The study area lies between Latitude 7.30˚N and Longitude 5.13˚E (Federal University of Technology, Akure, Ondo State) within the humid and tropical rainforest region of Nigeria [11] . It is 351 m above sea level and has a land area of about 2303 sq km [11] . Figure 1 shows the climograph of the main meteorological station around the study area. According to the Köppen-Geiger classification, the climate of the study area can be classified into Af (tropical rainforest) climate with significant rainfall for a larger part of the year and a short dry season. The average annual temperature is 26.7˚C, and the average annual rainfall is 1524 mm. The driest month is December, with 19 mm of rainfall. In September, the precipitation reaches its peak, with an average of 405 mm. The warmest month of the year is March, with an average temperature of 28.1˚C. At 25.2˚C on average, July is the coldest month of the year. The relative humidity ranges between 80% and 100% during the rainy season and less than 60% during the dry season period [12] .

Above ground biomass and carbon stocks of trees were estimated using allometric equation from [13] , which are suitable for measurements of trees. Estimation of tree biomass was based on the equation that relates biomass to diameter at breast height (Dbh) and wood density (WD). During the measurements in this study, the height of the chest was 1.30 m above the ground for each tree as used by [14] . In the sample plot, trees with minimum Dbh of 3 cm were identified, numbered and measured. Measurements taken include wood density (ρ) and diameter at breast height (Dbh) using the girth diameter tape. Random sampling method was adopted in establishing twenty sample trees consisting of five (5) species: Terminalia superba, Triplochiton scleroxylon, Clestopholis patens, Treculia rhinopetala and Antiaris africana on an acre of land. All the trees taken into consideration were ≥3 cm in trunk diameter. The goal of the sampling is to properly capture the variation in vegetation so that an unbiased estimate of carbon in the forest can be generated. Three soil samples were taken per tree sample; the following soil analyses were recorded: soil pH, soil organic carbon composition and the soil textural classification. Carbon stocks were determined using both the destructive and non-destructive methods of estimating the aboveground biomass and belowground biomass of forests landscapes.

In order to parameterize local allometric models, a total of twenty trees comprising of five species were sampled destructively. These trees were sampled within 1 acre of the study area. Sample trees were selected in such a way that their size range (height and Dbh) were as much as possible representative of the trees measured in the study area. After felling, diameter (Dbh) and height (H) (equal to the length of the stem) of each individual sample tree were measured. Fresh weight of stems, branches and leaves were determined using a weighing balance with an accuracy of ±0.1 kg. All the leaves of each sample tree were collected manually and the fresh weight was recorded. Fresh leaves samples were taken to the laboratory for determination of dry-weight/fresh-weight ratio (Dw/Fw). The leaf samples were dried at 70˚C ± 2˚C until a constant weight was achieved. Dry mass of the trees and leaves were determined based on the dry weight to fresh weight ratio as shown in Equation (1). Each wood sample was stored in a plastic bag to avoid water loss. Oven drying of wood samples for determination of fresh mass was done at 105˚C to constant weight. The biomass ratio of each wood sample was calculated by the relation between dry-weight (Dw) and fresh-weight (Fw):

B r = D w F w (1)

Root to shoot ratio (R:S) for each tree sample was determined by the relation between dry-weight (Dw) of stem and root:

R : S = D w ofRoot D w ofStem (2)

The belowground root biomass was estimated using trenches digging. Roots remaining in the soil were dug out as best as possible. Roots were divided into smaller parts by a saw and the soil was carefully removed by brushing. The fresh weight of the root system of each individual tree was determined and a sample was taken and oven-dried in the laboratory at 105˚C to constant weight. Woody biomass of standing trees was determined based on diameter at breast height and basic wood density [15] [16] . Allometric model from [13] was used to measure the carbon stocks of aboveground biomass in the young forest plot. This equation is suitable for estimation of carbon stock of above ground-biomass in tropical region. Allometric equation from [17] is a mathematical equation used to estimate the carbon stock of below-ground biomass. To calculate the carbon stock using allometric equation from [13] , two parameters were used; the wood density (ρ) and diameter at breast height (Dbh). The total above ground biomass of each tree was estimated using allometric model from [13] as stated in Equation (3);

AGB ( kg ) = ρ ∗ exp ( − 1.239 + 1.980 ln ( D ) + 0.207 ( ln ( D ) ) 2 − 0.0281 ( ln ( D ) ) 3 ) (3)

AGB = Aboveground biomass (kg),

ρ = Density (gcm⁻³),

D = Dbh (cm).

The maximum diameter at breast height for which this equation is reliable is 148 cm.

To estimate the carbon stock of the below-ground biomass (BGBC), the equation from [17] was used. The equation is stated thus;

BGBC = 0.235 × A GBIfAGBC > 62.5 tC / ha (4)

BGBC = 0.205 × AGBIfAGBC ≤ 62.5 tC / ha (5)

Since woody biomass is 50% carbon [18] , the carbon content of each component for individual tree was estimated by dividing the biomass by two. This method was also adopted by [19] .

Carbon = Biomass 2 ( tonnes ) (6)

The summary of the tree growth variables for the selected species is presented in Table 1. Their mean height, diameter at breast height and density are 441 cm, 4.53 cm and 0.83 g/cm³ respectively. However, Terminalia superba had the highest tree height and Dbh average of 520 cm and 5.8 cm respectively, while Triplochiton scleroxylon had the lowest tree height and Dbh average of 377.5 cm and 3.55 cm respectively. Wood density of selected tree species in the study area ranged from 0.67 g/cm³ for Terminalia superba species to 0.91 g/cm³ for Treculia rhinopetala species (Table 1). This study points out that the forest under consideration is a young forest as the Dbh of the trees measured and used is less than 10 cm i.e. Dbh > 10 cm [20] . However, this study was done for a plot of the study area which is equivalent to 1 hectare. For the distribution of trees by diameter class, the following observations were recorded; 15 trees had diameter class between 0 - 5 cm and 5 trees had diameter class between 5 - 10 cm. The total number of trees measured in this study for the study site was twenty trees. Table 2 demonstrates the distribution of the selected trees in the whole plot based on diameters.

TT: Total of trees; NT₁: Number of trees with diameter class 1 - 5 cm of Dbh; NT₂: Number of trees with diameter class 5 - 10 cm of Dbh.

The soils of the experimental site were predominantly Sandy Clay Loam according to USDA soil textural classification [21] . The descriptive statistics of particle size distribution of the site indicate that the soils generally have an average sand content of 51% ± 7.46% with an average silt content of 18% ± 5.67% and the average clay content is 31% ± 6.04% (Table 3). Meanwhile, values for the OMC at the experimental field are similar to those reported by [22] [23] . There was no significant difference in the mean values of pH down the soil layers but there was a slight decrease in the mean OMC with depth. High organic matter content was also observed in the forest soil (8.02% ± 2.88%) which is attributed to the continuous accumulation of undecayed and partially decomposed plant and animal residues in the surface soil. The presence of high nutrient in the forest land can help to support agro-forestry. Similar results were obtained by [24] in their work on land use effects on soil erodibility and hydraulic conductivity in Akure, Nigeria.

For the entire study area, it was observed that carbon stock is higher in above-ground biomass with 0.00407 t C compared to below-ground biomass with 0.00167 t C (Figure 2). From this total, the aboveground and belowground carbon stock represented 71% and 29% respectively. It was determined that in this tropical forest, the total carbon stock (AGBC + BGBC) was 0.00573 t C (Tables 4-6). Terminalia superba (0.00698 t C) has the highest carbon stocks while the Triplochiton scleroxylon contained the least stock of carbon (0.00212 t C). Figure 3 shows the average of AGBC and BGBC of selected species in the study area.

Regression analysis was carried out to generate three different models for the aboveground biomass with different input parameters (Figure 4). Equations (7)-(9) present the different models. An equation that relates AGB and BGB was also derived (Equation (10)).

1) New model I with Dbh, WD, and H as input

AGB = 10.26 + 0.00617 ( H ) − 5.731 ( Dbh ) − 9.39 ( ρ )                     + 0.7562 ( Dbh 2 ) − 0.001409 ( H × Dbh ) + 3.994 ( Dbh × ρ ) (7)

2) New model II with Dbh and WD as input

AGB = 12.946 − 6.074 ( Dbh ) − 10.88 ( ρ ) + 0.69103 ( Dbh 2 ) + 4.348 ( Dbh × ρ ) (8)

3) New model III with only Dbh as input

AGB = 5.178 − 3.012 ( Dbh ) + 0.7427 ( Dbh 2 ) (9)

BGB = 0.32 AGB (10)