2.1. The Study Area
This study was carried out in a rural area of Sekoukou and Dan Saga villages where the choice is based on the different eco-systemic variability. Sekoukou is located in the Tillaberi region, district of Kollo with 466 inhabitants (INS, 2014) and is distant around 41 km from the capital “Niamey”. Dan Saga is located in the Maradi region, district of Aguié with a population of 5000 inhabitants and distant at 661 km from Niamey. The area is characterized by a Sahelo-Sudanian climate with an average annual rainfall of 648.75 mm at Dan Saga and 400 mm at Sekoukou. The forest type in the community can be classified as Sudanian savannah in both. the average duration of the rainy season is 4 to 5 months (from June to September) and a dry season (October to May). temperature generally varies from 17°C to 43°C. Two types of soil characterize the two villages clay and sandy soils.
2.2. Sampling technique of forest inventory
Data collection was carried out during the start of the rainy season (June). However, to determine the ecosystem services, the estimation of the carbon stock through the diversity of the woody stand in the fields and characterization of woody vegetation on the one hand; systematic random sampling by transects were carried out on a total of 72 plots equidistant one after the other of at least 200 m (40 plots in Dan Saga and 32 plots in Sekoukou) (Saigle et al., 2016). The sampling unit is a rectangular plot of 50 m x 50 m, (2500 m²) on two east-west and north-south direction transects (Lawali et al., 2018). Each goes from the large central square of the village (Dan Saga and Sekoukou village) towards the limit of the land or a natural barrier. In each survey, an exhaustive census of the ligneous plants was carried out. For the installation of plots, the right triangle method (Pythagorean Theorem) or the 3-4-5 method considered was used. The measurements carried out focused on the dendrometry characteristics, namely: the diameter at 1.30 m (for trees) and 20 cm above the ground (for multi-stemmed shrubs), the height of the trees and the circumference at the base of the trunk. Concerning woody regeneration, the size of the plots also varies according to the types of vegetation; Five (5) meter plots were carried out in each plot to measure individuals with a circumference less than 10 cm (20 cm above the ground) are considered as suckers (young shoot or juvenile) (Thiombiano et al., 2016). The measurements were made using a flexible tape measure, GPS, Suunto clinometer, stakes, inventory cards, pencils, etc... (Saigle et al., 2016).
2.3. Survey
To determine the livelihoods of rural populations who live from the benefits offered by agroforestry parks, a survey concerned 131 people (68 in Dan Saga and 63 in Sekoukou). These 131 people surveyed constitute the representative sample (sample size) of the population obtained from the use of (Surveymonkey, 2014) method which has a parameter such as the size of the target population, default proportion of 0.5, sampling confidence interval (90%) and a 5% margin of error. The advantage of this method for quantitative surveys is that it allows a large population to indicate the size of the sample to obtain survey results representative of the entire population. The sample size is calculated with the following formula:
\(\text{n}=\frac{{\text{t}}_{\text{p}}^{2}⨯\text{p} \left(1-\text{p}\right)⨯\text{N}}{{\text{T}}_{\text{P}}^{2}⨯\text{P} \left(1-\text{P}\right)+ \left(\text{N}-1\right) \times {\text{Y}}^{2}}\) source (Surveymonkey., 2014)
n = Sample size
N = Total number of populations
P = proportion = 0.5
tp = confidence interval (for 90%, tp = 1.96)
Y = Margin of error (between 5 to10%)
2.4. Dendrometry parameters
Dendrometric parameters can be generated by plot or by plant community. They generally make it possible to appreciate species populations' Spatiotemporal dynamics and identify those that present conservation difficulties for appropriate action through figures or statistical tests. It is about: trees density (total number of trees in the plot/total area of the plot), trees diameter (at 1.30 m from the ground or at chest height were taken with a pi tape), regeneration rate (total number of young plants/ total population size source) (Morou et al., 2016). The tree's cover area (R (%) = π (Dm)² with Dm = average crown diameter and π = 3.14) is defined as the percentage of the ground surface that would be covered if the aerial parts of the species were projected vertically onto the ground (Saadou et al., 1997).
2.5. Diversity Index
Diversity indices are used for a better understanding of the structure and composition of plant communities. They take into account the relative abundances of different species. Species richness is often not sufficient to compare two communities because it does not take into account the relative dominance of each species (Mahamane et al., 1999).
Shannon index: H’= \(-\sum _{i=1}^{s}Pi.\frac{lnPi}{{ln}_{2}}.pi\) where
i = relative abundance of a species i
S = maximum number of species. Varies between [0; 5[ Source (Francour, 2000).
The Pielou regularity index: E=\(\frac{Hʹ}{{log}_{2}}\) source (Rocklin, 2003).
Where H' = Shannon's index and varies between [0; 1[
Sørensen index: Cs= \(\frac{2j}{2j+a+b}\) source (Mahamane et al., 2016), where:
J = number of common species
A = number of species found only in site A.
B = number of species found in site B. and varying between [0; 1[
Analyze of variance (ANOVA) in Dan Saga and Sekoukou
Calculations of the probabilities related to the testing of the different factors were made from the variances in the R-studio of the different terms in the ANOVA model. However, when normality is not verified, the Kruskal Wallis test is used. Furthermore, if the calculated probability value is ≤ at risk α (α = 0.05), one would conclude that there is a significant difference in the means of the character in the populations considered. In this case, the "provenance" factor is said to be significant and if the probability value is ≥ 0.05, then the difference is not significant.
2.6. Floristic wealth
Floristic richness is assessed from the number of families, gender, and species. Thus, the floristic list was established based on the APG III classification (2016) or phylogenetic classification, which is the fourth version of the botanical classification of angiosperms established by the Angiosperm Phylogeny Group. Thus, all the individuals encountered during the inventory are grouped by family, genus, and species.
2.7. List of inventoried species
An exhaustive list of species has been established to determine the number of individuals per species and to calculate the relative frequency of the species. Only the woody species including trees (height > 7 m), shrubs (height < 7 m) and natural regeneration of woody species were considered and their local and scientific names.
2.8. biomass and carbon stock estimation
Three techniques can be used to measure biomass and carbon in forests; field measurements, remote sensing-GIS (monitoring of field verification), and modeling. Thus, forest inventory was considered to estimate the total biomass (aboveground and underground) in the two villages. On the other hand, \({\text{C}\text{O}}_{2}\) is the unit of measurement used by international emission reduction organizations. Thus, for the total carbon stock in the two villages we had chosen Chave et al., (2014), modeling which takes into account the most used allometric equations in West Africa (Saigle et al., 2016). The amount of carbon for each carbon pool in each stratum is determined by multiplying the fresh biomass by the fraction of carbon in the dry biomass (0.47) and it is converted to tonnes of carbon per hectare (C/ha) (Saigle et al., 2016). Because dry wood contains about 50% carbon, this means that almost 25% of a living tree is made up of carbon, as the fraction commonly applied by default is 0.47 between dry biomass and living biomass (IPCC, 2006).
a. Aboveground biomass
AGB (kg) = 0. 0673 (ρ × D^2×H) ^0.976 Where ρ (g/cm^3) = wood density was obtained from ICRAF wood density Database (Ketterings et al., 2001).
AGB = aboveground biomass
H (m) = trees height (cm)
D (cm) = diameter in cm at breast height (1.3 m)
However, this formula (Chave et al., 2014) is only valid for trees with a diameter ≥ 5 cm.
b. underground biomass
If underground biomass is considered a carbon store, it can be calculated using the root-shoot ratio, which is the ratio of underground biomass to above-ground biomass.
UGB (kg) = AGB×R Where R = root growth ratio = 0. 25 developed by Saigle et al., (2016).
c. Total biomass
The total biomass of the tree in each village is the sum of the above-ground biomass plus the below-ground biomass with the formula:
TB (kg) = AGB + UGB where TB = Total biomass
2.9. Normalized Difference Vegetation Index (NDVI)
NDVI was used for the Spatiotemporal analysis to see and analyzed the dynamic of vegetation in Sekoukou and Dan Saga villages from 2001 to 2013. Thus, the NDVI vegetation Index, varying between − 1 and 1, is a normalized ratio between near-infrared and Red with the formula: NDVI= (PIR-R)/(PIR + R) Where PIR = reflectance in the near-infrared. R = reflectance in the red. The interval is comprised within]-1; 1[