Stratified sampling was based on the division of the area into homogeneous units according to land use. These homogenous units are subdivided into more homogeneous sub-areas in terms of ecological facies. For a given sub-area, a cluster of four (4) circular plots is installed twenty meters (20 m) from the cluster center along the cardinal directions. The cluster arrangement allows for the heterogeneity of the site to be taken into account. The network of permanent plots is designed to cover the entire area, ensuring the most homogeneous spatial distribution possible.

The high mangrove (HM) and the low mangrove (BM) are the identified strata. Based on accessibility (HM = 38,953 ha; BM = 29,823 ha), the number of plots to be surveyed is defined for each stratum (Table 1).

The network of permanent plots consists of 22 clusters of 88 plots located in the PRECEMA project area. The cluster is installed around a central point found by GPS. With the SUUNTO compass, the centers of the four circular plots are marked along the cardinal directions at 20 m from the cluster center. Each plot has a radius of 10 m (Figure 2).

The inventory protocol developed in 2016 was adopted. On an inventory form, the dendrometric and station characteristics are filled in, including the date, time of departure and arrival, the plot number, the attached village and the status of the vegetation. In addition to the scientific name of the species, the dendrometric characteristics are recorded: the diameter at the base at 5 cm (D0) and the diameter at 1.3 m from the ground (DHP) taken with a forestry compass, the height taken with a SUUNTO dendrometer, the cross-sectional diameter of the crown with a tape measure and the number of Rhizophora per foot counted.

The amount of carbon in the tree biomass is obtained following a dozen calculations previously made with the Excel spreadsheet. The database developed included:

· Average diameter by species (Rhizophora or Avicennia), by plot and by stratum,

· Above-ground biomass by species and by stratum,

· The average number of stems per species, per plot and per stratum,

· The number of contacts per species, per plot and per stratum,

· The density by species and stratum (density_{j HM/BM}),

· The ratio (R_j) of below-ground to above-ground biomass per hectare by species and stratum,

· Tree biomass/ha and by stratum (b_{TREES HM/BM}),

· The biomass of trees present in year t (B._tree,t) in grams of dry matter (gdm),

· The amount of carbon stored by the biomass trees (CO_{2 trees,t}) in year t (teqCO₂),

· The amount of CO₂ stored by the trees (ΔCO_{2 tree,t}).

This amount results from the change in the amount of carbon stored by dead wood at t in TeqCO₂ in high and low mangroves (ΔCO_{2 DEAD WOOD,t, HM/BM}) which is the amount of carbon stored by dead wood at year t in TeqCO₂ (CO_{2 DEAD WOOD,t,HM/BM}) following the equation:

ΔCO_{2 DEADWOOD,t} = ∑CO_{2 DEADWOOD,t}

To estimate the amount of carbon stored by dead wood at a year t, the conservative default equation in section 6.2 of AR-TOOL12 is used [10] :

CO_{2 DEADWOOD,t, HM/BM} = CO_{2 TREES,HM/BM,t} × DF

CO_{2 DEADWOOD,t,HM/BM} = Amount of carbon stored by deadwood in year t in TeqCO₂;

CO_{2 TREE,t} = Amount of carbon stored by tree biomass in year t in TCO₂/year);

DF = conservative ratio of carbon stored in dead wood to carbon stored in tree biomass in percent DF = 1%.

The amount of carbon stored by the soil in year t in TeqCO₂ is:

ΔCO_{2 SOL,t} = ∑44/12 × Proj area × dSOCt × 1 yr

Surface_Proj = Reforestation surface = 300 ha and management surface = 6000 ha;

dSOCt = Ratio of soil carbon stock change in one year in TeqCO₂/ha;

dSOCt = 0.5 tC for t₀ < t < t₂₀ and dSOCt = 0 tC for t > t₂₀.

The evolution of mangrove carbon sequestration was monitored by comparing the results of the assessment to those of the modeling projection made in 2016.

In 2021, the study was conducted in 20 clusters or 80 plots from the 22 clusters and 88 plots installed in 2016 (Table 2). The geographical coordinates of 2 clusters have not been found and the 3rd cluster, that of Dionewar, is bare.

Figure 3 is an illustration of the Dionewar bare cluster with 4 bare plots.

1) Specific diversity

The 3 main species encountered are Rhizophora racemosa, Rhizophora mangle and Avicennia africana (syn: Avicennia germinans). Table 3 shows a dominance of R. racemosa with 69.9% of total contacts and 76.5% of individuals on the high mangrove. It also remains dominant in the low mangrove with 61.8% of contacts.

2) Vertical structure

The subjects were classified according to height with 1.3 m amplitude. Figure 4 represents the distribution of mangrove individuals according to height classes. The vertical structure shows a predominance of individuals of height class between 1 and 2.3 m. This mangrove has a bushy habit with an average height of 1.91 m. The height distribution (in L) shows a right skewness (Skewness = 2.17) with a tail containing more observations than a normal distribution (Kurtosis = 4.07). The height peaks at 6.5 m.

3) Horizontal structure

Individuals were classified according to diameter with a range of 2 cm. Figure 5 represents the distribution of the inventoried individuals in diameter class. The lower classes (1 and 2) have almost all the individuals of the mangrove. The classes (7 and 8) of diameter between 13 and 17 cm have few individuals. Indeed,

the appearance of the figure reveals a young mangrove. The distribution is L-shaped or inverted J-shaped with an average diameter of 3.90 cm (right asymmetry) and 79.6% of subjects have a diameter less than 5 cm.

Figure 6 presents the coverage rates by locality. The highest rates are noted in the localities of Bambougar Malick, Keur Mbar, Mansarinko and Sadioga. Their recovery is above 50% (from 58.34% to 72.18%). On the other hand, the lowest rates are in Diogane, Falia, Niodior and Péthie with recoveries of less than 20% (from 06.03% to 14.52%). The average for all samples is 36.50% and the coefficient of variation (55.63%) attests to a heterogeneous vegetation cover. Note that each village name in this Figure 6 represents the plots inventoried.

· Coverage according to mangrove strata

Recovery was calculated for the high mangrove and low mangrove (Figure 7). The high mangrove has a higher average coverage (CV = 43.38%) than the low mangrove (CV = 61.64%) which has a more heterogeneous coverage.

The evolution of the carbon potential is followed on the one hand over five (05) years on the basis of evaluations carried out in 2016 and 2021 and on the other hand over fifteen (15) years on the basis of projections according to modeling

from 2011 to 2026.

1) Total carbon stock assessment in 2016 and 2021

The first evaluation was made in 2016 by the PRECEMA project and 5 years later, the second evaluation was made through this study of 2021. Thus the total carbon stock increased by 1353 TeqCO₂ (Figure 8) which corresponds to an average annual stock of 270.6 TeqCO₂. The carbon stock value (projected) is related to the period for 100% of the cases.

2) Average annual change in potential from 2016 to 2021 according to the assessment

The average annual change in carbon potential in 2016 is much higher than that of 2021 (Figure 9). In five (05) years, the mangrove has lost 20% of its potential.

3) Total carbon stock projection from 2011 to 2026 based on modeling

Overall, the projected carbon potential is increasing from 2011 (201,396 TeqCO₂) to 2026 (277,318 TeqCO₂) (Figure 10). For 2021, the model predicts a stock (270,289 TeqCO₂) slightly higher than the stock actually assessed (251,059 TeqCO₂), a gap of 7%. The value of carbon production (projected) is related to the period for 84% of the cases.

4) Average annual change in potential from 2011 to 2026 as projected

From 14,990 TeqCO₂ in 2011, the potential decreases to 826 TeqCO₂ in 2026, a decrease of 14,164 TeqCO₂ (Figure 11). Moreover, the quantity obtained with the projection for 2021 (2844 TeqCO₂) is higher than the one actually evaluated (1353 TeqCO₂) for the same year; that is to say a difference of 52%. The value of the projection is related to the period for 75% of the cases.

5) Average annual change in potential from 2011 to 2021 for the 19 clusters

The results from the 22 clusters were reported to 19 clusters as for the 2021 assessment. The average annual carbon change in the 15-year time interval is steadily decreasing (−2781.8). Also the potential obtained with the projection according to the modeling for 2021 remains always higher than the potential actually evaluated (Figure 12). The value of the carbon potential is related to the period for 76% of the cases.

In terms of specific diversity, 3 main species were inventoried: Rhizophora racemosa, Rhizophora mangle and Avicennia africana. In 2021, Rhizophora racemosa is dominant in all strata. In contrast, in 2016, Avicennia africana was dominant in the lower mangrove [10] . It occupied 27% of total contacts in the first assessment [10] and 9.6% in the second. Results reveal that A. africana is in the minority in 12/19 clusters inventoried. This species seems to be a victim of the effects of climate change and logging, while the dominance of R. racemosa would be related to its share in reforestation. Indeed, PRECEMA’s goal was to restore 50 ha of Avicennia reforestation (300,000 seedlings) and 250 ha of Rhizophora (1,250,000 propagules) [10] . During the survey, R. racemosa was found in 17/19 bunches and was dominant in 12 bunches with percentages varying from 57% for Bouly to 100% for Diogane. It is absent in the clusters of Baout and Bambougar Massamba and weak in Sadioga. For the vertical structure, the highest heights noted during the inventory in 2021 reach 5.5 m for A. africana and 6 m for R. racemosa. It is 5.8 m for A. africana and 7.8 m for R. racemosa [10] . The horizontal structure reveals high numbers in the smaller diameter classes in 2021. Indeed, for all clusters and all species combined, diameter classes between 4 cm and 9 cm are dominant [10] . This may indicate that woody individuals with stems larger than 05 cm in diameter are preferred for harvesting. The average recovery of mangrove stands is 36.50% with a coefficient of variation (CV) of 55.63% in 2021. It was 40.58% with a CV of 50.54% in 2016 [10] . This decrease in the cover rate reflects the level of degradation observed in the low mangrove, hence the relatively sparse state of the stand.

The data showed a disappearance of vegetation in some clusters testifying to the degradation of the mangrove. In addition, the loss of data for certain clusters calls into question the backup of project data. And the pattern of distribution of young individuals relates to the choice of reforestation and preservation sites. Indeed, if the reforestation site is very close to the houses, the plants are trampled by the cattle but also by the fishermen looking for space to maintain their fishing nets.

The total carbon stock increases overall during a 15-year period (2011-2026). In contrast, the annual change in potential shows a decline since 2012 [10] . Between 2011 and 2026 (projection), it goes from 14,990 TeqCO₂ to 826 TeqCO₂. For 2021, the total carbon stock assessed is still lower than the model projection. The small annual increase in stored carbon is thought to be due to the loss of reforested plants and limited monitoring of conservation areas. These include the adverse effects of climate change (drought, salinity, silting, erosion of Sangomar), anthropogenic actions, local governance and the lack of coordination between actors. The Sangomar rupture caused abrupt erosion of the mangrove with increased salinity, marine hydrodynamic forces, and sandy sedimentation [10] [19] . Indeed, the sites of Falia, Bambougar Massamba, Bassoul and Diogane have one of the four plots located in the sandy and bare tans. Despite reforestation, the mangrove has declined significantly in Djirnda and Bassoul with salinity, silting and erosion [14] . Sonwa [20] estimated carbon stocks of 243 t/ha in cocoa farms in southern Cameroon. However, some areas are well protected (Bouly, Keur Mbar, Diogane, Pethie, Keur Aliou Diop and Niodior) with restoration actions and natural regeneration of Rhizophora noted in the island of Sippo, in Ngallou Sam, Keur Mbar, Falia, Nema Bah, Niodior and in Avicennia in Mansarinko. Despite the threats, Faye [14] reveals the good condition of mangroves in landlocked and island areas.