**1. Introduction**

Urban green areas (green spaces) encompass all vegetation found in the urban environment, including blue spaces such as lakes or rivers and their adjacent greens and economic values [1]. The continuous changes in the natural ecosystem from disturbances, especially human activities, have been a global concern for decades. This has not only resulted in continuous reductions in its volume but has also led to a reduction in the services provided, such as water protection, flood control, air filtration and carbon

sequestration [2, 3]. Also, this has positioned urban green spaces as a better option for tree species conservation, since increasing prosperity and residential density require more green infrastructure (green areas) and trees to serve amenity functions [4]. In addition to the various critical functions that urban vegetation support in the ecosystem [5–7], the lives of urban residents depend on the availability and lushness of green areas as well as the abundance and diversity of urban trees [8]. Trees are a valuable asset to the urban community [9] which occur in many different sets of species, genera, orders and families with a variety of growth forms, shapes, vegetative and reproductive characteristics, leading to their great range of diversity [10]. Despite several documentations on the great potential for preserving high tree species diversity, several cities are still experiencing loss of green areas and trees diversity, especially in developing countries as a result of rapid economic and urban growth rate [11, 12]. This results in land cover conversion into other land-use forms which affects urban forest cover as well as the functions and services they provided. [13] noted that urban areas are responsible for more than 70% of the anthropogenic release of carbon dioxide and 76% of wood used for industrial purposes. Increasing urban green biomass contributes to an increase in atmospheric carbon sequestration in the urban's terrestrial biosphere. [14] ascertained that the largest single source of CO2 emissions comes from fossil fuel combustion, followed by industry, residential and commercial activities. Therefore, the huge release of emissions requires urgent mitigation measures.

However, the long-term ecosystem service provision by urban trees is dependent on diversity. Urban centers can also support the great number of trees diversity [15]. The connectivity of the urban forest patches in the urban landscape may be critical for maintaining the species population and diversity [16]. Trees' population and diversity can greatly be increased through human direct or indirect activities in the urban environment. These aids increase in connectivity and tree populations by planting more than one type of species in streets, parking lots, gardens and other areas within the urban centers. Species diversity is necessary for an adaptable urban ecosystem [17] which promotes resistance and resilience to disturbance, such as pests, fire outbreaks and destructions from various human activities. Therefore, the focus should be on increasing diversity to ensure ecosystem stability with more emphasis on the conservation of native species. The introduction of exotic species in urban areas can spread to neighboring rural areas as invasive plants, compete with and threaten the existence of native species and affect the ecosystem negatively by altering its processes [18, 19]. It has been predicted that current species could be at risk of extinction in the nearest future [20]. Therefore, the first step towards ensuring sustainable management of urban biodiversity is by quantifying the green infrastructure. This is done by conducting a tree inventory which may serve as a tool for the establishment of a baseline for setting management objectives by determining the resources present and where they are [21]. However, insufficient information about the status of green infrastructure in most urban cities in Nigeria remains a serious research gap, therefore this study was designed to assess the abundance and diversity of these trees within the three urban green areas of Osogbo.

## **2. Materials and method**

#### **2.1 Study area**

The study was conducted in Osogbo, the capital city of Osun state due to its commercial importance. Osogbo covers a total area of 144 km<sup>2</sup> and is located

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

within latitudes 7<sup>0</sup> 43´ N to 70 56´ N and longitudes 4<sup>0</sup> 33<sup>0</sup> E to 4<sup>0</sup> 350 E, with an elevation beyond 502 m above sea level [22]. The population size of Osogbo for 1991, 2006 and projected population for 2016 were 187,219, 288,455 and 395,500 respectively [23]. The climate of the study area is a tropical hinterland type with a mean annual rainfall of 1200 mm to 1400 mm and a mean annual temperature of about 27°C [24]. Osogbo falls within the lowland tropical rainforest vegetation characterized by multiple canopies and lianas, most of which had since given way to secondary forest and derived savannah [25] due to intensive cultivation and bush burning for several years. Tender forest trees become replaced with fire-tolerant species and vegetation changes in features within short distances.

#### **2.2 Woody trees sampling**

Purposive selection of the study sites was done based on the urban forest cover and study objectives where three green spaces were identified: Riparian vegetation (site A), Industrial site (site B) and Residential site (site C) after the reconnaissance survey inside the urban center. Ten sample plots of 25 m x 25 m each were selected at random at the three sites. Tree species (≥ DBH 10 cm, h ≥ 1.3 m) within the sampled plots were identified. The Diameter at Breast Height and height (≥ DBH 10 cm, h ≥ 1.3 m) of each tree were also measured. Saplings of woody tree species (≥1 cm < 10 cm) were also identified and recorded.

#### **2.3 Data analysis**

The tree data collected were analyzed to determine the following parameters:

#### *2.3.1 Biodiversity assessment*

Shannon-Weiner Diversity Index, Species evenness and Rényi diversity ordering technique were estimated using the R-statistical package. The Shannon -Weiner Index is well recognized for measuring wood species abundance and richness [26, 27]. It measures the average probability of where a specie will belong when randomly predicted individually [28] and this is considered in the study to assess specie abundance and richness. This was incorporated in the Rényi Diversity profile. Rényi Diversity Profile which is used to order the diversity of tree species within the different physiognomy [29, 30] was used in this study. Studies have shown that only one diversity index may not be sufficient enough to provide information on ordering sites from high to low. Rényi profile incorporated species richness, evenness, Shannon's index, Simpson's index and Berger-Parker's index in a simple index. R statistical package was employed for accurate and effective computation [31]. Species Relative Density (RD) was determined to assess species relative distributions across the different habitats.

Species relative density (RD) for each tree species was determined by:

$$RD\ (\%) = (N/ni) \ge 100\tag{1}$$

*ni* = number of individual species; *N* = Total number of species in the entire community.

#### *2.3.2 Growth characteristics and aboveground biomass of sampled urban trees*

Growth characteristics are important in the determination of the health status and biomass of trees and were measured using growth parameters such as girth size, basal area and volume. Species Relative Dominance (RDo %) was used to assess the relative space occupancy and management practice of forested lands.

• Species Relative Dominance (RDo %) [10]

RDo ¼ Total basal cover of individual species þ Total basal cover of all species � 100 (2)
