of peaks, trampling, irregularity of rainfall, grazing) as mentioned in several works [10] [13] [17] [59] [60] [61]. As a result of the high mortality rate among the regenerative cohort, Baraloto [57] points out that barely 1% of seedlings (germinated seeds) reach adulthood in tropical forests. Notwithstanding the natural events that affect natural regeneration in the forest, the share associated with humans would be preponderant in this ecosystem. The disturbance in the recruitment of taxa within

diameter classes (d^1.30 ≥ 10 cm), which is at the origin of the erratic curves for each of the species monitored, is irrefutable proof of recurrent anthropogenic interventions [10] [16] [20] [51] [52] [53] [54] [55] [62]. The corollaries of this ever-increasing and continuous anthropogenic pressure have a lasting impact on the natural regeneration of these taxa and concomitantly that of the entire ecosystem.

The natural regeneration of a forest is supported by the ratio of seedlings to large trees in the stand [63] [64] [65]. As such, natural regeneration is the basis for understanding the dynamics of woody vegetation, integrating the process of diametric recruitment, juvenile mortality, developmental stages and ecosystem survival [62] [66]. Under the dependence of several factors (light intensity, humidity, nature of the substrate, biological characteristics), natural regeneration would be a phenomenon sensitive to the variation of environmental parameters [67]. Correlatively, the modification of the parameters of the Loukaya ecosystem, following the various anthropic actions of the local residents, would explain some of the results of this study, particularly the mortality of young plants [1].

The Loukaya forest, which is much secondarised, looks like an ecosystem in full recovery, taking into account the high rate of young plants of heliophilic species (Ongokea gore and Syzygium brazzavillense). These stages of forest recovery are generally marked by a massive presence of pioneer heliophilic species [68]. A high level of anthropisation affects the physiognomy of the forest facies through the opening of the gaps. The corollaries are a variation in the flow of light reaching the ground, directly or through the heat, it brings generally considered as a determining factor in the renewal of forest stands [69] [70] [71]. This parameter would explain the high rate of seedlings in the study area; confirmed by a regeneration index greater than 100%; synonymous with good regeneration on the Rothe scale [48].

Also, it should be noted that the type of diaspora and the mode of dissemination effectively intervene in the process of densification of regenerants. The dominance of sarcochores in the Lukaya forest induces high densities of regenerants subject to predation and high mortality due to intraspecific competition at the base of the productive trees [57] [59] [72] who reports that in tropical forests confirms this hypothesis, only 1% of the individuals at the base of the productive trees reach maturity.

Although the natural regeneration index is satisfactory, the absence of individuals in certain diameter classes indicates that the process is subject to disturbances whose primary origin is human activity. Peri-urban forests are excellent places for supplying the surrounding populations with daily basic needs [73] [74] [75].

The spatial distribution of a species results from the interaction of biotic and abiotic factors, and as long as resources are not too limiting [76] [77]. Under these conditions, dissemination becomes the main spatial limitation and therefore determines the probability of recruitment [78] [79] [80]. As the Loukaya peri-urban forest is dominated by sarcochorous species, endozoochory is the main mode of spread. Human practices in peri-urban ecosystems, including hunting, are factors limiting spread, due to pressure on wildlife. Thus, the observed aggregate distribution of individuals of 2 ≤ d^0.20 < 10 cm would be the result of the limitation of dispersal [76] [80] [81] [82].

Anthropogenic activities in this ecosystem (hunting and cutting of trees and small woods) have a direct impact on biodiversity in general and indirectly on the dispersion of diasporas, especially sarcochores. These diasporas, which cannot ensure long-distance dispersal without animal involvement, give an aggregated spatial structure to the taxa of the regenerative cohort. Gaps or even windfalls are appropriate environments for the development of this phenomenon, as they provide optimal conditions for germination and growth of individuals [20] [83] [84]. Windfalls and open gaps resulting from natural and/or artificial events are important niches for regeneration [85].

Finally, it should be noted that the aggregate structure of the regenerative cohort evolves towards a random structure over time. Since the vital area is not extensible, recruitment within diameter classes affects densities, in the sense of regression. Indeed, the larger the trees, the lower their density [20] [62]. This evolution of structure would be the result of the mortality dependence mechanism of the individuals, which often corresponds to the transition from the young to the adult stage following intraspecific competition and that of water and mineral resources [86].