*Where* Ba = <sup>2</sup> /4

Importance Value Index relates how dominant a species is or the share of each species in a tree community.

• Importance Value Index (IVI) [31, 32]

$$\text{IVI} = \text{RD} + \text{RF} + \text{RD}\_o \tag{3}$$

*Where* RF = number of chances of a species occurrence **/** total number of plots x 100

Where, ni = number of individuals of species i, N = total number of all individual trees of all species in the entire community, Bai = basal area of all trees belonging to a particular species i, Ban = basal area of all trees in a habitat, RD = Relative Density, RDo = Relative Dominance, RF = Relative Frequency.

The improved pantropical biomass Equation for tropical rainforest developed by [33] was employed for the estimation of the above-ground biomass content of tree species.

$$\text{AGB}\_{\text{est}}\,(\text{Kg}) = 0.0673 \ge \left(\rho \text{D}^2 \text{H}\right)^{0.976} \tag{4}$$

AGB is aboveground biomass, ρ = wood density, D = Diameter at breast height, H = Height.

Aboveground carbon storage of tree species [7, 34].

$$\mathbf{AGB\_{carbon}} = \mathbf{AGB\_{est}} \,\mathbf{X} \,\mathbf{0.5} \tag{5}$$

The mean aboveground biomass of the three study sites was subjected to One-Way Analysis of Variance (ANOVA) to test for their significance difference and Least Significance Difference (LSD) when ANOVA was significant. Descriptive statistics was employed for the presentation of the results.

## **3. Results and discussion**

#### **3.1 Tree species abundance and distribution**

A total number of 124 individual trees consisting of 52 species, 43 genera and 22 families were identified in the three sites. In Site A, a total of 49 tree stems of 27 species were enumerated. Site B had 38 trees consisting of 18 species and site C had 37

## *Assessment of Diversity, Growth Characteristics and Aboveground Biomass of Tree Species… DOI: http://dx.doi.org/10.5772/intechopen.104982*

trees stem of 20 species. *Albizia zygia* of Mimosaceae family was the most frequently encountered species in sites A and B, while *Milicia excelsa* of Moraceae family was most abundant in site C. The results revealed the extent of urban trees management in the selected green areas. A total of 170 tree saplings belonging to 15 families and 28 species were also encountered in the sampling sites. The higher proportion of saplings is attributed to the regeneration process occurring in the different sites which are sufficient key to maintaining forest continuity and urban greening.

Generally, the percentage of native species was 67% while 31% was exotic. Site A had a higher percentage (81%) of native species than Sites B (72%) and C (40%). while Site C had the highest percentage of exotic species (60%). Studies on urban green areas and forestry have reported variations in the proportion of trees origin where residential areas had more exotic tree species with higher percentages of native trees in cities [6, 19]. The higher percentage of native species in this study could be attributed to cultural and traditional beliefs as well as indigenous knowledge of dwellers on the usefulness of the trees and the incentives that benefited from the native tree species [35]. They also act as mitigation for poverty. This could also be true of urban areas because not all urban dwellers are financially buoyant and satisfied as life in the urban can be more expensive. [36] also signaled this in a field survey within Ibadan metropolis, Oyo State, Southwestern Nigeria. The higher percentage of the trees serve medicinal value (29%), followed by miscellaneous values (23%) such as ornament, clothing, dye-making, paper, tobacco, cosmetics and fencing. Trees used primarily as fuel and charcoal are 9% of the tree species population. Tree species used as a food source (15%) are trees that supply fruits, seeds and condiments, or spices. Their products may also be sold during fruit seasons (**Figure 1**). Trees for shade and support (10%) are used on farmlands for the protection of crops against harsh environmental conditions and as support for stem tubers. Trees used for construction and furniture trees constitute 99% of the total population. Also, primarily 5% of the tree species enumerated help in soil nutrient enhancement and protection.

Diversity indices are a more compact method of comparing the diversity (variety) of species. Shannon-Wiener diversity estimation for the three sites has values within the expected range of 1.5 - 3.5 [10, 28, 37]. This shows that the sites are rich and diverse in tree species. Site A had the highest diversity index value of 3.15, while Site B had the lowest diversity index value of 2.55. Describing species diversity as a single

**Figure 1.** *Ecosystem Services of Tree Species within the study sites.*

**Figure 2.**

*Rényi diversity profile. H = Rényi diversity profile, alpha = diversity parameter, RIP = riparian vegetation, RES = residential site, IND = industrial site.*

value has been reported to compromise much of the detailed structure of a community and that different measures may lead to different rankings among communities [38, 39]. Therefore, a diversity profile that portrays the simultaneous values of a large collection of diversity indices in a single diversity spectrum has been recommended. Rényi diversity profile revealed that the riparian site was the most diverse. Ordering of the sites by Rényi Profile diversity followed; Riparian > Residential > Industrial site (**Figure 2**). Comparing the slope of the three-diversity profile, it is revealed that riparian sites and residential sites have similar and higher species evenness than the industrial site. The more horizontal the shape of the side profile, the higher the species evenness [30, 31]. However, the Riparian and Industrial sites have more connectivity (**Figure 3**) to the number of tree species common to both of them, The nine [9] tree species common to both sites are *Albizia adianthifolia, A. zygia, Brachystegia eurycoma, Ceiba pentandra, Ficus exasperata, Gliricidia sepium, Holarrhena floribunda, Lecaniodiscus cupanioides* and *Margaritaria discoidea.* The agricultural practice was also common within the two sites (Riparian and Industrial sites) in the past. This could be affirmed by the presence of tree species which are usually found within regrowth vegetation previously used for agricultural practice. Species evenness, as a basic component of diversity that measures the equitability of species spread [40], was observed to be highest in site A (0.9681), followed by site C (0.9529) and site B (0.8826). More evenly distribution of trees within the riparian vegetation could be attributed to less competition for space among the tree species and high competition among the tree species within the industrial site as a result of stem proximity which could have also led to competition.

The growth characteristics of the trees in each site varied with growth parameters. Growth variables were estimated as 1.18 m<sup>2</sup> , 5.01 m<sup>2</sup> and 11.06 m<sup>2</sup> (basal area), and 13.49 m<sup>3</sup> , 64.03 m<sup>3</sup> and 122.39 m3 (volume) for sites A, B and C respectively. Site A had the highest percentage of small-sized trees (65.31%) while large and largest-sized trees were absent. Industrial sites had 42.11% of the trees within the small-sized range and only 2.63% within the largest-sized range, while site C had 43.24% and 10.81% within the medium and largest-sized ranges respectively (**Figure 4a**). Generally, a

*Assessment of Diversity, Growth Characteristics and Aboveground Biomass of Tree Species… DOI: http://dx.doi.org/10.5772/intechopen.104982*

**Figure 3.** *Clustering analysis of trees diversity within the three sites.*

#### **Figure 4.**

*(a) Size-class distribution (mean sd) and (b) height-class distribution (mean sd) of trees enumerated.*

higher percentage of the trees (41%) were within the class of 10.3-20.2 m. Site A had no record for trees heights within class 40.3-50.2 m (**Figure 4b**). Site B had only 10.5% of trees within class 1.3-10.2 m. The variation in the DBH and heights contributed to the differences in estimated AGB (**Table 1**). The first four species that contributed to the highest growth characteristics in the three sites are presented in **Table 2**. *C. pentandra* in site A; *Chrysophyllum albidum* in site B and *Brachystegia eurycoma* in site C contributed to the highest basal area, volume and RDo. However, the highest RDe, RF and IVI were contributed to by *A. zygia* in sites A and B, and *M. excelsa* in sites C. The variations explain the management practices for trees preservation and exploitation in the different sites. From the field survey, it was observed that the riparian vegetation is an open area for public use (agriculture, logging and settlement). The residential site is a restricted area to loggers and forest exploiters. The industrial site is also restricted to exploiters but faced with the challenge of encroachment by secret exploiters. Forest resources exploitation for services such as medicine, food and cosmetics may have contributed to reduced trees growth. The depletion of these economically important species populations as habitat degradation and over-exploitation are the two main causative agents [41, 42]. The multipurpose utility of species could also indicate high pressure on them. Moreover, native species with the least abundance could be considered vulnerable in their different habitats. This agrees with a study carried out by [10]


#### **Table 1.**

*Aboveground biomass, size and height of top ten and least ten tree species in the study location.*

that species with low relative density and relative dominance are at the top list of vulnerable species under threat of extinction.

The highest aboveground biomass was recorded in site C, while site A had the least value (**Table 1**). The size class 21-50 cm contributed most to the tree aboveground biomass in site C with a mean value of 5062.29 730.90 Kg C ha<sup>1</sup> (5.06 0.73 t C ha<sup>1</sup> ), while site C recorded the highest aboveground value for >100 cm with a mean value 134531.84 29018.85 Kg C ha<sup>1</sup> 134.53 29.02 t C ha<sup>1</sup> ). Generally, the size class 10-20 cm contributed the least to the aboveground biomass in all the land use (**Table 3**). The contributions of ten species with the highest aboveground biomass were recorded in **Table 1** where *C. albidum* (site C) had the highest AGB. This is attributed to its large size and height. (131 cm and 47 m respectively). *M. excelsa* also had a mean DBH of 88.6 cm and a height of 28.2 m while its carbon content was estimated as 94,237.25 Kg C ha **<sup>1</sup>** (94.24 t C ha <sup>1</sup> ). These are examples of native species with high carbon content. Protective and maintenance measures are necessary for the conservation of native species that could promote the uptake of high carbon in the atmosphere. Moreover, the highest AGB values for site C were contributed to by the highest girth sizes of the trees which also relates to the management practice of tree preservation within the site. Size,


*Assessment of Diversity, Growth Characteristics and Aboveground Biomass of Tree Species… DOI: http://dx.doi.org/10.5772/intechopen.104982*

#### **Table 2.**

*The first four species with highest growth characteristics in the different sites.*


**Table 3.**

*Size-class distribution of tree aboveground biomass (kg C ha<sup>1</sup> ) recorded across the study sites.*

age and species are major factors that influence the amount of carbon that trees can absorb. [43] reported that matured trees can absorb up to 48 lbs. of CO2 per year. [44] reported that at maturity, trees can store approximately 1000 times more than saplings. The result of the Analysis of Variance (ANOVA) performed on the aboveground biomass among the three study sites revealed that there was no significant difference between sites A and B. There was also no significant difference between sites B and C. However, a significant difference occurred between sites A and C at probability level 0.01 (99% confidence interval).
