Assessment of Tree Species Diversity in the University of Lagos , Akoka , Lagos

This study was aimed at assessing and providing management options for tree species growing on Akoka Campus of University of Lagos, Southern-Western Nigeria by collecting data from four (4) randomly selected 50m x 50m plots in the study area. The assessment was carried out through extensive field survey with the inventory of all tree species, including their mean diameters at breast height (DBH), heights (H). A total of 67 woody tree species within 27 families were represented in the study area. The family Fabaceae had the highest frequency (14 species) representing 20.59% of total species enumerated in all the plots followed by Moraceae which consists of 7 species (10.29%). The result revealed high values for tree species diversity and also provided necessary information such as tree height and diameter at breast height, which will be used in proper management and maintenance of the tree species numbered; hence, proper management and sustainability of the tree species can be achieved. DOI: https://dx.doi.org/10.4314/jasem.v22i1.3 Copyright: Copyright © 2017 Ogunyebi 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 11 November 2017; received in revised form 14 December 2017; accepted 30 December 2017

Nigeria vegetation is one of the most endowed in Africa, as almost all the vegetation types that exists in other African countries are found widely distributed in different geopolitical zones of the country, this is favoured by the variations in climate and geographic features, which harbours about 7895 species of plant; this makes her one of the richest countries in the continent in terms of biodiversity Adeyemi and Ogundipe, 2012;Pelemo et al., (2011).
However, these habitats are under threat from advancing civilization and other unsustainable human activities, the attitude of the populace towards conservation is relatively poor; thereby resulting to inevitable loss of genetic resources at all levels.Ideally, conservation of biodiversity is supposed to be an intrinsic responsibility for all mankind according to IUCN, but this is far from the case, as the rate of destructive anthropogenic activities on the floras and biodiversity at large escalates daily with nearly 90% of forest in Nigeria cleared (Kabiru, 2008).In 2010 Nigeria had 9 million hectares of forest, 336,000 hectares of which were primary forest (FAO, 2010).But the continual existence of this forest is in doubt, as several authors such as Batta et al., 2013;Pelemo et al., 2011;Ladipo (2010), have lamented the rate of deforestation in the country which is estimated at 3.5% per year, translating to a loss of 350,000-400,000 ha of forest land per year and the entire Nigeria's forest land area now is about 10%, which is well below FAO's recommended national minimum of 25%.Perhaps this is threatening about 484 plant species in 112 families of 7895 plant species estimated in the country with extinction (Pelemo et al., 2011).Unarguably, one of the persistent problems associated with deforestation is the selective exploitation of some targeted species for economic, social and spiritual paraphernalia, and trees are mostly targeted (Alamu and Agbeja, 2011).
In Nigeria, there are about 560 species of trees (Ihenyen et al., 2009).However continued existence of these trees species is in jeopardy; because deforestation, logging and other various forms of unsustainable activities have drastically increased in recent times, thereby posing appreciable risk of local extinction to some species.Indeed, the tree species growing in this study area, situated in the most commercial and urbanized city of Nigeria and accommodates about 10% of the entire population of the country (Pelemo et al., 2011), are however not spared from the aforementioned threats.Rapid increase in population has led to the development of several infrastructural facilities so as to provide comfort to insatiable human's wants; which of course has led to direct and indirect destruction of almost all the ancestral vegetation in and within the study area and its proximate vegetation.Urbanization and other commercial activities are socioeconomic problems, and are too difficult to be controlled.Attempt to list the tree species that exist in this area has been made, but only skeletal attempt has been made so far Shonubi and Okusanya, 2007;Adekanmbi and Ogundipe (2009).This research is aimed at Ogunyebi, AL; Abiodun, MJ; Oludoye, OO; Omoyajowo, KO; Fayenuwo, GA; Fingesi, TS enumerating and identifying different tree species in each sampling point pertaining to the study in order to provide a detailed assessment of the remnant plants species, so as to decide on adequate management options and conservation measures for sustainability.

MATERIALS AND METHODS
Study Area: The study was conducted in Akoka campus of University of Lagos, Yaba, Lagos, South-Western Nigeria (Fig. 1).The area which is largely surrounded by the scenic view of the Lagos lagoon comprises a total of 802 acres (3.25km 2 ) of land.It is located on longitude 3°24'E and latitude 6°30'N and on elevation of 40-90m, which makes flooding difficult.The vegetation in this area is half cleared and developed, and the remainder is represented by mangrove vegetation and most of the species have diminished in number and density (Nodza et al., 2014).It has an undulating terrain, half of which represents buildings, with various fresh water channels and creeks passing across at different location of this area.A large area of mangrove swamps, roughly 50%, dominates the vegetation.In the north and southeast lies the brackish lagoon water which supports a typical terrestrial habitat, and experiences less human disturbance while in the south and southwest lays the fresh water, where the soil is highly rich and supports a rich flora which is highly favoured by the climate type much disturbed by human activities.A rich lush of grasses species such as Panicum subalbidum, Panicum maximum, Andropogon spp, Sporobolus spp, Andropogon spp; herbaceous plants such as Chromolaena odorata, Ipomoea aquatic, Combretum spp, Asystasia gangentica, Typha australis, and Sedges dominated by Kyllinga erecta, Cyperus esculentus and Cyperus javanicus, amongst others, were well represented in each sampling plot.
Data Collection: A Global Position System (GPS) was used during the sampling period.The study area was divided into four sampling plots.In each of the sampling plot, trees were enumerated in a 50x50m plot, samples were collected and the tree species enumerated were identified.
The heights of the various tree species encountered were measured using Haga Altimeter and diameter at breast height (DBH) was taken using a Diameter Tape at 1.3meters above the ground level.
Haga Altimeter was used to measure the tree heights within a fixed distance to the tree by the trigonometric principle and slope in percentage.A suitable scale was selected by rotating the adjustment disk at the front of the device and a reference tape was installed at the tree to determine the distance optically.Consequently, the following measurements were taken; (i).The corresponding height scale was selected turning the adjustment disk; in this case 15m scale was used (i.e.standing at 15m away from the tree to be measured).(ii).the bottom of the tree was sighted through the Haga Altimeter and then locked (the pointer needle) for the value gotten to be recorded.The same procedure was carried out for the crown of the tree.(iii).the difference between the value obtained for the crown of the tree and that of the bottom of the tree was calculated to determine the tree height.

Diameter Tape (D-TAPE):
A diameter tape (D-tape) was used to measure diameter of the trees at breast height (DBH).The DBH was measured at a fixed height of 1.3m (130 cm) above the ground level.The DBH was measured up high the tree (at 1.3m) to avoid measuring a tree's butt swell.Thus the height and diameter values were used to determine the volume of the trees.

Determination of DBH:
The diameter at breast height of the trees was determined using a Diameter Tape to take the diameter of the trees at 1.3meters above ground level.
Data Analysis: The data were analysed using the Shannon-Wiener diversity index.The Shannon-Wiener Diversity Index, which accounts for species richness and how the species are distributed, is derived from the relation; R '= -Σ • ( ) =1 Where: '=Shannon-Weiner index; R=Number of species; =Proportion of individuals or abundance of the th species expressed as a proportion of the total number of individuals of all species ( = log base10).
Also, the mean of the diameters at breast height (DBH) and total height of the different species of trees were taken.The analysis of the frequency of the individual families was done by plotting a chart of the frequency of each family against the total number of families encountered in the study area.

RESULTS AND DISCUSSIONS
A total of 67 woody tree species with varying heights and diameter at breast height (all belonging to 57 genera and 26 families) were enumerated (Table 1).The number of tree species encountered in a sample survey was adopted as a surrogate for the actual species richness in this study (Adekunle et al., 2013).

Polyalthia longifolia Annonaceae
The low annual precipitation rate and climate of the study area may have contributed to average tree growth rates and low to medium tree basal area.The diversity index ( ') for the tree species was calculated to be 4.204, for all the species enumerated, and this is high, representing a diverse and equally distributed community.Thus this statistics also illustrate the great numbers of exotic plants that have become established in native habitats (Nodza et al., 2014).In order to measure the tree species diversity of a forest, there is need to know the total area of the forest and the area occupied by each species, this information was made available from the inventory data and the actual survey carried out (Fig. 2).The study provides a baseline for the management of protected areas in developing countries and it shows the potential of in situ method in conservation.Forest habitats play a central role in the functioning of the biosphere, as they are the origin of many cultivated plants and animals.To examine the efficacy of nature reserves as a means of biodiversity conservation, ecologist can use such information from this study on rare and common tree species alike to help manage habitat as well as provide cultural resource values of these trees; the qualitative characters related with density, dominance and diversity of these trees could well act as indicators of changes and susceptibility to anthropogenic stressors among various vegetation categories and their formations, which could be further interpreted as distinct flora habitats.
The frequency distribution of tree species suggested that most of them had low frequency as would be expected of typical species-abundance distributions.Tree structure and diversity could play an important role in climate regulation as both could influence forest biomass production and hence determine the forest' capacity for carbon storage.Findings in this study could help rationalize the need for an ecologically sound fallow period; this will ensure renewal of at least some tree species and some fauna habitat element (Kumar et al., 2006).
During the survey, it was observed that some trees needed to be pruned because they posed potential risk/threat to the people and properties on the University community.Some species enumerated showed signs of decay, disease and deformity.If these species were properly managed and maintained, they would have really exhibited their aesthetic values.

Conclusion:
Trees have existed within the University of Lagos campus since its establishment; afforestation has also taken place due to aesthetic values and academic purpose.Overtime, lack of proper management caused trees to overgrow, while some are dying, thereby causing potential risk to both property and life within the University community.Repairs of these damage cost a lot; in view of these, proper tree management and maintenance options should be exercised to ensure sustainability of these trees and also mitigate future hazards.

Fig 1 :
Fig 1: Map showing study area; fresh and mangrove swamp of University of Lagos

Fig 2 :
Fig 2: Families of Tree Species and the Frequency at which they occur within the Study Area

Table 1 :
Mean Diameter at Breast Height (DBH) and Total Height of Tree Species enumerated.

Table 2 :
Family of each tree species encountered in the study area

Table 3 :
Frequency of each family enumerated in the study area