Brazzaville, located between 4˚ - 5˚S and 15˚ - 16˚E (Figure 2), extends for 1/3 on a plain, formerly marshy, and for 2/3 on a plateau 20 m higher, which dominates the Congo River [30] [31]. The climate is of the low Congolese type [30]; corresponding to the AW4 type according to Köppen’s classification [32] - [37].

Its characteristics can be summarized as: 1) a mean annual temperature close to 25˚C (Figure 3) with a small thermal amplitude of 5˚C to 6˚C. March and April are the hottest months, on the other hand, July and August are the coolest [38] [39]; 2) an average annual rainfall of 1200 mm. Rainfall occurs from October to May with a very pronounced slowdown in January and February. The wettest months are generally March, April and November (Figure 3). The months of June to September are partly dry, resulting in stable air [31]; 3) a dry and cool season from June to September, which alternates with a wet and hot season from October to May [31]; 4) the relative humidity is particularly high, always above 70%. An absolute minimum is noted in August and September, and a relative minimum in February and March. The minima vary between 50% and 60%; the maxima vary between 88% and 94%. Evaporation shows a relative maximum from February to March and an absolute maximum in August and September [21]; 5) the mean annual insolation oscillates between 1700 and 2000 hours. The insolation curve shows two maxima in April-May and November, and a minimum in July (Figure 3). Two tributaries of the Congo River, the Djiri in the north and the Djoué in the west water Brazzaville [31]. In addition to these main rivers, the Tsiémé in the north, Madoukou Tiékélé and the Mfoa in the centre flow into the Congo River. To the west, the M’filou is a tributary of the Djoué.

The vegetation consists of shrubby savanna and riparian forests rich in timber species. The work of Olson et al. [40] shows that the study site is located in the Afrotropic ecoregion and more precisely in the tropical and subtropical moist broadleaf forests zone. According to African chorology [41], this flora and

vegetation belongs to the Guinean-Congolese Region, in particular to the Lower-Guinea endemism subcentre. By integrating Congo’s phytogeographic data into the White [41] and Kimpouni et al. [42] show that the flora and vegetation of Brazzaville belongs to the Léfini District, in the Lower Congolo-Zambezian transition Sector of the Lower-Guinea Domain within the Guineo-Congolese Region. Thus, this vegetation, composed of shrubland, woodland and forests, corresponds to the drier peripheral semi-evergreen Guineo-Congolian rain forest, and Zambezian dry evergreen forest and transition woodland Grassland and wooded grassland [41]. Brazzaville’s urban forest ecosystem is a low, m esophilic, tropophilic and dry formation. This ecosystem is almost without dominant strata, except for the one formed by introduced and generally isolated species such as Millettia laurentii, Albizia adianthifolia, Terminalia superba, Samanea saman, Hamanaea courbaril, Enterolobium cyclocarpum and Erythrophleum suaveolens. An atypical facies of African rainforest formations, the urban forest is made up of forest galleries and mesophilic islets resulting from the degradation of the Patte d’Oie forest [14] [15] [24]. In addition to these natural woody formations, Brazzaville has developed artificial forests, mainly of Eucalyptus sp. and Pinus sp. To this forest cover are added avenue trees (Acacia mangium, Terminalia superba, Terminalia mantaly, Peltophorum pterocarpum...).

The subsoil includes important series of mainly sandstone and sandy soils: the Batéké and Bambio plateau series (silicified sandstone, sands); the Stanley Pool series and the Carnot sandstones consisting of sandstone and argillite [31]. The soils of Brazzaville are ferrallitic, highly desaturated, impoverished and develop on sandy materials, poor in exchangeable bases, permeable to water [43].

The plant material studied consists of woody and herbaceous species recorded in the Moukonzi-Ngouaka district. This floristic cohort is made up of non-native indigenous species of Brazzaville flora on the one hand, and allochtonous species from the Guineo-Congolese region on the other. The identification of the material is carried out in situ for common taxa and if necessary at the botany laboratory of the École normale supérieure (ENS) or even at the national herbarium (IEC), located at the Institut National de Recherche en Sciences Exactes et Naturelles (IRSEN) in Brazzaville. The basis for identification remains the floral fascicles of Gabon, Cameroon, Central Africa (DRC, Rwanda, Burundi) and Rwanda, Nzayilu Nti: Guide des arbres et arbustes de la région de Kinshasa-Brazzaville [44], the catalogue of vascular plants in Congo [45], Hortica [46] and Tropica [47], 1001 garden plants in Singapore [48]. At the taxonomic level: (1) the ordination of families follows the systems of APG IV [49] and PPG I [50]; (2) and the nomenclature adopted is that of Lebrun and Stork [51], for African taxa.

It is based on two complementary phases, on the one hand, the bibliographical compilation and interviews with the owners and/or occupants of the parcels, and on the other hand the botanical prospection, parcel by parcel.

The floristic inventory (Table 1 and Figure 4) shows 281 species classified in three main groups: ferns (0.71%); Gymnosperms (1.78%); Magnoliids: Monocots (22.06%), and Eudicots (75.44%). According to the biological forms, this flora is divided into phanerophytes (63.57%), chamephytes (6.07%), geophytes (9.64%), therophytes (17.50%), and hemicryptophytes (3.21%). The tree portion is dominated by nanophanerophytes (28.21%), which are ahead of microphanerophytes (17.86%) and mesophanerophytes (3.18%). The proportion of mesophanerophytes is 3 times higher than the nanophanerophytes and microphanerophytes combined, in relation to the number of individuals counted. In terms of species diversity, the preponderant families are Fabaceae (10.32%) and Euphorbiaceae (7.47%). All other families (97.40%) have a contribution between 0.36% and

Legend: biological types (TB), hemicrytophyte (Hc), mesophanerophyte (Msph), microphanerophyte (Mcph), nanophanerophyte (Nnph), climbing phanerophyte (Phgr), prostrate phanerophyte (Phr), rhizomatous geophyte (Gr), climbing tuberous geophyte (Gtgr), bulbous geophyte (Gb), tuberous geophyte (Gt), erect therophyte (Thd), creeping therophyte (Thr), climbing therophyte (Thgr), erect chamephyte (Chd), creeping chamephyte (Chgr); Status: Spontaneous (spont), cultivated (Cult); Phytogeographic type (TP), Palaeotropical (Paleo), Pantropical (Pant), Cosmopolitan (cosm), Afrotropical (Aftr), AfroMalagasy (Afma), African-American (AfAm), Lower-Guinean (CG), Guineo-Congolese (GC), Sudano-Zambezian (SZ); Usage: Ornamental (orn), medicinal (Med), food (alim); Part use: leaf (fe), rhizome (Rh), meristem (ti), tuber (tub), fruit (fr), stem and/or root bark (Éc. t/r), root (ra), bulb (bulb), stigma (stig), seed (Gr).

4.98%. It should be noted that 32.47% of the families have a diversity rate of 0.36%, either one species.

Three biodiversity conservation statuses have been registered in Brazzaville, notably in the Moukonzi-Ngouaka district. Of the 281 taxa: 67.27% are cultivated and therefore under the close control of the actors; the portion of plants

whose naturalization is almost complete but which are still being cultivated is 20.73%; and finally, 12% of the said taxa are spontaneous, the majority of which are ruderals and nitrophytes. These proportions include introductions from local and foreign migrations.

Phytogeographic data show a clear preponderance of pantropical taxa, which cover more than 2/3 of the inventory (Figure 5). The endemism of the Guinean-Congolese regional centre, constituting the local base element, is poorly represented with less than 1/3 of the taxa inventoried. All other types of phytogeographic distribution, taken as a whole, represent 13.59%. Data on the geographical origin of the taxa show that they originate from three major foci: the Neotropic, notably South and Central America with 33.59%; the eastern part of the Paleotropic, especially the Asian and Malagasy regions with 21.64%; and finally, the African portion with 30.60%.

Of the 281 species recorded, 60.30% are exotic, many of which are naturalized. The spontaneous cohort of 39.27% is composed of 3.27% of the species present in the flora of the Brazzaville Patte d’Oie forest, 18.54% of ruderal taxa and nitrophytes, and finally the 2 main Congolese ecosystems. The savannas, in addition to those of the Léfini phytogeographic district, contribute 3.27%. The dense humid forest underpins this flora at 14.18%.

This census highlights four taxa, including three non-natives (Mangifera indica, Persea americana, Carica papaya) and one native (Dacryodes edulis) widely cultivated. The parcel preponderance of each taxon varies on average from 1/3 to ½. Ornamentals, without any other use, are less prized and their presence seems to be a fad or just a snobbery of the populations.

The interviews focused specifically on the reasons for introducing and/or cultivating these plants in their parcel. Ninety-one percent of the survey revealed that this practice is underpinned by the satisfaction of 1) socio-cultural needs

(traditional medicine, cultuals, etc.) and 2) food needs. Taking into account the totality of the data, 79.56% of the informants favored the food aspect, 89.92% traditional medicine and cultuals, and 22.34% the aesthetic aspect.

The socio-cultural benefits, without being exhaustive, are provided by savannah taxa such as Bridelia ferruginea, Sarcocephalus latifolius, Millettia versicolor... and forest taxa such as Trilepisium madagascariense, Dioscorea spp., Acanthus montanus, Lannea welwitschii, Canarium schweinfurthii, Buchholzia macrophylla... As for food requirements, we can mention Elaeis guineensis, Trilepisium madagascariense, Canarium schweinfurthii, Glyphaea brevis, Cola spp...

Finally, in spite of the local non-native species of the flora of Brazzaville, for the satisfaction of these needs, the populations use naturalized taxa such as Mangifera indica, Persea americana, Sambucus mexicana, Spondias spp., Annona spp., polyscias spp., Carica papaya, Eugenia uniflora...

The benefits resulting from the use of this flora are capitalized in three well-hierarchical categories. Traditional phytotherapy is favored by 89.83% of the respondents, while nutrition follows with 79.56% and, finally, aesthetics with 22.33%. Informants did not express an opinion on all other services, which received 0%.

Data on the use of taxa divided them into two groups: 1) those specific to one of the 3 categories, and 2) those with multiple uses. Multiple-use taxa occur concomitantly in either category. For example, ornamental plants, which cover 48.36% of the taxa, are simultaneously involved in the medicinal aspect to the extent of 4%. This is also true for food species whose contribution is 28.36%, with a traditional medicine intervention rate of 8%. Finally, the medicinal cohort, with 35.27% of taxa, is involved in feeding and ornamentation (Figure 6). It should be noted that multiple-use species are the most sought-after and maintained by the population.

These floristic data on uses highlight two categories of ecosystem services which are (1) harvesting services with food and traditional medicine, (2) and socio-cultural with the aesthetic value of ornamental plants. In addition to these palpable aspects, populations indirectly exploit other ecosystem services. The

fact that their benefits are seasonal and/or occasional, or even intangible, means that their influence has less impact on people. The benefit-based floristic analysis shows that multiple-use species are well represented in numbers, but not in terms of individuals per taxon. These taxa cover all categories and several ecosystem services such as food, traditional therapy, handicrafts, shade, shelterbelts, income generation, fuelwood, spiritual well-being and aesthetics. The species used are Dacryodes edulis, Mangifera indica, Persea americana, Elaeis guineensis, Buchholzia macrophylla, Gardenia jovis-tonantis, Polyscia spp., Euphorbia spp., canarium schweinfurthii...

The history of humankind is made up of encounters and exchanges of all kinds, including plants useful in more than one way [1] [9] [10] [11] [12] [13] [56]. As a result of continental drift, the new centres of differentiated floristic speciation in the new territories have experienced a floristic explosion whose taxa are spread throughout the rest of the world [6] [7] [8]. Initially confined to new territories, these taxa have overcome adaptation and naturalization mechanisms to the point of competing with native flora [14] [62] [63] [64]. The history of plant migration throughout the world is deeply coupled with that of man and these taxa provide evidence of the economic and cultural factors that govern their selection [6]. The colonial period was marked by a process of massive introduction of species into the “new territories”, based on an aesthetic affectation, the creation of botanical gardens, supply points for slave ships, and the establishment of colonial plantations, especially cash crops [5] [6] [7]. Exchanges between phytogeographical regions of the world, at any scale, generate floristic changes [5] [8]. Notwithstanding the exchanges at the level of empires and regions, those that take place within the territories of a region are not negligible. These very small-scale exchanges are generally based on rural exodus, the corollary of which is impoverishment. The difficulty of satisfying primary needs on a daily basis is such that populations resort to the values of their socio-cultural base. This phenomenon explains the introduction of several taxa from one phytogeographical district to another. In the present case, the taxa Bridelia ferruginea, Millettia versicolor, Gardenia jovis-tonantis, Hymenocardia acida, Trilepisium madagascariense, Myrianthus arboreus, Mondia whitei, Cola acuminata... are a tangible example of very low-level exchange and perpetuation of the values of the socio-cultural base.

Like the majority of sub-Saharan cities, especially working-class neighborhoods, Moukonzi-Ngouaka is home to an exhdered population, most of whom live below the poverty line of less than US$1 a day [65]. Indeed, the majority of the population works in the informal sector, for financial satisfaction. The disorganization of this sector is such that it is equivalent to daily subsistence. The corollaries are the impoverishment of society, especially of the most vulnerable social stratum, which cannot meet basic food and health needs due to prohibitive costs [15] [18] [66] [67]. Faced with this reality, the population most often resorts to traditional knowledge to meet basic needs, as several studies have noted [18] [19] [20] [66]. As the use of traditional knowledge is linked to useful plants (food and phytotherapeutic), the massive presence of non-native species with proven medicinal and dietary properties in Moukonzi-Ngouaka is nothing other than a result of internal migration or even the rural exodus of populations [67] [68]. This duality authenticates the existence of savannah taxa (Bridelia ferruginea, Sarcocephalus latifolius, Millettia versicolor, Hymenocardia acida, Gardenia jovis-tonantis, Elaeis guineensis...), forest taxa (Mondia whitei, Myrianthus arboreus, Trilepisium madagascariense, Canarium schweinfurthii, Terminalia superba, Cola acuminata, Buchholzia macrophylla, Markhamia tomentosa, Caloncoba welwitschii, Dioscorea spp...) and wetlands, both savannah and forest (Cyperus articulatus, Costus afer, Brillantaisia patula, Acanthus montanus...).

An analysis of data from the traditional African and particularly Congolese pharmacopoeia points to the use of several of these plants for the same purposes in their geographical area [2] [62] [69] [70] [71]. This universal value associated with plants is maintained by the displacement of populations, which is the origin of exchanges and the mixing of knowledge. This phenomenon is said to be the main source of use of non-native species outside their natural range [2] [4] [14]. Without being exhaustive, the taxa referenced in this study are Peltophorum pterocarpum, Senna siamea, Annona muricata, Eucalyptus spp., Eugenia uniflora, Psidium guayava, Hura crepitans, Jatropha curcas...

Ecosystems of all types and compositions provide humans and humanity with a variety of benefits, expressed in terms of ecosystem goods and services [27] [28]. This value is expressed in tangible, and therefore palpable, or intangible ways and is a foundation for the sustainability of ecosystems and the social and economic well-being of human populations [72] [73] [74]. In addition to the above-mentioned aspects, the integration of exotic taxa into the local flora has beneficial effects, equivalent to added value [75] [76] [77]. Plants provide direct benefits to the population (food security, artisanal material, fuelwood, source of income and phytopharmacognosia) and indirect benefits (climate regulation, carbon sequestration, pollution detection, dust sequestration, etc.) to the community [78] [79] [80].

The analysis of use data coupled with the ecosystem categories and services shows harvesting with food and phytotherapy as ecosystem services, and socio-cultural with aesthetic value. No other services, direct or indirect, particularly regulation services, are included in the benefits derived from arboriculture, the basis of urban forestry [14] [81] [82] [83]. The population’s vision on the introduction of plants into their environment is mainly influenced by practices rooted in traditional knowledge. Daily observation shows that urban arboriculture represents a non-negligible source of fuelwood and income for the most vulnerable populations. In spite of this recognized value, the fact that it is seasonal and occasional would overshadow its importance.

Urban populations living far from forests would be expected to be less impacted by ecosystem services [84] [85]. As the city of Brazzaville has developed on a formerly marshy terrain dominated by a savannah plateau and mesophilic forests, landscape modelling has accompanied its development [15]. An analysis of the current flora shows that the evolution of plant cover has been to the detriment of spontaneous biodiversity. As the introduction of these plants is intimately linked to human history, the inventory reveals a disparity in the floristic composition from one parcel to another. Except for the taxa involved in the delimitation of properties as living hedges, which are more or less constant, all the others reflect the socio-cultural base of the inhabitants and, better still, the endogenous knowledge that lies dormant in each of them [85] [86]. This environment thus constituted and resulting from the exploitation of the value of biodiversity, associated with the socio-cultural bases, would correspond to the limit of a socio-ecosystem [87] [88] [89] [90]. The anthropogenic floristic cohort is involved in two categories of ecosystem services, either harvesting or socio-cultural. In harvesting, the focus is on food aspects and traditional phytotherapy, whereas aesthetics is the only ecosystem service recognised in the socio-cultural category. The benefits generated by these ecosystem services are such that they obscure the income from the sale of the products of these plants, especially the fruits. This consideration would arise from the fact that the seasonality of these revenues, although additional and subsequent, does not override the daily phytotherapeutic and food values. Finally, since the values expressed by populations on ecosystem services are based on Non-Timber Forest Products (NTFPs), urban forestry, notwithstanding its ecological functions, can be a lever for poverty alleviation [91] [92] [93] [94]. From a harvesting and socio-cultural point of view, let us recognize the place of wood energy and the source of income, two elements fundamental to the survival provided by urban arboriculture as a unit of socio-ecological systems [92] [95] [96].

Urban forestry, of which arboriculture is the major pillar, provides functions that are essential to the proper functioning of the city and even of humanity, all things considered [97]. As urban cities are essential sources of heat island emissions, plants and especially woody plants sequester carbon and release oxygen through physiological metabolic processes [64] [80] [98]. Through these mechanisms, plants effectively combat urban heat islands while maintaining human well-being, air quality and of course zonal or even regional climate regulation [79] [98] [99] [100].

Urban arboriculture could also have positive effects on health, in a city where the population has less than 0.5 m² of green space per person [98] [101]. Notwithstanding the success of unspoken intangible phenomena, people derive benefits from plants, described as secondary, such as shade, windbreak role and spiritual well-being. In addition to these, several others, more subtle and fundamental, organize their daily existence [74] [100] [102].

In research, all methods are valid under two conditions: 1) clearly identify the objective; 2) marginalize subjective factors. The different stages of the study have led to distinguishing 1) native taxa from exotics by analysing speciation centers; 2) the knowledge acquired from the mixing of knowledge for local populations; 3) to understand the benefits derived from this double introduction of plants and the reasons for which they are introduced; 4) the perpetuation of the socio-cultural base is the primary unit that feeds it via internal migration; 5) finally, floristic variability is a function of individual or even clan knowledge. In addition to this highlighted material aspect, the immaterial aspects of urban arboriculture are not lagging behind. This part of the research highlights the gap between the actors on the benefits generated by urban arboriculture and its exploitation.