Afforestation/Reforestation Based on Gmelina Arborea (Verbenaceae) in Tropical Africa: Floristic and Structural Analysis, Carbon Storage and Economic Value (Cameroon)

The study was carried out in three selected plantations. Sampling was made infive 100x20 m plots per site. Overall, 32 species, 36 genera and 17 families were surveyed. Hymenocardia acida, Combretum adenogonium, Daniellia oliveri, Entada africana, Terminalia macroptera, T. laxiflora, Lannea schimperi, Lophira lanceolata, Maytenus senegalensis, Ochna schweinfurthiana, Protea madiensis, Psorospermum senegalense, Piliostigma thonningii, Sarcocephalus latifolius and Securidaca longepedunculata were the most important species. The richness index ranged from 2.53±0.05-7.74±0.03. Shannon index was 3 in all sites. Density ranged from 98±2.01-253±10.23 stems/ha. Basal area was statistically significant among the sites (p<0.001). All sites were floristically similar (k>70 %). The vertical structure showed three types of figures; L shape, symmetrical bell shape and unsymmetrical bell shape. These structures confirmed a good regeneration of timbers in the sites. There was a positive correlation of the species dispersal in the sites (p<0.001). The amount of AGB was 23.50±0.38 t C/ha. The amount of C sequestration was 86.28±16.57 t CO2eq/ha. The ecosystem service payments ranged between 258.87±24.88-8629.25±248.16 €/ha with the lowest values for CDM price and the highest for REDD+price. A financing of such projects is required in the frame of creating adaptation and attenuation measures to global warming effects.


Data Collection
A Randomized Complete Block Design (RCBD) was used to collect data for statistical analysis. The RCBD is one of the most widely used experimental designs in forestry research. The design is especially suited for field experiments where the number of treatments is not large and there exists a conspicuous factor based on which homogenous sets of experimental units can be identified (Jayaraman, 1999). The primary distinguishing feature of the RCBD is the presence of blocks of equal size, each of which contains all the treatments. For this study, three treatments representing three sites of Gmelina plantations were considered. Each transect was used replication; five transects of 100 m x

Vegetation Analysis
The ecological importance of each species in the studies stands was demonstrated using parameters such as relative frequency, relative abundance (%), relative dominance (%), density, basal area as well as the importance value index.
-Relative dominance is the percentage share of the basal area of a given species out of the total measured stems basal areas for all species; -Relative abundance is the percentage of the abundance of each species out of the total stem numbers for all species; -Relative frequency is the percentage of the frequency (the percentage of the total number of plots containing the species compared to all plots) of a species compared to the total frequencies of all the species added together; -The Importance Value Index (IVI) of each species was computed by summing the relative frequency, relative abundance and relative dominance as follow: IVI=relative Dominance (species) +relative Abundance (species) +relative Frequency (species) . with BA: basal area (m²/ha), d: diameter (m), C: (m) circumference.
-Species richness and diversity for stand were calculated using a popular index of alpha diversity: N =2 H , 2 is the basis of logarithm, H is the Shannon index and N is the effective species richness; Shannon's index: ISH=-∑ LOG 2 , with ni=number of the species i, N=number of all species; ISH is expressed in bit and Pielou's equitability: EQ= (Shannon, 1949;Magurran, 1988).
-Coefficient of similarity of Sorensen (K) was used to reveal floristic similarity between the stands: K= with a=number of species of the statement 1, b=number of species of the statement 2, c=number of species common to the 2 statements.
-Floristic structure (size-class distribution): to catch the structure of the plantations, we used dbh and species height. For the size-class diameter distribution in the understories of the stands, timbers were grouped in class of diameters with amplitude of 10 cm. For the size-class height distribution, we adopted Letouzey's method; here individuals were grouped into class of height with amplitude 2 cm.
The structure shapes have led to distinguished stems of regeneration, stems of future, mean stems and big trees. By so doing, the aspect of the evolution of species in the understories was forecasted through histograms of distribution.

Biomass and Carbon Sequestration Potential Estimates
-Above Ground Biomass (AGB) was estimated from the dbh assessed during the vegetation survey. We used existing allometric equations to evaluate biomass: B=0.066*D 2.59 , B=biomass in kg; D=dbh in cm, with 8≤D≤48 cm (Ketterings et al., 2001).
-The carbon sequestration potential (VCO 2eq ) was estimated using the ratio 44/12 corresponding to the CO 2 /C report.
-Economic values: Many carbon markets were put in place since 2000. However, we opted for the CDM, Voluntary market and REDD+prices which the mean prices are 3€/tCO 2 eq; 4.7€/t CO 2 eq and 100€/t CO 2 eq respectively (Chenost et al., 2010; Ecosystems Marketplace, 2017).

Statistical Analysis
All statistical analyses were performed with STATGRAPHICS plus version 5.0 (2016) for Windows and R software. The significance and correlation tests were performed using the One-way analysis of    Verbenaceae in sites 1 and 3, Combretaceae in site 2 have also contributed the most in family index value (Table 3).

Species Distribution and Floristic Similarity among the Study Sites
Using STATGRAPHICS plus version 5.0, the cluster analysis (AFC) between species in relation with abundance in the study sites showed a strong correlation among the sites ((Dnl=2; F=54.34; p≤0.001); Figure 4A). This correlation translates a floristic similarity among the sites (Sorensen's coefficient of similarity, k>70 %). Some species are weekly represented in the study population; these species are accidental and form a cloud of points ( Figure 4B). In contrast, Gmelina arborea, Lophira lanceolata, Gardenia aqualla, Terminalia mollis, Sarcocephalus latifolius, Strychnos spinosa are the most abundant species in the Gmelina stands.

Floristic Diversity and Structure of Gmelina Stands
Species richness ranged from 2.53±0.05 to 7.74±0.03 with the highest in site 3 (p<0.001  The size-class distributions following the height, the dbh and the circumference were significantly different amongst the study sites (<0.001). In fact, the Figure 5A showed that the floristic structure using the height presented a symmetric bell Shape; the majority of individuals being represented in size-class 8-10 m in height.
Following the size-class diameter, the analysis of Figure 5B showed that Gmelina stands exhibited a classic exponential decay distribution curve (of Shape "L" or "J" if inverted) reflecting the predominance of individuals with small diameters; this structure shows a good regeneration in the stands.
The floristic structure showed an unsymmetrical bell Shape using the circumferences; the most represented class being the size-class 70-80 cm.

Dendrometry Parameters, Carbon Storage and Its Economic Value
The species number, the mean density, the mean height, the mean diameter and biomass varied significantly amongst the sites (Kruskal-Wallis test; p<0.001). In contrast, there was no significant difference of above ground biomass amongst the study sites (Kruskal-Wallis test; p>0.001). The values of these studied parameters are given in Table 5. The lowest values of the studied parameters were found in site 2. Carbon stocks ranged from 22.72±0.36 to 24.63±0.47 t C/ha with the highest value in site 3.