1 Introduction

The loss of woody biodiversity is caused by anthropic and natural activities resulting in serious environmental issues. For example, urbanization through the expansion of urban infrastructure development leads to the destruction of tree species, habitat loss, and the extinction of urban forests [1, 2]. These anthropic activities lead to biodiversity loss, soil erosion, the emergence of various diseases, influencing the impacts of climate change and associated environmental problems. However, many studies reported the importance and use of trees in urban areas [3, 4]. For instance, the urban forest is the source of food production [5], contributes to the mitigation of urban air pollution [6], climate change mitigation [3], and the enhancement of the urban population’s well-being [7, 8]. In addition, urban trees in school areas improve air quality, regulates school temperatures, heighten the beauty of the schools, thereby improving students’ academic performance [9] and urban food production [10] which make a city green and sustainable as highlighted by the United Nations Sustainable Development Goals (SDGs) 11 and Paris Agreement which calls for more climate solutions such as green city solutions like tree planting programs in urban areas.

Additionally, schoolyards’ trees reduce conflict and physical violence in urban areas [11], and their presence in urban spaces also contributes to the smooth running of physical activities [12]. Despite the various importance urban forest brings to bear; only a few studies have investigated the role of trees in the urban schoolyards and the services they provide, with previous studies focusing on only the role of trees on campuses. For instance, the campuses have been reported to be sites of biodiversity conservation [13, 14]. Furthermore, [15] discovered that the University of Sumatera Utara, Campus of Medan in Indonesia has 121 tree species from 37 families and also provides numerous services such as wood and latex production as well as absorption of polluted air. Additionally, [16] also indicated that the public primary schools represent 10% of tree cover in Kenya.

Moreover, studying the students’ perception of plant and animal species in their schoolyards is also rare in the West Africa Sahel region. This has necessitated the need to assess the students’ perception of plant and animal species in their schools. This can be achieved by the formation of student groups such as green students, which [17] underlined that the concept of eco-literacy as botanical knowledge is necessary for sustainable natural resources management. For example, woody plants have been used for school greening purposes in Niger since the colonial period [18]. The schoolyards in the cities of Niamey and Maradi are an important educational hub in terms of the number of schools and students in Niger. The trees in the schoolyards of these two cities have been characterized slightly as urban forest types in Niamey and Maradi cities [19] with natural forest patches and artificial tree plantations (Fig. 1). These urban green spaces provide a suitable, healthier, and peaceful learning environment. In addition, most of the schools in Niger are now working to meet Sustainable Development Goals (SDGs) by creating urban green infrastructure in their schoolyards, the trees in schoolyards contribute to quality education through the forest biodiversity for environmental education and urban forest in schools (SDG4) and also enhance the academic performance of student and schools’ employees’ productivity [9]. Consequently, the trees in schoolyards promote the city's sustainability (SDG11), climate actions (SDG13), and the conservation of terrestrial biodiversity (SDG15). The tree diversity in the schoolyards of Maradi and Niamey is shown in Fig. 1.

Fig. 1
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Tree species diversity in the schoolyards of Maradi and Niamey (source Moussa Soulé)

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Previous studies on urban schoolyards have highlighted numerous contributions of schoolyards biodiversity in mitigating climate change, regulating the microclimate of the schools as well as in improving the performance of the student and the beauty of the cities. However, these studies did not investigate the diversity and values of woody species at different educational levels (primary, secondary and tertiary schools) and students’ perception of plant and animal species in their schools. Similarly, there is no study, which explored the determinants of the botanical knowledge of tree species of the students with regard to the schoolyards of their studying areas, the role of urban schoolyards in biodiversity conservation, and carbon storage, especially in the West African context. This study attempts to close this gap by investigating the woody species richness in the urban schoolyards forest in the cities of Maradi and Niamey in the Niger Republic. By doing so, the study provides a baseline reference situation from Maradi and Niamey two cities of the Niger republic with contrasting urban school environments. Therefore, this study attempts to fill this gap by (1) investigating the woody species richness and carbon sequestration potential in the urban schoolyards forest in the cities of Maradi and Niamey in the Niger Republic and (2) highlighting the perception of schoolyard vegetation. Specifically, this study seeks to answer the following questions: (1) Could urban schoolyards' biodiversity contribute to the reduction in the loss of woody species in the Sahel region? (2) What is the carbon sequestration potential of woody species in the schoolyards of these cities? (3) What socio-economic and ecological benefits are provided by these urban schools’ woody plants? (4) How do the students and school managers perceive the schoolyard's trees?

2 Materials and methods

2.1 Study area

This study was conducted in Maradi and Niamey, which are two Sahel cities in the Niger Republic. Niamey is the political capital of the Niger republic located between 13.5116° N, and 2.1254° E and accounts for a population of 1,203,766 inhabitants [20]. The second city Maradi is a city located between 13.5010° N, and 7.1036° E and accounts for approximately 326,804 inhabitants in 2021. The choice of the two cities in this study is explained by their differences in socio-economic, political characteristics, and agroecological structures. For instance, the average maximum temperature in Niamey is about 35.9 °C and the average minimum temperature is 19.90 °C, with Maradi is having an average maximum temperature of 36.7 °C and an average minimum is about 24 °C [20]. Additionally, the urban forestry activities in the two cities are made up of trees planted in public and private spaces and preserving historical woody vegetation [19]. Figure 2 provides an overview of the geographical location of the two cities and the schoolyards' woody species in primary, secondary, and tertiary schools across Maradi and Niamey cities.

Fig. 2
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Study area

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2.2 Data collection

This study used urban forestry inventory, based on quota and purposive sampling approach. Accordingly, three educational levels (primary schools, secondary schools, and tertiary schools) were considered and two types of schools (private and public) were included. The study sample consisted of 30 schools in both two cities (Maradi and Niamey). From each school, dendrometric data were collected from a plot of 50 m × 50 m as recommended for the agroforestry systems in West Africa [21]. In each plot of 2500 m2, parameters such as tree diameter at 1.30 m, total woody plant height, crown diameters, and name of the species were collected. In addition, the centre of the plot was recorded by a means of GPS (Garmin GPSMAP 64x, Handheld GPS, and Garmin GPSMAP 64 Worldwide with High-Sensitivity GPS and GLONASS Receiver). The wood density for biomass estimation was performed using [22] and an ethnobotanical survey was used to collect data from the students, school managers, and their perception of the existence of trees in their schools. The book called Trees, Shrubs, and Lianas of West African Dry Zones, was used to identify tree species [23]. The tools for data collection for this study were the forest inventory sheet used for forest data collection, individual questionnaires (individual interviews) for the ethnobotany data collection, and observations such as taking pictures with our phone’s cameras. The questionnaires and inventory sheet were in the French language.

2.3 Data analysis

2.3.1 Determination of ecological indicator

Descriptive statistics such as frequency, percentage and inferential statistics were used to draw the systematic composition such as tree species richness, families, and genera, species origins (native or exotic) across the two urban schoolyards strata. Accordingly, the following ecological indicators were determined.

  1. a.

    Percentage of the planted or preserved stem in schoolyards: it corresponds to the total number of stems counted in a given school divided by the total number of the stem recorded in the primary, secondary and tertiary schools. For this indicator, the dominance was calculated based on the percentage by inferring the percentage.

  2. b.

    Stem density (Stem/ha): it refers to the total number of individual stems of a given species per unit area (ha);

  3. c.

    Crown area (Ca, m2) was calculated assuming an elliptical crown shape using (Ca) (m2) = π((d1/2)x(d2/2), Where d1 is the largest crown diameter (m) and (d2) is the diameter perpendicular to the larger crown diameter (m).

  4. d.

    Tree cover (%) was calculated as = Ca (m2) (100)/Plot size (m2);

  5. e.

    Basal area, Basal area was calculated as BD (m2) = D2 * (π/4), where π is pi = 3.14, and D is diameter of 1.3 m.

  6. f.

    Shannon–Wiener diversity index: H’ =  − ∑ [(ni/N)*ln (ni/N)], where ni is the number of individuals of species i, N is the total number of individuals in per land-use unit and ln is the natural logarithm.

  7. g.

    Pielou’s evenness (J’): J = H’/H’max, where H’ is the Shannon diversity index and H’max is the maximum possible value of H’.

  8. h.

    Sorensen index was calculated to compare the similarity between the Niamey and Maradi using the formula described in (Thiombiano et al., 2017): K = 2C/(2C + A + B), where A is the number of species in one area (e.g. Niamey), B is the number of species in another area (e.g. Maradi) and C is the number of species common to the study sites. Excel was used for some calculations, tabulation and drawing graphs.

2.3.2 Estimation of schoolyards carbon stock

The above ground biomass (AGB) was used to estimate the carbon stock by using the general allometric model developed by [24]. For tropical vegetation, the above-ground biomass AGB = 0.0673(ρD2H)0.976 where AGB = refers to the aboveground biomass in kg, ρ = wood density (gcm−3), D = diameter in cm at breast height (1.3 m), H = total trees height (m). The below-ground biomass was estimated using of root shoot ratio of 0.25 developed by [25] for the tropical vegetation and the carbon conversion factor of 0.47 [22].

2.3.3 Determinants of the students’ botanical knowledge of the schoolyards trees species

A regression analysis was conducted to determine the factors affecting the students’ botanical knowledge of the tree species in the schoolyard. In this study, the number of tree species known by the student was considered as a dependent variable (Y) and other selected factors such as age and sex of the students, the types of the schools, the type of ecosystem services obtained by the students from the trees in the schoolyards, the number of vertebrate in the schoolyards, the number of invertebrate hosted by the trees in the schoolyards, the school education level and the location of the schools were considered the independent variables of the model. The dependent variable of the model is a continuous variable and refers to the number of trees species known by the student (Y). The explanatory variables were the age of the student (X1), the number of vertebrates hosted by the trees the (X2) and the number of invertebrates hosted by the trees of the trees of schoolyards (X3). Other explanatory variables were included X4 is a categorical representing the type of school; it scores 1 if the school is public and 2 otherwise, X5 the location of the schools, which scores 1 if the school is located in Niamey and 2 if it is located in Maradi, X6 refers to the school educational levels, which the value 1 if the educational level is a primary school, 2 if it is a secondary school and 3 if the educational level in tertiary school. Likewise, X7 represents the type of ecosystem service perceived by the students from the trees of the schoolyards, which scores 1 if intangible ecosystem services, 2 if tangible ecosystem services, and take 3 if they perceive both tangible and non-tangibles ecosystem services. The regression model used to determine the factors affecting the students’ botanical knowledge of the tree species in the schoolyard is as follow:

$$\mathrm{Y}={\mathrm{a}}_{0}+{\sum }_{\mathrm{n}=1}^{\mathrm{n}}\mathrm{Xn}$$

where the dependent variables (Y) represents students’ botanical knowledge of the tree species in the schoolyard in term of number of trees species and Xn other selected explanatory variables of the regression model such as age and sex of the students, the type of school, school educational level, the location of the school.

The laboratories of the University of Maradi and Abdou Moumouni University were used to identify unknown species by the means of tree species photos. The tree species classification and nomenclature were conducted by the mean of Angiosperm Phylogeny Group IV botanical classification system [26]. Furthermore, a taxonomically comprehensive phylogeny was used for the legume tree species, the new subfamily classification of the Leguminosae [27] and the animal species were identified with the support from specialists using photos taken during the collection of the data on the field.

Prior to the statistical analysis, the Ryan-Joiner test and Levene’ test were used to check the normality and homogeneity of the data in order to use the parametric analysis. Kruskal Wallis test was used to the difference in the number of tree species across the school level at alpha 0.05 level of significance.

3 Results

3.1 Sociodemographic characteristics of the respondents

The sociodemographic characteristics of the respondents are presented in Table 1. The most respondents were male (52.00%), aged between 16–23 years (49.00%) and studied in public school (63.17%) (Table 1).

Table 1 Sociodemographic characteristics of the students (n = 600)
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3.2 Structure of the urban forests in the urban schoolyards

3.2.1 Floristic composition of the urban schoolyards in the two cities

There are 62 woody species classified into 31 families in the schoolyards of Niamey, with Fabaceae being the dominant botanical family (15 species) (Annex 1). In addition, the neem tree (Azadirachta indica A. Juss.) accounted for 58.40% of the total stem recorded in the schoolyards (Table 2), with about 53% of the total plant species (33 species) of exotic origin, and 26 species (42%) were food trees in the schoolyards of Niamey. Furthermore, the number of tree species varied significantly according to the types in Niamey (public, private) (H = 5.77 DF = 2, P = 0.056). The overall Shannon index was 2.17 ± 0.79 and the overall evenness of 0.58 ± 0.10 as. A similar trend was observed in the schoolyards at Maradi, where about 82 woody species belong to 29 families with 33% of woody plants species being of exotic origin (Table 2 and Annex 2). The number of planted stems accounted for 88% and 74% in Niamey and Maradi city respectively with an overall Shannon index of 2.40 ± 0.63 and 0.58 ± 0.18 as overall evenness. The similarity index was 64% indicating high similarity between the floras of the schoolyards in the two cities.

Table 2 Tree biodiversity urban forest in the schoolyards
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3.2.2 Parameters of the structure of urban forests in the schoolyards

The results showed that the tree cover was higher in Maradi primary school (49.71%) than in Niamey primary school (38.40%) (Table 3). Additionally, in Niamey, primary schools had the highest values for the basal area (16.92 m2/ha) and the highest carbon density (48.91 t/ha), whilst Maradi primary schools had the highest stem density (135 stem/ha) (Table 3). The basal area varied significantly between the two cities across the school types (H = 3.86 DF = 1; P = 0.050), however, there are no significant differences in the tree cover, stem density and carbon density across the schools in Maradi and Niamey schools.

Table 3 Parameters of the structure of urban forest in the schoolyards of Maradi and Niamey
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3.2.3 Students’ knowledge of the tree biodiversity and zoological diversity in their schoolyards

The students’ knowledge of their schoolyard’s biodiversity (Botanical and zoological knowledge) and ecosystem services rendered by the tree species in their schools are summarized in the Table 4. It showed that most of students (85.50%) knew between 0 and 5 different tree species, 74% of them knew between 0 and 2 vertebrae. In addition, 93.67% of the students knew between 0 and 2 invertebrates, 57.00% of them were aware of intangible ecosystem services provided by the schoolyard trees.

Table 4 Students’ knowledge of the tree species and zoological diversity in their schoolyards (n = 600)
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3.2.4 Relations between schools, schoolyards trees biodiversity ecosystem services and hosted animals

Pearson's correlation test was conducted to examine the relationship between the type of schools, the educational level, the number of tree species, vertebrates and invertebrates in the schoolyards and the type of ecosystem services provided by the trees in the schoolyard across the two cities. Table 3 showed that there were negative, weak but significant relationships between the type of schools (public/private) and the number of tree species in the schoolyards (r = -0.1244**), the type of schools and the number of vertebrates (r = -0.1129**), the number of vertebrates and invertebrates (r = -0.1283**) on the trees of the schoolyards.

Moreover, the results showed that there were weak, positive but significant relationships between the types of schools and the number of invertebrates (r = 0.1011000*) in the schoolyards, the number of tree species and the number of vertebrates in the schoolyards (r = 0.1320), the number of tree species in schoolyards and the number of invertebrates in the schoolyards (r = 0.1776**), the number of tree species in schoolyards and the trees benefits obtained by the students (r = 0.2700**). Furthermore, Table 5 it showed that there was a week, positive and significant relationships between the number of vertebrates and the location of the school (r = 0.2662**).

Table 5 Relationship between schools, tree biodiversity and hosted animals and city (n = 600)
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3.3 Determinants of the students’ botanical knowledge of the schoolyards tree species

The results showed that the p-value for the regression model is equal to 0.0000 while its correctness equals to is 0.2059 and the Chi-square equals 22.05%. These values indicate that although the value of the Chi-square of the model is low, its variables had a significance value of 0.0000. It can therefore be concluded that the model is efficient to determine the determinants of student botanical knowledge of trees species in the schoolyard. Accordingly, the regression equation expressing the determinants of the students’ knowledge of tree species in their schoolyards is as follows:

$$\mathrm{NTS}=-0.66385+0.3236992\mathrm{X}2+0.4114956\mathrm{X}3-0.4014161\mathrm{X}4+2.578733\mathrm{X}7\left(1\right)+ 3.51309\mathrm{X}7 (2)$$

A positive sign implies that the associated variables affect positively the students’ botanical knowledge of tree species in their schoolyard whereas a negative sign indicates the associated variables affect negatively the students' botanical knowledge of tree species in their schoolyards. The results of the regression analysis showed that the number of vertebrates in the schoolyard trees and the number of invertebrates existing in the schoolyards trees and the type of benefits obtained by students from the trees in their schoolyards affected positively and significantly the botanical knowledge of tree species of the schoolyard. In addition, the type of schools (public/private) and their locations affected negatively and significantly the students’ botanical knowledge of the tree species in their schoolyards.

With regard to public students, being a student in private schools decreases the students’ knowledge of the number of the tree species of the schoolyard by -0.40. This implies that the student of public school had better botanical knowledge of tree species of the schoolyards than those in private schools.

An increase by 1 unit of the students’ knowledge of intangible ecosystem services provided by schoolyards trees by 1 unit increases their knowledge of trees species by 2.578733 (Table 6) whilst an increase by a unit of the students’ knowledge of tangible and benefits provided by the trees species of the schoolyard increases tree species by 3.5 (Ceteris paribus).

Table 6 Determinants of the students’ botanical knowledge of the tree species of the schoolyards
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On the other hand, with reference to Niamey the location of school in Maradi decreases the students’ botanical knowledge of the tree species of the schoolyards by 0.2891124 (Ceteris Paribus). Accordingly, the students studying in Niamey had better botanical knowledge of the tree species of schoolyard than those studying in Maradi. schools. Then, an increase in the number of vertebrates and invertebrates known by one unit increases the students’ botanical knowledge of the tree species of the schoolyards by 0.3236992 and 0.4114956 respectively (Table 6).

4 Discussion

The overall Shannon indices in Niamey and Maradi showed that the schoolyards of these two cities have high woody species diversity. Previously, [28] noted that overall Shannon indices with values greater than 2 indicate medium–high diversity. Similarly, [29] stated that such Shannon values show commonly low spatial competition among tree species within a given site. The high woody species diversity in these schoolyards could result from the natural regeneration of the fruit consumed by the students and the dissemination of tree seeds by the birds. In this context, [30] noted that bats are key agents in dispersing the seeds of Azadirachta indica trees. Additionally, tree species like Azadirachta indica have high germinative capacity and might be utilized for various purposes. For example, [31] mentioned that Azadirachta indica is a high seed production capacity and germinative capacity tree, which was is to reduce the air pollution. Also, the dominance of Azadirachta indica trees in the two cities’ schoolyards resulted from programs such as Green Sahel National tree plantation days in the Niger republic, which promoted the planting of these trees during that period. [32] stated that the city’s climate conditions are favourable for the growth and propagation of Azadirachta indica trees. Additionally, the schoolyard’s tree biodiversity possessed both native and exotic floras, which makes them an important reserve of tree species. For instance, in Niamey, the schoolyards accounted for 62 woody species grouped into 31 families in the schoolyards of Niamey with Fabaceae being the dominant botanical family (15 species) (Annex 1) 53% of the total plant species (33 species) with the exotic origin, and 26 species (42%) being food trees. A similar trend was observed in Maradi, which accounted for 82 woody species belonging to 29 families while 33% of woody plant species were of exotic origin (Table 2). Similarly, [19, 33] indicated that the urban areas are important sources of tree species diversity and contain both native and exotic species. These tree floristic compositions in the schoolyards of Niamey and Maradi could enhance the academic performance of the students. [34] reported that the schoolyards' tree species and trees’ composition affected the academic performance of primary school students.

Additionally, the study showed that Fabaceae was the most dominant botanical family in the schoolyards. Some of the tree species belonging to this family such as Tamarindus indica, Dialium guineense, and some gum species are multipurpose trees. In addition, the dominance of Fabaceae in these schoolyards indicates the Sudano-Sahelian vegetation. Similarly, [35] and [36] highlighted that Fabaceae spreads in large soil and climate ecological drylands, which indicates the diversity of angiosperm in the area. Also, [14] reported that Fabaceae is the most dominant family in the forest of the campus of Sambalpur university.

On the other hand, the study showed that the students perceived only the intangible ecosystem services rendered by the tree species in the schools. This is due to fact that the schoolyard’s trees are used as shades during break time and hot weather and also for student's basic learned lessons from school. This observation has been made by [37] which could be explained by the major service perceived by the students is shade in both cities. Similarly, [38,39,40] reported that urban trees played a key role in carbon sequestration. Similarly, [41] emphasized that protecting the trees from destruction slows down the impacts of climate change which is a key to meeting the SDG 13 (climate action).

With regards to the students’ botanical knowledge of the schoolyard’s tree species and animals existing in the schoolyards, the study revealed that most of the students knew few tree species. This is an indication that the students were botanically illiterate about their schoolyard’s tree biodiversity, which could affect the promotion of green education. In addition, this low botanical knowledge of the students might be due to the fact that the tree floras of the schoolyards are mostly made up of exotic tree species. This could be also due to the lack of interest in plants as reported [42] which is a leading factor in botanical illiteracy. Previously, [17] noted that it is crucial for to school authority to initiate tree conservation for sustainable natural resource management. The school authorities need to use the school tree vegetation for teaching purposes such as teaching the local environment components with a focus on tree species in the schoolyards for a botanical education such as teaching the importance of tree species to the students and naming the tree species to their schools via ecological excursion. There is also a need to involve the students in the tree planting in their schools and allow them to put the names of each tree species in their schools.

Furthermore, the findings of our study showed that the student had low zoological knowledge of the animals (vertebrates and invertebrates) living in their schoolyards. The low knowledge of the student regarding the zoological diversity of their schoolyards could result from the absence of the animals during the day as well as the extinction of some animals due to students’ noise. There is to promote zoological education via the ecological excursion in the schoolyards, to visit the Zoos. The school managers can also construct the school’ botanical garden with some animals for ecological education which is key to sustainability education.

The study showed that the number of vertebrates and of invertebrates existing in the schoolyards and the ecosystem services provided by these schoolyard’s trees to the students affected positively and significantly the student botanical knowledge of tree species. Accordingly, these factors contribute to enhancing the student knowledge of the schoolyards tree species and therefore increase the students’ botanical literacy as noted by [43, 44]. Since the study revealed that most students had low botanical knowledge of the tree species existing in their schoolyards, this might pose a serious hindrance to the students’ discussion about tree species, which can affect negatively the student’s observational skills and conceptual learning as noted by [45, 46]. Furthermore, poor botanical knowledge could augment the risk of the students consuming toxic fruits [47].

In addition, the type of school (public/private) and the location of the school affected negatively and significantly the student botanical knowledge of tree species. With regard to private schools in the two cities (Maradi and Niamey), the students had low botanical knowledge of tree species existing in their schoolyards. This could be due to the fact that the teachers in private schools did not carry out few outdoor activities in their school forests. This result corroborates that of [48,49,50] who reported that the students have low botanical knowledge in the nearby environment. In addition, the students studying in Niamey had better botanical knowledge of the tree species of their schoolyards than those in Maradi schools. This might be explained by that the accessibility of the student to NTIC and other information channels (TY & radio) in Niamey could be better than in Maradi. There is a need to use the tree in their schoolyards for teaching purposes in order to increase green awareness and environmental education in schools.

Our findings revealed the use of exotic food tree species such as mango tree, Annona squamosa, Carica papaya, Moringa species etc. which is key to food security and nutrition for the students. This also highlights the urban food forestry in the schoolyards in the two cities such as the existence of school food forests like the campus mango forest at the University of Niamey and other schools which is key to promoting the school sustainability key for quality education. In addition to that our results show the use of indigenous food tree species such as Balanites aegyptiaca, Lannea microcarpa, Sclerocarya birrea, etc. in the school urban forest which can help to conserve biodiversity like native birds [51] and their fruits constitute a great source of minerals and vitamins which can help to fight food insecurity and malnutrition in the schools. In short, our study demonstrates the role of urban schoolyards in biodiversity conservation and the carbon sequestration potential of the trees in the schoolyards which are key to meeting the SDGs for sustainable cities.

4.1 The study implications with the Sustainable Development Goals (SDGs)

Forests in urban schoolyards are central to meeting the Sustainable Development Goals. Accordingly, the results of the current study demonstrate that some cultivation of the food trees such as Moringa species, (Fig. 1, photo B) the layer of urban food forests such as canopy layer (large fruit and nut trees) which is composed of Carica papaya, Moringa oleifera and mango trees. The second layer is an herbaceous layer composed of vegetable plants such as lettuce (Lactuca sativa). The two layers are part of seven layers of the urban forest garden, which is central element of urban food forestry [52] which constitutes a means of promoting to the zero poverty (SGD 1) and zero hunger (SDG 2). Additionally, the leaves and the fruits of fodder tree species such as the fruits of Faidherbia albida in the schoolyards could contribute the raise funds through a sale that improve the quality of the education (SDG 4) and food security for promoting a Smart City Model. Our study reported the diversity of the food tree species (Annex 1 and 2) used in the schoolyards in the urban areas which can rise the urban food forestry practices in the schoolyards as means of promoting Smart cities, where people are allowed to harvest nuts, root, fruits, leaves and vegetables that can contribute the urban climate change adaptation (SDG 13).

Furthermore, the results of the regression analysis showed the existence of strong relationship between the number of trees species and intangible ecosystem services rendered by the schoolyard’s forest. One of the key implications of this relationship is the regulation of microclimate through the lowing of the temperature of the school and beyond of the city. Therefore, it contributes to climate change mitigation (SDG 13) especially during the hot periods in dryland zones like the West Africa Sahel cities. This role of urban forests in regulating the urban temperature plays a key role in promoting good health and well-being (SDG 3) for sustainable and heathier cities (SDG 11) as the urban forests are solutions to the urban island heat [53] for urbanite’ well-being. This relationship highlights also the urban forest in the schoolyard of the two cities present an opportunity for urban air pollution removal, which improve the health and wellbeing of the urban citizens. This service could be confirmed due the dominance of the neem trees in the urban schoolyards in the two cities as neem tree has been pointed out that neem tree is a good natural air filter (air pollution removal central to the urban heath) and, absorber of CO2 (climate change mitigation), Oxygen producer (urban health) and used for soil restoration [31].

The use of food plants in the schoolyards as income generating activity is an opportunity for green job creation (SDG 8) for fighting poverty and enhance the resilience of urban areas. For instance, the harvest of fruit, leaves, nut and vegetables in the urban areas can boost the food market and provide jobs to the young people and women as the two cities the sale of leaves, fruits, nut and some vegetables are gender oriented commercial activities. This can promote the gender empowerment (SDG 5) and fix young people from joining the terrorist and other serene of urban delinquency for promoting peaceful cohesion (SDG 16) in the urban areas which is paramount to the urban sustainability mostly in the cities like Niamey and Maradi where urbanites are facing security challenges. The use of native tree species in the urban forestry has been highlighted to provide many services such as carbon sequestration, beauty of city, cultural services and enhancer of biodiversity conservation (SDG 14 and 15) due to the fact that local plants are habitat to the local living things. For instance, the local tree species are used as cultural services such as in manning some neighbourhoods of the cities in Niamey and Maradi. For exemple, the local name of Faidherbia albida is Gao in Hausa language, which is used for names of some neighbourhoods in Niamey and Maradi such as Dan Gao (which means the son of Faidherbia albida). This highlights the use of local tree as spatial identifier. The presence of the local trees in the schoolyards can educate the urban students about the local name of the tree species which is central to botanical education.

5 Conclusions

This study documented the taxonomy, the diversity, and the relevance of tree’s existence as green infrastructure elements in the school (primary, secondary, and tertiary). Our study has characterized the structure of unforgotten urban forest types, which is a collection of trees in urban schoolyards. The study identified three school forest types in the urban educational system, which are primary school urban forest, secondary school urban forest, and tertiary urban school forest with their different structural and compositional values. Our findings highlighted that school urban forests had high structural values, which are mostly high woody species richness with myriad of ecosystem services across school types in Niamey and Maradi. The results have demonstrated the role of urban schoolyards in tree biodiversity conservation, which has to be considered in the urban forestry analysis. Our study recommends the use of multipurpose woody plant species in urban greening school initiatives for quality education. Furthermore, the study recommends a further investigation, which will look at the effects of the school urban forest structure on the students’ academic performance and the allergenic potential of woody species found in urban schools. Our study has demonstrated planting trees and conserving natural forests in schoolyards is a double win for biodiversity conservation and trapping carbon. However, increasing demand for new academic buildings, parking places and other developmental processes within the schoolyards in the two cities are some menaces associated with these urban forest types. Hence, the present work can act as baseline information regarding the importance of schoolyards in tree biodiversity and carbon conservation, which can help in making appropriate management strategies for the conservation of these urban school forests. Our study complements the international efforts of proposing city green solutions as nature-based solutions to fight climate change and make healthier and sustainable urban areas. Finally, our study recommends that the school authorities incorporate multipurpose tree planting in your school’s environmental plans and the use of Rainwater Harvesting Systems for the school urban forestry in two cities.