Amenity Trees Diversity in Selected Tertiary Institutions within Port Harcourt Metropolis , Rivers State , Nigeria

Trees are a central component of most urban communities, providing diverse benefits such as improving air quality, increasing aesthetic appeal, preventing erosion and providing habitat for wildlife. However, there is inadequate information on the distribution and diversity of these trees within our academic environments. This study investigates the distribution, frequency and species diversity of amenity trees within the main campuses of tertiary institutions in River State, Nigeria. The selected tertiary institutions are University of Port-Harcourt (Institution A) and Rivers State University (Institution B). Five (5) major roads in each campus were randomly selected, and 200m along each road was selected as sample areas. Ten (10) meter was marked from the edge of the road, and complete inventory was taken of all trees within the marked sample area. Diameter at breast height (DBH), crown diameter and tree height were measured. Shannon-Weiner and Simpson diversity index were used to calculate amenity trees diversity, while Margalef specie richness was used to calculate the species index. A total of 539 trees were surveyed on both campuses, comprising 26 different species and 10 families. Casuarina equisetifolia had the highest frequency with 92 trees in B. Simpson diversity index showed higher diversity (D = 0.78) in B amenity trees than A (D = 0.81). Shannon-Weiner species evenness was 0.71 and 0.76 for Institutions A and B respectively. Margalef specie richness index were 2.90 for A and 1.40 for B. This study provides baseline information for ecosystem management of urban forest tree species within campuses. For sustainable management of trees on campuses, frequent inventory and survey should be conducted to establish their abundance, distribution and diversity. DOI: https://dx.doi.org/10.4314/jasem.v24i12.25 Copyright: Copyright © 2020 Oladele et al. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 10 October 2020; Revised: 30 November 2020; Accepted: 20 December 2020

Trees are the unsung heroes of our urban environment whose roles cannot be over emphasized. They provide countless services such as conservation of species, soil erosion control and flora and fauna habitat preservation (Armenteras et al., 2009). According to Ezeabasili et al., (2015) urban forestry involves the planting and maintaining of a group of trees within a built environment. These built environments could be cities, residential towns, institutional areas such as schools, churches, mosques etc. Trees in academic institutions located in cities are part of urban forests (Raji and Babalola, 2018). University of Port Harcourt is a typical urban environment. Over the years, urban environment has experienced tremendous increase in human population which has led to different anthropogenic disturbance on the trees (Omoro et al., 2010). The University being an academic area is highly populated and is an example of a typical urban environment. The report of United-Nations (2006) stated that over the next four decades all of the world's population growth will occur in urban areas and over 60 percent of the global population will reside in urban areas. Kumar et al., (2006) started that management interventions is required to maintain the overall biodiversity, productivity, and sustainability. All over the world, overexploitation has been noted as a major environmental and economic problem resulting in the rapid loss of tree diversity (Mani and Parthasarathy, 2006). Suratman (2012), stated that information on composition, diversity of tree species and species-rich communities is of key importance in the planning and implementation of biodiversity conservation efforts. Diversity is an indicator that allows appreciating links between the richness and the abundance of individuals' trees; it reflects the degree of heterogeneity or stability of vegetation. Biodiversity conservation depends basically on the knowledge of the structure, species richness, and the ecological characteristics of vegetation. Quantitative information of these variables is important in understanding the status of the forest estate, decision-making, planning and implementation of conservation strategy (Suratman, 2012). Ogwu et al., (2016) reported that sustainable development advocates that humans and biodiversity coexist side by side. A university campus should be promoted as a model environment for sustainable development. Knowledge on tree species diversity and distribution would serve as baseline information to know the status of the tree species in the study area; which is basic in understanding regeneration processes, such as tree growth, tree mortality, under-storey development, and the spread of disturbances (Francisco et al., 2017). To protect and prevent trees decline in our urban environment, it is essential to examine the current status of species diversity, composition and abundance of tree species as this will provide guidance for management and conservation. This study is aimed at assessing tree species diversity and distribution within two tertiary institutions in Port Harcourt, Rivers State, Nigeria with the view to providing baseline information useful to developing conservation strategy. Data collection: Data for this study were collected through sampling the avenue roads in the main campuses of the universities. 200 meters each of five major roads in each campus was selected through a random sampling and data were collected from the amenity trees in the selected roads. The service of Taxonomist was employed in the identification of the trees species. Each of the trees found in the campuses were identified to species level with their scientific name, local name and family. Tree growth parameters were measured on the trees. The measured parameter includes tree height, diameter at breast height and crown diameter. Stem diameters, crown diameter and total height were measured using diameter tape, meter tape and Haga altimeter respectively.

MATERIALS AND METHOD
Data analysis: Plant population and species diversity were analyzed using the following species diversity indices: Shannon-Weiner Index of Diversity (H') -Species diversity within the study area were analyzed using the diversity index of Shannon and Weaver (1949), the index varies depending on the number of species present. It is higher when there are more species, indicating greater diversity. Shannon-Weiner Index of diversity (H') was calculated following Kent and Coker (1992) and Magurran (2004): Where: = S = number of individuals of one species; N = total number of all individuals in the site and In = logarithm to base e Simpson index is a measure of species dominance. As biodiversity increases, the Simpson index decreases. Therefore to get a clear picture of species dominance, simpson index is given as; (Simpson, 1949) Where: n is the total number of organisms of a particular species and N is the total number of individual of all species.
Tree species richness was analyzed using Margalef species richness index (d), which is used as a simple measure of species richness according to Margalef (1958).
Where S = total number of species; N = total number of individuals in the site and In = natural logarithm.
Relative Density (RD), is used to assess species relative distribution of the town using; Where ni = the number of species and N = the total number of trees in the town.  (table 2). Trees species identified in the survey were higher for A (17) than B (9). However, Institution B had a total of 292 individual trees while Institution A had 247 trees.   (Table 3).   Floristic composition of amenity trees in the study area: The taxonomy, diversity and distribution of tree growth variables for amenity trees in the study area have been documented. The urban trees play a vital part in the health and welfare of people who lives and works in the environment thereby promoting sustainable development. The study reveals that the sampled university campuses possess different species of trees. This finding is in agreement with the report of Babalola and Raji (2016) who also observed diverse tree species in academic area. The abundance and diversity of trees in the academic area may be as a result of the need to provide aesthetic environment and beautiful landscape for the University as well as provision of shade for student and staff. Generally trees are abundant in university environment because of the benefits derived from urban trees such as increased property values (Panduro and Veie, 2013;Raji, 2016 andRosato et al., 2017), control rainfall runoff and flooding (Livesley et al., 2016), improve wellbeing (Hall and Hodges, 2011;Taylor et al., 2001), purify the air and provide of shade (Agbelade et al., 2016) and are desired for beautification (Bello and Yacim, 2014). Mangifera indica, Delonix regia, Elaeis guineensis, Terminalia spp. e.t.c. were some of the species found in the study area. Aigbokhan (2014) and Ogwu et al., (2016) also found similar species in their study area due to the services such trees provides. However, the flora composition of the University Campuses especially University of Port Harcourt are under threat from infrastructural development and other anthropogenic activities. Similar findings have been reported by Wardle et al., (2004) and Ogwu et al., (2016). Chazdon, (2003), reported that as human population continue to grow, land use intensity increases and the negative effects of deforestation are likely to worsen hence sustainable management techniques are required to maintain the biodiversity and productivity of ecosystems (Reddy and Elanchezhian, 2008).

Diversity of amenity trees in Study areas:
In this study biodiversity index was used to assess the level of biodiversity in the study area. According to Agbelade et al., (2016) biodiversity indices are generated to bring the diversity and abundance of species in different habitats to similar scale for comparison and the higher the value, the greater the species richness. Kacholi (2014) reports that understanding of tree species diversity, composition, and structure is very important in assessing sustainability of forest, species conservation, and ecosystems management.  Ogwu et al., (2016) for trees in University of Benin. The biodiversity indices obtained by Raji and Babalola (2018) for trees in University of Ilorin is similar with the finding of this study. Tree diversity is vital to tropical Forest biodiversity, because tree provide homes and resources to a wide variety of plant and animal species.
Growth parameters of amenity trees: The measurement and prediction of dbh, height and crown diameter of urban tree species will enable arborists, researchers, and urban forest managers to model costs and benefits, analyze alternative management scenarios, and determine the best management practices for sustainable urban forests (McPherson et al., 2005). This study describes the amenity tree composition and distribution of the selected universities, giving dbh, height and crown diameter of tree species community. Tree growth variables were generally higher for Institution A compared to B. The growth of trees in the studied universities is an indication of the level of conservation of tree species which urban forest can offer. This growth may be as a result of control and strict prohibition of tree felling within the campuses.
Conclusion: Basic information on amenity trees in University of Port Harcourt and River State University which can be used for the development of species databases in both institutions has been revealed. The research conducted a species diversity analysis proving the study area as diverse with medicinal and socio-economically importance species. Necessary action should be undertaken towards the sustainability of these trees and conservation of urban forest at large. Frequent inventory should be conducted for trees on campus to note their abundance, distribution and diversity.