**2. Methodology**

#### **2.1 Study area**

The research was conducted in three protected forests (Omo biosphere reserve (BR), Akure strict nature reserve (SNR) and Okomu national park (NP)), in May 2021, located in the southern part of Nigeria. Strict nature reserve (SNR) is prominent among the methods for *in-situ* conservation of biodiversity in Nigeria in particular and the world in general [1]. SNRs are created to protect representative samples of natural ecosystems for preservation of biodiversity and ecological processes, scientific study, environmental monitoring, education and the maintenance of genetic resources in a dynamic and evolutionary state [12]. Biosphere reserve (BR) is areas of terrestrial and coastal ecosystems promoting solutions to reconcile the conservation of biodiversity with its sustainable use. They are internationally recognized, nominated by national governments and remain under the sovereign jurisdiction of the states where they are located. An example of biosphere reserve in Nigeria is the Omo biosphere reserve (BR). The Omo biosphere reserve is located in Ijebu area of Ogun State, Southwest, Nigeria. Established in 1977, roughly 6000 people live within the biosphere reserve boundaries, which cover a total area of 130,600 hectares. There are several national parks established by the Federal Government (FG) of Nigeria to serve as tourist attraction centres and conservation of plants and animals. These national parks consist of diverse species of fauna and flora that contribute to the ecosystem development and conservation status. Nigeria national park service is the body that oversees and responsible for preserving, enhancing, protecting and managing vegetation and wild animals in the national parks of Nigeria. Nigeria national park service works closely with the Nigerian tourism development corporation in the handling and management of the national parks. These national parks cover a total land area of approximately 20,156 km2 , which is about 3% of Nigeria's total land area. Okomu national park covers a land area of 200 hectares in Edo State, about 60 km Northwest of Benin City, Nigeria. The area is bounded by latitudes 6.08° and 6.30° N and longitudes 5.01° and 5.27° E. The climate of the region is characterized by a double maximal year-round rainfall pattern with a mean annual rainfall of about 2200 mm, which peaks between May and October and a mean monthly temperature of 27°C. Tropical hardwood tree species in the area include *Celtis zenkeri,Triplochiton scleroxylon, Pycnanthus angolensis*, *Alstonia congoensis, Khaya ivorensis* (African mahogany) and *Lovoa trichilioides* (African walnut).

#### **2.2 Method of data collection**

These three (3) protected forests were selected (Omo BR, Akure SNR and Okomu NP) for this study. In each protected forest, biodiversity and tree growth data were collected from two lines transects of 1000 m each in length laid approximately at the

centre of the protected forest [3, 6]. The two line transects were separated by a distance of at least 1000 m. Four temporary sample plots of 25 m � 25 m were laid at alternate sides along each transect at every 250 m interval. Thus, there were 4 sample plots per transect, 8 per protected forest and twenty-four (24) for this study. Within each plot, all living trees with Dbh ≥ 10 cm were identified, their Db, Dbh measured with girth tape and Dm, Dt and total heights measured with Spiegel Relaskop [1, 6].

#### **2.3 Data analysis for species diversity**

Species relative density (RD) used to determine species relative distribution was computed using:

$$RD = \left(\frac{\text{ni}}{\text{N}}\right) \times 100\tag{1}$$

where: RD (%) = species relative density; ni = number of individuals of species i and N = total number of all individual trees of all species in the entire forest reserve. Species relative dominance (RDo) was estimated using:

$$RD\_0 = \left(\frac{\sum Ba\_i \times 100}{\sum Ba\_n}\right) \tag{2}$$

where: Bai = basal area of all trees belonging to a particular species *i* and Ban = basal area of all individual trees.

Importance value index (IVI) of each species was computed with the relationship:

$$\text{IVI} = \left(\frac{\text{RD} + \text{RD}\_0}{2}\right) \tag{3}$$

Species diversity index (H<sup>0</sup> ) was computed using the Shannon-Wiener diversity index below:

$$\mathcal{H}' = -\sum\_{i=1}^{s} \text{Pi} \, \text{In}(\text{Pi}) \tag{4}$$

where: H<sup>0</sup> = Shannon-Wiener diversity index; S = total number of species in the protected forest; pi = proportion of S made up of the ith species and ln = natural logarithm. Simpson Concentration index

$$\mathbf{D} = \sum \left(\frac{\text{ni}}{\text{Ni}}\right)^2 = \mathbf{D} = \frac{1}{\sum\_{i=1}^s \mathbf{P}\_i^2} \tag{5}$$

In the Simpson index, p is the proportion (n/N) of individuals of one particular species found (n) divided by the total number of individuals found (N), Σ is still the sum of the calculations and (S) is the number of species.

Shannon's maximum diversity index was calculated using:

$$\mathbf{H}\_{\text{Max}} = \text{Ln } (\mathbf{S}) \tag{6}$$

*Phyto-Sociological Attributes, Ecosystem Services and Conservation Dynamics of Three… DOI: http://dx.doi.org/10.5772/intechopen.106433*

Where: Hmax = Shannon's maximum diversity index and S = total number of species in the protected forest.

Species evenness in each plot was determined using Shannon's equitability (*EH*), which was obtained using:

$$\mathbf{E}\_{\rm H} = \frac{\mathbf{H}'}{\mathbf{H}\_{\rm Max}} = \frac{\sum\_{i=1}^{s} \mathbf{P}\_{i} \text{Ln } \mathbf{P}\_{i}}{\text{Ln } (\rm S)} \tag{7}$$

#### **2.4 Forest conservation structure**

The following computations were computed for forest structure analyses. The basal area of each tree in the protected forest was calculated using

$$\text{BA} = \frac{\pi D^2}{4} \tag{8}$$

Where: BA = Basal area (m<sup>2</sup> ), D = Diameter at breast height (cm) and л = pie (3.142). The total basal area for the plot was obtained by adding basal area of all trees in the forest reserve. Volume of individual trees was estimated using

$$\text{V} = \text{\textpi h} \frac{Db^2 + 4(Dm^2) + Dt^2}{24} \tag{9}$$

where: V = Tree volume (m<sup>3</sup> ), π = 3.142, h = tree height (m) measurement and Db, Dm and Dt = tree cross-sectional area at the base, at the middle and top of merchantable height, respectively. The total volume for the forest reserve was obtained by adding all individual trees volume computed.

#### **2.5 Biomass and carbon stock**

In determining the total carbon (TC) stocks, estimation of AGB and BGB were computed. Biomass expansion factor (BEF) of 1.74 was used to estimate tree aboveground biomass for tropical rainforest [13], multiple by volume over bark (m<sup>3</sup> /ha) and wood density (kg/m<sup>3</sup> ).

$$\text{Above} - \text{Ground Bonas} \ (\text{AGB}) = \text{BEF} \times \text{VOB} \times \text{WD} \tag{10}$$

Where, BEF = Biomass expansion factor; VOB = Volume over bark (m<sup>3</sup> ) and WD = Wood density (kg�<sup>2</sup> ). Wood density for tree species was acquired from Global Wood Density Database. Arithmetic mean of (0.60 gcm3) for a tropical African forest was used for species that were not found in the database following [14]. The carbon stock of the protected forests was determined by a fraction of 50% of biomass.

$$\text{AGC} = \text{AGB} \times \text{0.5} \tag{11}$$

Thus, above-ground carbon (AGC) was calculated as a conversion factor of 0.5 multiplied by AGB.

$$\mathbf{BGB} = \mathbf{AGB} \times \mathbf{0.2} \tag{12}$$

where below-ground biomass was computed as 20% of AGB following MacDicken [15]; IPCC [16], using a synthesis of global data and a conservative ratio shoot-to-root biomass of 5:1 [17].

$$\text{TCS} = \text{AGC} + \text{BGC} \tag{13}$$

The estimation of carbon content in BGC is the same as that of AGC Eq. 12. Total carbon storage (TCS ton ha�<sup>1</sup> ) stock was calculated by summing up the carbon stock of AGC and BGC following [18].
