Issue 4 (204)/2025

ALSE and ACS Style
Ogunade, J.; Onilude, Q.; Onyekwelu, J.; Ogunwande, O.; Yaduma, Z. Institutional contribution to urban biodiversity conservation, productivity, and carbon sequestration: a case study of the Forestry Research Institute of Nigeria. Journal of Applied Life Sciences and Environment 2025, 58 (4), 577-599.
https://doi.org/10.46909/alse-583194

AMA Style
Ogunade J, Onilude Q, Onyekwelu J, Ogunwande O, Yaduma Z. Institutional contribution to urban biodiversity conservation, productivity, and carbon sequestration: a case study of the Forestry Research Institute of Nigeria. Journal of Applied Life Sciences and Environment. 2025; 58 (4): 577-599. 
https://doi.org/10.46909/alse-583194

Chicago/Turabian Style
Ogunade, Joshua, Quadri Onilude1, Jonathan Onyekwelu,Olusola Ogunwande and Zacharia Yaduma. 2025. “Institutional contribution to urban biodiversity conservation, productivity, and carbon sequestration: a case study of the Forestry Research Institute of Nigeria” Journal of Applied Life Sciences and Environment 58, no. 4: 577-599.
https://doi.org/10.46909/alse-583194

Institutional contribution to urban biodiversity conservation, productivity, and carbon sequestration: a case study of the Forestry Research Institute of Nigeria

Joshua OGUNADE¹, Quadri ONILUDE^1*, Jonathan ONYEKWELU², Olusola OGUNWANDE¹ and Zacharia YADUMA¹

¹Forestry Research Institute of Nigeria [FRIN], Jericho Hill, 200116, Ibadan, Nigeria;

²Federal University of Technology Akure [FUTA], P.M.B. 704, Akure, Nigeria; 

*Correspondence: omoonilu@gmail.com 

Received: Jul. 18, 2025. Revised: Nov. 06, 2025. Accepted: Nov. 12, 2025. Published online: Jan. 12, 2026

ABSTRACT. Urban trees within developed areas provide essential ecosystem services that include carbon sequestration. But growing urban populations put pressure on vegetated urban ecosystem services and biodiversity. This study examined tree diversity, biomass, and carbon storage within the developed areas of the Forestry Research Institute of Nigeria (FRIN). A complete enumeration of 326 trees distributed among 57 tree species and 29 families was conducted. Biodiversity indices were computed using standard procedures, while biomass and carbon storage were estimated using a non-destructive method. Most species (61.4%) were indigenous, while 38.6% were exotic. Pinus caribaea, Khaya senegalensis, Entandophragma angolense, and Gmelina arborea were among the dominant tree species. About 65.6% of the trees were very stable given their low slenderness coefficient. The high biodiversity index values (species richness = 57; Shannon-Wiener diversity index = 3.84; Margalef index = 9.68) suggest that the developed areas of FRIN have good biodiversity conservation status. Total basal area and volume productions were 49.54 m² ha^-1 and 660.32 m³ ha^-1, respectively, corresponding to 447.90 tons ha^-1 of biomass and 223.95 tons ha^-1 of carbon. Despite their small land size, the developed areas of FRIN are a significant carbon sink compared to similar institutional landscapes in Nigeria. This study highlights the need for institutional green-space management strategies to be integrated into national climate change adaptation and biodiversity conservation policies for enhanced ecological resilience and sustainability.

Keywords: biodiversity conservation; carbon sink; developed area; FRIN; urban vegetation.

 

INTRODUCTION

Globally, biodiversity is decreasing at unprecedented rates due to continued unsustainable human activities (Camilo et al., 2022; UNEP, 2024). This decline is projected to continue due to a lack of abatement and ongoing biologically destructive anthropogenic practices. Interventions such as afforestation and reforestation programmes have proved insufficient to reverse the trends (FDA, 2024). The 2024 Forest Declaration Assessment (FDA) report revealed that the rate of global forest loss in 2023 was 45% higher than the target required to end deforestation by 2030. This high rate of forest loss, has resulted in worldwide decline in flora and fauna species population and ecosystem integrity (FAO and UNEP, 2020; Şeren and Çelekli, 2024).

In Nigeria, urban and rural populations are growing at annual average rates of 4.5% and 1.4%, respectively (Akpan and Ebong, 2021; Olalude et al., 2024; Onyekwelu, 2013). The rapid population growth in cities across Nigeria and other developing countries is partially attributed to high rural-urban migration rates (Agbelade et al., 2016; Onyekwelu, 2013). According to the United Nations, Department of Economic and Social Affairs (UN DESA, 2013), about 60% of urban areas that will exist in 2050 have not yet been built, which is an indication that urban populations will be much higher in the near future. The world has become an urban society, with over 50% of humanity currently living within urban areas and more than 70% expected to reside in urban areas by the year 2050 (Liu et al., 2020; UN DESA, 2018).

Tree planting and conservation are essential tools for addressing climate change challenges as well as other environmental concerns. Conservation activities have multiple benefits, including biodiversity conservation, ecosystem protection, human well-being improvement, among others benefits (Goncalves-Souza et al., 2021; FAO, 2025; Mi et al., 2023; Naidoo et al., 2019; UNCCD, 2025; Wang et al., 2025). Information on the synergy between biodiversity and carbon storage has shown that urban trees, being an integral part of terrestrial ecosystem, reduce vegetation loss, buffers local climate variability and enhances ecosystem productivity, ultimately enlarging long-term carbon storage (Duncanson et al., 2023; UNEP, 2021; Wang et al., 2025; Weiskopf et al., 2024).

Urban forestry is the art, science, and technology of managing trees and forest resources in and around some urban community ecosystems for the psychological, sociological, aesthetic, economic, and environmental advantages trees bring to the society (FAO, 2020; UNEP, 2021). It plays a critical role in achieving the UN Sustainable Development Goals (SDGs 11, 13, and 15) (UN DESA, 2022). Urban forestry plays an important role in addressing various environmental, ecological and socio-economic challenges associated with urbanization. Urban trees are integral components of urban landscape, which could provide important ecosystem services like food, fruits/seeds, vegetables, carbon sequestration, environmental and micro-climate regulation (Agbelade et al., 2017; Fuwape and Onyekwelu, 2011). Managing urban trees helps conserve carbon stocks, mitigate air pollution, urban heat island effect and enhance their sequestration, making it a cost-effective and eco-friendly strategy for mitigating climate change (Livesley et al., 2016; Wang et al., 2025).

In recent times, institutions in urban areas have become involved in tree planting, biodiversity conservation, natural resources management, among others, thereby contributing to the success of urban forestry. For instance, Oyerinde et al. (2018) reported avenue trees in selected tertiary institutions in Nigeria. Ajayi et al. (2020) and Abdulmuhyi et al. (2022) reported abundance and diversity of trees in academic institutions.

Globally, academic institutions contribute immensely to carbon sequestration through their forested landscapes by focusing on three broad but interconnected areas: capacity for carbon storage; biodiversity provision, and a wide range of ecosystem goods and services; and their function as dynamic living laboratories for research, education, and community engagement (Filho et al., 2024; UNESCO, 2025). Forests established by institutions can be publicly and privately owned and may include avenue trees, trees in parks, school premises, research institutes, religious institutions, residential yards, among others location (Enisan et al., 2020).

Forestry Research Institute of Nigeria (FRIN), an ideal model of institutional urban forestry with managed vegetation that together represent a significant urban forest ecosystem. FRIN contributes not only to research and education but also to biodiversity conservation, carbon sequestration, and ecosystem services provision within urban landscapes. Traditional institutions like sacred forests located around human settlements promote ecosystem sustainability through biodiversity conservation, carbon sequestration and provision of numerous ecosystem services (Davi et al., 2021; Onyekwelu et al., 2024). Despite the increasing involvement of institutions in tree planting and management, information on their contributions to biodiversity conservation and ecosystem services as components of urban forestry is lacking. Past studies (Agbelade et al., 2022; Ogunkunle and Jimoh, 2019) mainly assessed tree species richness, density, and composition in urban green spaces.

However, these studies failed to integrate biodiversity conservation with tree productivity metrics. This limits the capacity to holistically evaluate the ecological importance and benefits of institutional green vegetation. Also, most trees in institutions were originally planted to conserve natural landscapes, with little known about their effectiveness for carbon storage.

Given the predicted dramatic reduction in biodiversity over the coming decades (CBD), 2019; UNEP, 2024), collaborative global and institutional actions are essential to reverse this trend and promote conservation. Few studies in Nigeria have integrated biodiversity indices with carbon stock estimation, and most of them focused separately on species composition or biomass productivity, thereby limiting understanding of how institutional green spaces contribute simultaneously to biodiversity conservation and carbon sequestration (Akomolafe et al., 2025; Ubaekwe, 2020).

To this end, this study was designed to assess the contributions of the Forestry Research Institute of Nigeria (FRIN, 2025) to urban biodiversity conservation, productivity, and carbon sequestration. It is hoped that the empirical evidence will guide policy and challenge other institutions (governmental and private) to actively engage in integrated urban forestry strategies to support national and global climate adaptation goals.

 

MATERIALS AND METHODS

The study area

This study was conducted within the premises of FRIN located in an urban area at Government Reserved Area (GRA), Jericho Hill, Ibadan North in Oyo State, Nigeria (can be accessed at https://frin.gov.ng/contact-us/). It lies between 7° 25´ and 7° 55´ N and 3° 9´ and 3° 53´ E. The study area experiences two distinct seasons. The rainy season runs April–October, while the dry season spans November-March. The climate is usually warm, with mean daily temperature ranging 18.7–34.4 °C. Mean annual rainfall ranges 1,400–1,500 mm. Relative humidity ranges from about 60% between December and February to 82% between June and September (Lawal et al., 2020). The soil texture ranges from loamy sand in surface horizons to sandy loam and sandy clay loam in subsurface horizons (Ishola et al., 2021; Onemayin et al., 2020). These soils correspond mainly to Acrisols or Alfisols, which are typical of humid tropical regions (Food and Agriculture Organization-United Nations Educational, Scientific and Cultural Organization, 2015; United States Department of Agriculture, 2014). FRIN covers a total land area of 104.89 ha comprised of developed areas (offices and staff quarters), forest plantation, natural forest, and fallow land. The developed areas cover about 19.73 ha of the entire land and consists of hard surfaces (e.g., paved) and landscape with beautiful ornamental flowers, shrubs, grasses, and trees (indigenous and exotic species). The institution is well guarded with boundaries (brick fence), buildings are linked by a good road network, lawns, and pedestrian walkways. The integration of trees and vegetation within the developed areas of FRIN enhances air quality, supplies non-timber products, and functions as windbreaks that protect infrastructures from damage while also creating a peaceful, beautiful, and serene environment.

Brief history of FRIN

The institute was established as the Federal Department of Forestry Research in 1954, but in 1977 its status was changed to Research Institute, a status it maintains to date. The mandate of FRIN is to conduct research into all aspects of forestry, forest product utilisation, watershed management, wildlife, agroforestry and to train technical personnel in forestry and related fields. The focus of FRIN is to apply scientific knowledge to the management and conservation of forest resources in Nigeria in a way that meets the needs of present and future generations (Onyekwelu, 2024).

In addition to research at its headquarters at Ibadan, FRIN research activities are carried out at 17 hubs and research stations spread across all ecological zones of Nigeria. One of the means through which FRIN strives to meet its mandate is the establishment of demonstration forest plantations within its headquarters and stations. Forest plantation establishment at FRIN headquarters commenced about 25 years ago. Since then, indigenous and exotic tree species have been established, with Nauclea diderrichii, Terminalia superba, Terminalia ivorensis, Gmelina arborea, and Khaya senegalensis being the prominent tree species.

Data collection, processing, and analysis

Data collection was limited to the developed areas of FRIN. All trees with a diameter at breast height (DBH) ≥10 cm were counted. Tree identification (by scientific and common names) was undertaken by an experienced taxonomist. Species identifications were later confirmed at the Forest Herbarium Ibadan located at FRIN headquarters. Keay (1989) was used as a guide for tree nomenclature as well as family placement. The following measurements were made on all trees: diameter at 1.3 m above the ground surface using a diameter tape; diameters at the base, middle, and top of the tree; tree height (measured from the base of the tree to the top of the tree using a spiegel relaskop); marketable height (usable length of a tree’s bole measured from the ground to a specified upper diameter limit); crown height (the vertical distance from the ground to the lowest live branch that forms part of the tree’s crown); and crown diameter (the average width of the spread of a tree crown).

Biodiversity indices

Eight biodiversity indices were computed using the methods suggested and/or adopted by Onyekwelu et al. (2008, 2021) and others.

• Species relative density (RD, %) is an index for assessing species relative distribution within an ecosystem (Equation 1).

where n_i is the number of individuals of species i, and N is the total number of all individuals of all species in the study area.

• Species relative dominance (Rdo, %) is useful for assessing the relative space occupancy of a species (Equation 2).

where Ba_i is the basal area of all trees of species i, and Ba_n is the total basal area of all species.

• Menhinick’s index (D) measures species richness in the ecosystem (Equation 3).

where S is the number of species, and N is the number of individuals.

• Shannon-Wiener diversity index (H’) accounts for species richness and evenness (Equation 4)

where S is the number of species in the study area, p_i is the proportion of S consisting of the i^th species, and ln is the natural logarithm.

• Shannon’s equitability index (E_H) measures species evenness (Equation 5).

• Importance value index (IVI, %), or coverage index is used to determine the overall importance of each species in the community (Equation 6).

This index shows how abundant, widespread and dominant each species is.

• Simpson’s Diversity Index (D) measures species diversity by considering both richness (number of species) and evenness (relative abundance) (Onilude et al., 2020). It is calculated using Equation 7.

where n_i is the number of individuals belonging to species i, and N is the total number of individuals observed across all species.

The value of 1 – D (Simpson’s índex) was then used to represent diversity, where values closer to 1 indicate higher diversity and values near 0 indicate lower diversity.

• Margalef’s Index (d) was used to calculate the species richness. The Margalef’s index (d) is independent of sample size. It is based on the relationship between total number of species (S) and total number of individuals (N) (Adio et al., 2019) (Equation 8).

where S is the total number of species, N is the total number of individuals, and ln is the natural logarithm.

Growth and yield

Three statistics were calculated to assess tree size and shape.

• Basal area (BA) of each tree was estimated using the equation from Husch et al. (2003) (Equation 9)

where π is 3.142 (constant). The total basal area for the developed areas was computed as the sum of the basal areas of all trees. Basal area per ha was obtained by dividing the total basal area by the number of hectares covered by the developed areas of FRIN.

• Volume (Vol) of each tree was estimated using Newton formula (Husch et al., 2003) (Equation 10):

where H is the tree height (m), D_b is the diameter at the base (cm), D_m is the diameter at the middle (cm), D_t is the diameter at the top (cm). The total volume of trees in the developed areas was calculated by adding the volumes of all trees. The volume per ha was obtained by dividing the total volume by the number of hectares covered by the developed areas of FRIN. It is important to note that, for monodic tree species, volume estimation was based on the main trunk of the tree, while in sympodic species, the most vigorous and robust stem was selected for measurement.

• Tree slenderness coefficient (SLC) was estimated by dividing the height of a tree by its DBH (Equation 11).

The tree slenderness coefficient often serves as an index of tree stability (Onilude and Adesoye, 2007). The SLC of each trees was categorised in one of three coefficient classes:

1. High slenderness coefficient: Slenderness coefficient Class 1: SLC > 99,
2. Moderate slenderness coefficient: Slenderness coefficient Class 2: 70 < SLC < 99,
3. Low slenderness coefficient: Slenderness coefficient Class 3: SLC < 70

(Onilude and Adesoye, 2007).

Estimation of aboveground biomass and carbon stock

The aboveground biomass (AGB) of standing trees per ha estimated using a non-destructive method was obtained by summing the biomass of all trees in the developed areas of FRIN and dividing the sum by the number of hectares covered by the developed areas (Equation 12).

where BEF is the Biomass Expansion Factor; VOB is the volume over bark (m³), and WD is the wood density (g/cm³). Since species-specific biomass equations were not used, the BEF of 1.3 was used, based on Intergovernmental Panel on Climate Change (2019) guidelines for tropical moist forests. Each tree species’ wood density was obtained from the Global Wood Density Database (Zanne et al., 2009).

The carbon stock (Cs) of any forest tree is usually estimated as 50% of its biomass (Sharma et al., 2020; Onyekwelu et al., 2024). Therefore, the amount of carbon sequestered by each tree was calculated by multiplying the AGB by 0.5

Paleontological Statistics (PAST) software package version 3.12 (Hammer et al., 2001) was used to assess the biodiversity indices. Microsoft Excel software was used to process the growth and yield, biomass, and carbon stock data.

 

RESULTS

Our results suggest that the developed areas of FRIN contribute greatly to biodiversity conservation due to presence of 57 tree species distributed among 29 families. A total of 326 trees were measured within the developed areas of the institute, which are predominantly Fabaceae (19.3%), Meliaceae (10.5%), Sterculiaceae (5.3%), Leguminosae (5.3%), Combretaceae (5.3%), Moraceae (5.3%), and Sapindaceae (5.3%) (Table 1).

A high percentage of the tree species are indigenous (61.4%), while 38.6% of the species are exotic. Tree species with a high frequency of occurrence are E. angolense (n =51), P. caribaea (n = 44), Terminalia mantaly (n = 26), K. senegalensis (n = 23), and Albizia lebbeck (n = 22) (Table 1).

Other tree species with appreciable representation were Azadirachta indica (n = 12), Ptericarpus osun (n = 10), Morinda lucida (n = 8), T. superba (n = 7), and Triplochitin scleroxylon (n = 6). Several tree species, Acacia holosericea, Cedrela odorata, Chrysophyllum albidum, Cola gigantea, Hildergadia barteri, Newbouldia leavis, and others, are represented by only one individual. Many of the tree species are economically important tropical rainforest species in Nigeria. However, there are a few savanna or transition zone tree species, such as Parkia biglobosa and Vitex doniana, and agricultural species, such as Mangifera indica and Anacardium occidentale.

Tree species with a high RDo were P. caribaea (17.75%), K. senegalensis (11.62%), E. angolense (5.99%), G. arborea (4.32%), and A. indica (4.21%) (Table 1). Most of these tree species also emerged as being very important in the floristic composition of the study area, based on the IVI. Tree species with a high IVI included P. caribaea (15.62%), E. angolense (10.82%), K. senegalensis (9.34%), T. mantaly (5.48%), and A. lebbeck (4.51%) (Table 1).

The SLC of individual trees varied greatly, ranging 8.33–318.52, which indicated that there were very stable trees as well as very unstable trees within the study area. While most of the trees are stable, a small percentage of the trees were very unstable (Figure 2).

Using the categories of Onilude and Adesoye (2007), 65.6% of the trees (n = 214) have a low SLC (very stable trees), 16.3% (n = 53) have a moderate SLC (moderately stable trees), and 18.1% (n = 59) have a high SLC (unstable trees).

The DBH of the trees ranged 10.8–169.5 cm. The DBH frequency distribution followed an inverted-J pattern (Figure 3) similar to the DBH frequency distribution pattern of trees in natural tropical forest ecosystems (Adekunle, 2013; Onilude and Adesoye, 2007; Onyekwelu et al., 2021, 2024). About 53% of the trees had a DBH less than 30 cm, 29.9% had a DBH of 30–60 cm, and only 1.2% had a DBH greater than 100 cm. About 56% of the trees had height less than 20 m, 46.8% were 20–40 m high, and 7.7% were less than 40 m high. Tree basal area varied from 0.009 m² to 2.26 m², while tree volume ranged 0.042– 27.09 m³, indicating the presence of small trees as well as very big trees, which is supported by the wide range of tree crown diameter from 2.31 m to 25.47 m. Mean tree DBH and height were 37.2 cm and 20.7 m, respectively (Table 2). Mean crown height and crown diameter were 8.48 m and 9.81 m, respectively. Total basal area and volume of all trees in the developed areas of FRIN were 49.54 m² ha^-1 and 660.32 m³ ha^-1, respectively (Table 2). The relationship between DBH and biomass followed a negative trend (Figure 4). The moderately high R² indicates that DBH accounted for about 65% of the variation in biomass.

 

Table 1
Diversity indices of tree species found within the built-up areas at FRIN headquarters

Family	Species	Freq	Ba (m2)	RD (%)	RDo (%)	IVI	IUCN	Habitat status
Fabaceae	Acacia holosericea	1	0.64	0.31	1.28	0.80	LC	Exotic
Leguminosae	Afzelia africana	4	0.81	1.23	1.64	1.43	VU	Native
Leguminosae	Albizia lebbeck	22	1.12	6.75	2.26	4.51	LC	Exotic
Anacardiaceae	Anacardium occidentale	2	0.40	0.61	0.82	0.72	LC	Exotic
Combretaceae	Anogeissus leiocarpa	3	1.52	0.92	3.08	2.00	LC	Native
Moraceae	Antiaris africana	4	0.27	1.23	0.54	0.88	LC	Native
Araucariaceae	Araucaria araucana	2	0.62	0.61	1.26	0.94	EN	Exotic
Meliaceae	Azadiracha indica	12	2.09	3.68	4.21	3.95	LC	Exotic
Balanitaceae	Balanite egyptia	1	0.10	0.31	0.20	0.25	LC	Native
Sapindaceae	Blighia sapida	2	0.23	0.61	0.46	0.54	LC	Nativee
Malvaceae	Bombax bouonopozense	2	0.10	0.61	0.21	0.41	LC	Native
Fabaceae	Cassia fistula	2	0.23	0.61	0.46	0.54	LC	Exotic
Meliaceae	Cedrela odorata	1	0.02	0.31	0.04	0.18	VU	Exotic
Bombacaceae	Ceiba pentandra	2	0.05	0.61	0.10	0.36	LC	Native
Ulmaceae	Celtis zenkeri	2	0.12	0.61	0.23	0.42	LC	Native
Sapotaceae	Chrysophyllum albidum	1	0.03	0.31	0.06	0.19	LC	Nativee
Sterculiaceae	Cola gigentia	1	0.45	0.31	0.91	0.61	LC	Native
Boraginaceae	Cordia aliodora	8	0.77	2.45	1.55	2.00	LC	Exotic
Burseraceae	Dacryodes edulis	3	0.62	0.92	1.25	1.08	LC	Nativee
Fabaceae	Delonix regia	5	0.74	1.53	1.50	1.52	LC	Exotic
Meliaceae	Ekebergia senegalensis	3	1.33	0.92	2.68	1.80	LC	Native
Meliaceae	Entadophragma angolense	51	2.97	15.64	5.99	10.82	NT	Native
Meliaceae	Entandrophragma cylindricum	2	0.68	0.61	1.38	0.99	VU	Native
Myrtaceae	Eucalptus camaldulensis	3	0.27	0.92	0.55	0.74	NT	Exotic
Myrtaceae	Eucalyptus torelliana	3	0.58	0.92	1.16	1.04	LC	Exotic
Fabaceae	Gliricidia sepium	2	0.06	0.61	0.12	0.36	LC	Exotic
Verbenaceae	Gmelina arborea	7	2.14	2.15	4.32	3.24	LC	Exotic
Sterculiaceae	Hildergadia barteri	1	0.01	0.31	0.03	0.17	NE	Native
Irvingiaceae	Irvingia gabonensis	2	0.09	0.61	0.19	0.40	VU	Nativee
Meliaceae	khaya senegalensis	23	5.76	7.06	11.62	9.34	VU	Native
Sapindaceae	Lecaniodiscus cupanioides	1	0.01	0.31	0.03	0.17	LC	Nativee
Fabaceae	Leuceana leucocephala	1	0.04	0.31	0.07	0.19	LC	Exotic
Anacardiaceae	Mangifera indica	2	0.36	0.61	0.72	0.67	LC	Exotic
Moraceae	Milicia excelsa	3	0.74	0.92	1.50	1.21	NT	Native
Annonaceae	Monodora myristica	1	0.26	0.31	0.52	0.41	LC	Nativee
Rubiaceae	Morinda lucida	8	0.71	2.45	1.43	1.94	LC	Native
Moringaceae	Moringa oleifera	3	0.10	0.92	0.21	0.57	LC	Exotic
Rubiaceae	Nauclea diderrichii	2	0.55	0.61	1.10	0.86	NT	Nativee
Bignoniaceae	Newbouldia leavis	1	0.03	0.31	0.06	0.19	LC	Nativee
Fabaceae	Parkia biglobosa	2	0.33	0.61	0.66	0.64	LC	Native
Fabaceae	Peltophorum pyetocarpum	1	0.43	0.31	0.87	0.59	LC	Native
Fabaceae	Pericopsis elata	4	1.41	1.23	2.84	2.03	EN	Native
Pinaceae	Pinus caribaea	44	8.80	13.50	17.75	15.63	LC	Exotic
Leguminosae	Pterocarpus osun	10	0.92	3.07	1.86	2.46	LC	Nativee
Fabaceae	Senna siamea	6	0.64	1.84	1.30	1.57	LC	Exotic
Dipterocarpaceae	Shorea roxburghii	3	0.61	0.92	1.22	1.07	EN	Exotic
Solanaceae	Solandra maxima	2	0.15	0.61	0.30	0.46	NE	Exotic
Combretaceae	Terminalia mantaly	26	1.48	7.98	2.99	5.48	LC	Exotice
Combretaceae	Terminalia superba	7	1.91	2.15	3.85	3.00	LC	Native
Fabaceae	Tetrapleura tetraptera	3	0.22	0.92	0.45	0.69	LC	Nativee
Moraceae	Treculia africana	5	0.96	1.53	1.94	1.73	LC	Nativee
Sterculiaceae	Triplochitin scleroxylon	6	2.05	1.84	4.14	2.99	LC	Native
Lamiaceae	Vitex doniana	1	0.06	0.31	0.12	0.21	LC	Native
Apocynaceae	Voacanga africana	1	0.03	0.31	0.06	0.18	LC	Native
Fabaceae	Xylia xylocarpa	4	1.20	1.23	2.43	1.83	LC	Exotic
Sapindaceae	Zanha goungensis	1	0.72	0.31	1.46	0.88	LC	Native
Rutaceae	Zanthoxylum zanthoxyloides	1	0.03	0.31	0.06	0.18	NE	Native

Note: Freq = Frequency, BA = Basal area, RD = Relative density, RDo = Relative dominance, IVI = Importance value index, LC – Least concern; NT – Near threatened; EN – Endangered; NE – Not evaluated; VU – Vulnerable. Habitat status – Native: naturally occurring species within the region; Exotic: species introduced from outside the region. Subscript “e” denotes endemism (e.g., Nativee = endemic to the local bioregion; Exotice = endemic to its natural region of origin).

 

 

 

Table 2
Biodiversity indices, stand growth, and yield characteristics of trees within the developed areas of the Forestry Research Institute of Nigeria

Biodiversity indices and growth variables	Values
No. of species	57
No. of trees	326
No. of families	29
Shannon-Weiner Index (H’)	3.84
Simpson diversity index	0.93
Menhinick’s index	3.16
Margalef index	9.68
Shannon equitability index	0.81
Dominance index	0.07
Mean diameter at breast height (cm)	37.2
Mean total height (m)	20.7
Mean crown height (m)	8.48
Mean crown diameter (m)	9.81
Total basal area (m2 ha-1)	49.54
Total volume (m3 ha-1)	660.32
Mean tree slenderness ratio	68.62%
Total aboveground biomass (tons ha-1)	373.25
Total biomass (tons ha-1)	447.90
Total carbon stock (tons ha-1)	223.95
Native to Nigeria	35
Exotic to Nigeria	22

 

The developed areas of FRIN have high tree species diversity as indicated by the high H’ of 3.84 and Simpson index of 0.93 (Table 2). Menhinick’s index value was 3.16. The low D index of 0.06619 is an indication that no single species dominated the study area. Instead, the tree species were relatively and evenly distributed, as indicated by the results of the Margalef index and Shannon Equitability index values of 9.68 and 0.81, respectively.

FRIN’s developed areas contain species with variable International Union for Conservation of Nature (IUCN) conservation status. According to IUCN classification, 5.3% of the tree species are endangered, 73.7% are in the least concern category, 7% are near threatened, and 8.8% are vulnerable; 5.3% are not categorised.

A wide range of tree biomass values were recorded in the study area, due mainly to the variable tree sizes. While biomass per tree was as low as 0.0133 tons for some trees, it was as high as between 10–20 tons for others. Tree species with very high biomass were K. senegalensis (19.51 tons), Milicia excelsa (12.95 tons), Shorea roxburghii (8.83 tons), Anoigessus leocarpus (8.57 tons), Entandrophragma cylindricum (8.34 tons), indicating their high carbon sequestration potential.

On the other hand, some trees species had substantially lower; these included Ceiba pentandra (0.0133 tons), M. lucida (0.0280 tons), Antiaris africana (0.0292 tons), Delonix regia (0.0331 tons), and P. caribaea, among many others.

The trees within the developed areas of FRIN had a total biomass and carbon stock of 447.90 tons ha^-1 and 223.95 tons ha^-1 respectively.

The developed areas of FRIN had good growth characteristics with higher total height and greater DBH and basal area compared to the statistics from other sites (Table 3).

However, the developed areas of FRIN had lower biomass and carbon storage compared to most of the other sites. This discrepancy may be attributed to the lower tree density in FRIN’s developed areas compared to most of the study areas in Table 3.

 

DISCUSSION

Urban tree composition and species richness

Due to the current rising populations in urban areas and the pressure exerted on limited resources, the well-being of the increased human populace is secured by the presence of urban vegetation to provide essential ecosystem goods and services. The families and tree species in in the developed areas of FRIN are characteristic of the Nigerian tropical forest region. There is agreement in the published literature (Adekunle, 2006; Agbelade and Onyekwelu, 2020; Onyekwelu et al., 2008, 2024) that the Nigerian tropical forest region is largely dominated by the same tree families found to dominate the floristic composition in the study area. The number of families in the study area is higher or within the range reported for various tropical forest ecosystems, especially in Nigeria (Adekunle et al., 2013; Agbelade et al., 2017; Dangulla et al., 2021; Nero et al., 2024; Onyekwelu et al., 2024).

 

 

Table 3
Comparison of biodiversity indices and carbon storage of trees within the developed areas of the Forestry Research Institute of Nigeria (FRIN) with various published results

Variable	FRIN	Port Harcourt	Ilorin	Kumasi Zoological Garden	Developed area in Zaria, Nigeria	Sacred groves in Nigeria	Urban green sites in India
No. of species	57	37	46	43	37	41–85	–
Tree density	326	746	556	185 ha-1	252	311–423	–
No. of family	29	19	18	15	21	22–32	–
Shannon-Weiner Index	3.84	3.39	3.61	2.42	3.1	2.63–3.55	–
Mean dbh (cm)	37.2	44.84	49.74	51	48.9	24.4–31.9	–
Mean total height (m)	20.7	11.1	13.35	20	19.1	9.8–13.7	–
Maximum dbh (cm)	169.5	98	96.4	122	–	105–185	–
Total basal area (m2 ha-1)	49.54	16.39	22.21	–	–	–	–
Total volume (m3 ha-1)	660.32	409.78	745.44	–	–	245.0–343.1	–
Total biomass (tons ha-1)	447.9	67,979.1	91,512.5	–	407.2	78.8–231.9	545.4
Total Carbon (tons ha-1)	224	33,989.5	45,756.3	53.9 mg C ha− 1	203.6	43.9–115.9	272.2
Reference	This study	Agbelade and Onyekwelu, 2020		Nero et al., 2024	Dangulla et al., 2021	Onyekwelu et al., 2024	Pradhan et al., 2022

The species richness in FRIN’s developed areas is within the range reported for forested sites in Nigeria (Adekunle, 2006; Lowe, 1997; Onyekwelu and Olusola, 2014; Onyekwelu et al., 2024). A high number of trees have been reported for urban forests. Moshood et al. (2022a) enumerated 3,225 trees in Ilorin, Nigeria, and Agbelade and Onyekwelu (2020) reported 746 and 556 trees in Port Harcourt and Ilorin urban forests, respectively, which are higher than the 326 trees in the developed areas of FRIN. However, there could be size differences among study areas. The five recreation centres in Ibadan studied by Bolanle-Ojo et al. (2020) had higher number of trees than FRIN’s developed areas. The low number of trees in FRIN’s developed areas can be attributed to its small size (19.73 ha). Given that the developed areas of FRIN are dispersed among administrative, residential, laboratory, library, workshops, recreation and sporting facilities, and other buildings with few vacant places, the presence of 326 trees should be appreciated and considered substantial.

Indigenous vs. exotic tree species in the study area

The higher proportion of indigenous tree species (61.4%) in the study area compared to exotic species (38.6%) is an indication of the role and contribution of FRIN to the conservation of tree species, especially the native species, which are important given their ecological importance. Some cities and developed areas are known to have higher percentage of exotic than indigenous species, thus showing FRIN’s efforts in conservation of indigenous tree species. The higher frequency of indigenous tree species in our study could be attributed to the commitment of FRIN to fulfilment of its mandate of promoting the conservation of native species. This also enhances ecological resilience, as native trees are better adapted to local climatic and edaphic conditions, supporting greater biodiversity and ecosystem stability (Chazdon et al., 2016; Onyekwelu, 2024). Some of the indigenous tree species were intentionally planted at FRIN, with the intention of introducing them to visitors, thus the developed areas of FRIN can also be regarded as an arboretum of a sort. The presence of endemic species within FRIN enhances its conservation significance. Endemic species represent unique genetic resources that have evolved in response to the local environment. Their occurrence reflects the ecological integrity and biogeographical uniqueness of FRIN.

Exotic tree species accounted for 63.4% of all species in the developed areas of Zaria, Nigeria (Dangulla et al., 2021), which is substantially more than the 35.1% reported in our study area. About 80% of the trees used in public spaces in Lima, Peru were non-native species (Moreno et al., 2024). Similar to our results, Aremu et al. (2023) found a higher percentage of indigenous tree species (mean 67%) in urban green areas of Osogbo, Nigeria. Conversely, a high proportion of exotic species could increase vulnerability to pests, diseases, and environmental stressors, and in some cases, lead to invasive tendencies that can alter native community structures (Pysek et al., 2020). Hence, the higher frequency of indigenous species in our study area highlights FRIN’s contribution to sustainable urban forest management and ecological stability.

Stability and structural suitability of tree species in the study area

Despite our study area having moderate species richness, only a few tree species dominated the floristic composition. Five species (P. caribaea – 15.6%, E. angolense – 10.8%, K. senegalensis – 9.3%, T. mantaly – 5.5%, and A. lebbeck – 4.5%) accounted for about half (45.8%) of the IVI and by implication the dominance of the study area. Among these species, only T. mantaly is widely used as avenue trees in urban and developed areas (Agbelade and Onyekwelu, 2020; Nero et al., 2024). The other dominant tree species are typical native tropical timber species, thus their dominance in FRIN’s developed areas can be ascribed to intentional native species conservation efforts by FRIN.

Several authors have demonstrated that trees with low SLC values are less susceptible to wind induced throw down or damage (Onilude and Adesoye, 2007; Ige, 2017; Moshood et al., 2022b; Olajiire-Ajayi et al., 2024). The high percentage of very stable trees (i.e., with low SLC values) in our study area is an indication of good management and proof of the suitability of the trees for developed environments since the likelihood of their throw down or damage by wind or rainstorm, and possible harm to human lives and property, is very low. Cautious measures must be taken concerning trees with a high SLC, which constitute about 18.1% of the trees within the developed areas of FRIN, since they have been shown to be very unstable and highly susceptible to wind throw and damage (Ige, 2017; Olajiire-Ajayi et al., 2024). High risk to wind-throw and damage pose a significant challenge for trees with high SLC in developed areas. During heavy wind or rainstorms, these unstable trees can cause harm to humans if they are thrown down or their branches are broken. Thus, efforts must be made by FRIN management to improve their stability and prevent or minimise the risks to humans and properties. In the worst case scenario, these unstable trees can be removed.

Biodiversity indices and conservation status

The high H’ and other biodiversity indices (e.g., Simpson diversity index – 0.93, Margalef index – 9.68), and Shannon Equitability index – 0.81) of FRIN’s developed areas are indications of the good biodiversity conservation status of the study area. A check of the published literature revealed that the H’ in our study area is higher and comparable to those of some cities and developed areas. Abuja and Ibadan urban forests had an H’ of 3.56 and 3.35, respectively (Agbelade et al., 2016a, b) while Port Harcourt urban forests recorded an H’ of 3.39 (Agbelade and Onyekwelu, 2020). Agbelade et al. (2022) reported H’ values of 3.08–3.61 for three cities in north-central Nigeria. The developed area of Zaria had an H’ of 3.1 (Dangulla et al., 2021). Nero et al. (2024) found H’ values of 1.68–2.77 for botanical gardens and parks in Ghana. The H’ of the developed areas of FRIN was also comparable or higher than those of some tropical forest ecosystems in Nigeria (Adekunle et al., 2013; Onyekwelu et al., 2008).

Species evenness (0.81) in our study area was higher than the ranges given by Bolanle-Ojo et al. (2020) and Agbelade et al. (2022) but within the ranges reported by Onyekwelu et al. (2024) and Nero et al. (2024). Although high species evenness was observed, an uneven distribution of trees within some species within the developed areas was noticed, indicating a skewed distribution. These patterns are common in tropical forests, where a few species are ecologically dominant while numerous others persist at low numbers per ha (Ojo, 2004; Onyekwelu et al., 2008; Adekunle et al., 2006). This was confirmed by the low D value of 0.0662.

The high values of biodiversity indices in our study area can be attributed to the continuous planting of various tree species within the FRIN premises. Although FRIN’s developed areas have only experienced minimal expansion in size to date, tree planting has been going on since its inception. However, planting is only done when the need arises, and the rate of planting and replacement of dead trees is low. The latter can and should be improved upon. The distribution of tree species among the IUCN status categories revealed a predominance of species with stable populations (least concern). However, the presence of species in the endangered, near threatened, and vulnerable categories reveals the need for improved conservation efforts and strategies.

Biomass and carbón sequestration potentials of tree species in the study area

Biomass plays a very important and sensitive role as an indicator to determine or predict rate of carbon sequestration or carbon storage. It can also be used as an indicator of forest health such as ecosystem productivity and resilience. Esperon-Rodriguez et al. (2022) showed that cities are planting trees for resilience through urban greening initiatives as governments understand the importance of urban forests in improving the quality of life and mitigating climate change. Climate regulation through carbon sequestration has become one of the crucial ecosystem services of forests. Many tree species in FRIN have the capacity to store large amounts of biomass and thus sequester large amounts of carbon. These species include K. senegalensis (19.56 tons), M. excelsa (12.95 tons), S. roxburghii (8.83 tons), Anogeissus leiocarpus (8.57 tons), E. cylindricum (8.34 tons), Afzelia africana (8.10 tons), Pericopsis elata (7.88 tons). These tree species have been noted to have the characteristics of attaining very large dimensions upon maturity (Keay, 1989), making them ideal candidates for carbon sequestration. Anogeissus leiocarpus is a very strong tree species with high calorific value that is often preferred for charcoal production because it burns slowly, which suggests heavy carbon storage within it tissue. Jumawan et al. (2024) reported that Ficus benjamina, Eucalyptus deglupta, G. arborea, and Cocos nucifera produced high aboveground biomass in Bood Promontory and Eco-Park in the Philippines. In Port Harcourt, Hevea brasiliensis and A. occidentale had high biomass accumulation, while Hura crepitans and A. lebbeck accumulated high biomass in Ilorin (Agbelade and Onyekwelu, 2020). In Lima, Peru, Ficus pertusa, Erythrina falcata, and Eucalyptus camaldulensis had the highest carbon trapping rates (Moreno et al., 2024). These trees species have the capacity to store a large amount of biomass and thus contribute significantly to carbon storage and atmospheric carbon sequestration.

The total biomass and carbon storage in the developed areas of FRIN was 447.90 tons and 223.95 tons, respectively. Given the small size of these areas, this can be considered a substantial contribution to atmospheric carbon sequestration. Moreno et al. (2024) reported carbon storage of 2,859.0 tons in 43 ha of urban green spaces in Lima, Peru, which is much higher than the 223.95 tons in the 19.73 ha of FRIN’s developed areas. The urban green spaces in Lima had amuch higher number of trees (28,123) (Moreno et al., 2024) compared to our study area (326), which can explain the higher carbon sink potential in Lima than in the developed areas of FRIN. The 203.60 tons of carbon stored in the developed area of Zaria (Dangulla et al., 2021) compares favourably to the carbon stored in the developed areas of FRIN.

Thus, despite its small land size, the developed areas of FRIN can be considered an excellent source of carbon sequestration and climate change mitigation.

Comparison with other studies

A comparison of the results of our study with the results in published literature (Table 3) revealed that the developed areas of FRIN have good biodiversity conservation status, good productivity, and good carbon storage potential.

Although the developed areas of FRIN had a lower tree population compared to most of the other studies, it had higher tree species richness than most of the other sites (Table 3). It had a comparable species richness with the sacred groves in Nigeria (Onyekwelu et al., 2024). The H’ of FRIN’s developed areas was higher than that of Kumasi Zoological Garden, Ghana (Nero et al., 2024), the developed area in Zaria (Dangulla et al., 2021) and some sacred grove sites in Nigeria (Onyekwelu et al., 2024).

 

CONCLUSIONS

The developed areas of FRIN have high biodiversity conservation status, good productivity, and good carbon sink potential. The number of tree species (57) and the high diversity indices of the areas are higher or comparable to those of some cites, indicating its significant contributions to urban biodiversity.

The high proportion of indigenous tree species (61.4%) shows the commitment and contribution of FRIN to native species conservation. Most of the trees (65.6%) are suitable for developed environments since the likelihood of their throw-down or damage by storms and causing harm to life and property is very low due to their low SLC. Many trees in FRIN stored large amount of carbon, making them ideal candidates for atmospheric carbon sequestration. Total basal area, volume, biomass, and carbon productions were 49.54 m², 660.32 m³, 447.90 tons ha^-1, and 223.95 tons ha^-1, respectively.

Despite the small land size, the developed areas of FRIN serve as significant carbon sinks with notable potential for climate change mitigation initiatives. Documentation of the endemic taxa within FRIN affirms its role as a microrefuge for regional biodiversity and underlines the need for targeted conservation.

Expanding the green spaces within FRIN can help to improve urban sustainability and biodiversity. Findings from this study could inform Nigeria’s National Biodiversity Action Plan and support evidence based urban greening and climate mitigation policies. Future research should incorporate remote sensing validation, full propagation analysis, long-term carbon monitoring, and spatial modeling to improve accuracy and scalability of institutional carbon biodiversity assessments.

There is a need for FRIN management to strengthen conservation practices, expand monitoring of biodiversity, and carbon stocks. This can be used to attract policy support, partnerships, and funding to be able to enhance FRIN’s role in the urban environment’s sustainability.

 

Acknowledgment: We are grateful to all the anonymous reviewers for their feedback on the paper and their thoughtful suggestions on all aspects of the paper.

Funding: There was no external funding for this study.

Author contributions: Conceptualisation: JO, JC; Methodology: JC, QA; Data collection: QA, JO, JC; Analysis: QA, JC; Data curation: QA, OA; Writing: QA, JO; Review: QA, ZB; Supervision: ZB. All authors declare that they have read and approved the publication of the manuscript in this present form.

Data availability statement: The data presented in this study are available on request from the corresponding author (omoonilu@gmail.com).

Conflicts of interest: There is no conflict of interest.

REFERENCES

Abdulmuhyi, N.I.; Sani, M.M.; Hussaini, L.A.; Nendirmwa, D.S. Checklist of trees and shrubs at the Naraguta campus of the University of Jos. Journal of Agriculture and Ecology Research International 2022, 23(5), 24-30. https://doi.org/10.9734/jaeri/2022/v23i530236

Adekunle, V.A.J. Conservation of tree species diversity in tropical rainforest ecosystem of southwest Nigeria. Journal of Tropical Forest Science 2006, 18(2), 91-101.

Adekunle, V.A.J.; Olagoke, A.O.; Akindele, S.O. Tree species diversity and structure of a Nigerian strict nature reserve. Tropical Ecology 2013, 54(3), 275-289.

Agbelade, A.D.; Onyekwelu, J.C. Tree species diversity, volume yield, biomass and carbon sequestration in urban forests in two Nigerian cities. Urban Ecosystem 2020, 23(2), 957-970. https://doi.org/10.1007/s11252-020-00994-4

Agbelade, A.D.; Onyekwelu, J.C.; Oyun, M.B. Tree Species Richness, Diversity, and Vegetation Index for Federal Capital Territory, Abuja, Nigeria. International Journal of Forestry Research 2017, 1. https://doi.org/10.1155/2017/4549756

Agbelade, A.D.; Onyekwelu, J.C.; Oyun, M.B. Tree Species Diversity and their Benefits in Urban and Peri-Urban Areas of Abuja and Minna, Nigeria. Applied Tropical Agriculture 2016a, 21(3), 27-36.

Agbelade, A.D.; Onyekwelu, J.C.; Apogbona, O. (2016b). Assessment of Urban Forest Tree Species Population and Diversity in Ibadan, Nigeria. Environment and Ecology Research 2016b, 4(4), 185-192. https://doi.org/10.13189/eer.2016.040401

Agbelade, A.D.; Onyekwelu, J.C.; John, A.A.; Adedayo, J.; Alabi, T. Assessing the conservation status, biodiversity potentials and economic contribution of urban tree Ecosystems in Nigerian Cities. Urban Ecosystems 2022, 25, 165-178. https://doi.org/10.1007/s11252-021-01137-z

Ajayi, I.K.; Kayode, J.; Ademiluyi, B.O. Study on urban trees in Ekiti State University, Ado-Ekiti, Nigeria: 1. Structure and composition. Budapest International Research in Exact Sciences (BirEx) Journal 2020, 2(2), 147-156. https://doi.org/10.33258/birex.v2i2.872

Akomolafe, G.F.; Mustafa, Y.S.; Ilyas, S.; Atoki, G.D.; Udeh, L.C.; Osabwa, I. Woody Plant Diversity and Carbon Storage Assessments in Urban Secondary Schools, Lafia, Nasarawa State, Nigeria. FULafia Journal of Science & Technology 2025, 9(1), 64-71. https://doi.org/10.62050/fjst2025.v9n1.487

Akpan, S.B.; Ebong, V.O. Agricultural land use and population growth in Nigeria. The need for synergy for a sustainable agricultural production. Journal of Agribusiness and Rural Development 2021, 3(61), 261-270. https://doi.org/10.17306/J.JARD.2021.01424

Aremu, O.; Adetoro, O.; Awotoye, O. (2023). Assessment of Diversity, Growth Characteristics and Aboveground Biomass of Tree Species in Selected Urban Green Areas of Osogbo, Osun State. In Forest Degradation Under Global Change. Samec, P. (ed). IntechOpen, London, United Kingdom. https://doi.org/10.5772/intechopen.104982

Bolanle-Ojo, O.T.; Falana, A.R.; Bolanle-Ojo, O.I.; Levan, C. Assessment of tree species diversity and benefits in selected recreation centres for biodiversity conservation in Ibadan Metropolis, Nigeria. Notulae Scientia Biologicae 2020, 12(1), 100-113. https://doi.org/10.15835/nsb12110561

Camilo, A.R.M.; Forster, T.; Sietchiping, R.; Githiri, G.; Hillel, O.; Alvarado, V. Managing Urban-Rural Linkages for Biodiversity: An Integrated Territorial Approach. Position Paper for CBD COP 15 for Global Framework for Biodiversity (GBF) targets, United Nations Human Settlements Programme (UN-Habitat), 2022, 43pp. https://unhabitat.org/sites/default/files/2022/12/managing_urban-rural_linkages_for_biodiversity.pdf

 CBD. Convention on Biological Diversity. Extinction Crisis. Centre for Biological Diversity, 2019. http://www.biologicaldiversity.org/programs/biodiversity/elements_of_biodiversity/extinction_crisis/index.html. (accessed on 6 June 2025).

Chazdon, R.L.; Broadbent, E.N.; Rozendaal, D.M.A.; Bongers, F.; Zambrano, A.M.A.; Aide, T.M.; Balvanera, P.; Becknell, J.M.; Boukili, V.; Brancalion, P.H.S. Carbon sequestration potential of second-growth forest regeneration in the tropics. Nature Climate Change 2016, 6(6), 730-734. https://doi.org/10.1038/nclimate3004 

Dangulla, M.; Manaf, L.A.; Ramli, M.F.; Yacob, M.R.; Namadi, S. Exploring urban tree diversity and carbon stocks in Zaria Metropolis, North Western Nigeria. Applied Geography 2021, 127, 102385. https://doi.org/10.1016/j.apgeog.2021.102385

Davi, N.B.; Lepcha, N.T.; Mahalik, S.S.; Dutta, D.; Tsanglao, B.L. Urban sacred grove forests are potential carbon stores: a case study from Sikkim Himalaya. Environmental Challanges 2021, 4, 100072. https://doi.org/10.1016/j.envc.2021.100072

Duncanson, L.; Liang, M.; Leitold, V.; Armston, J.; Krishna Moorthy, S.M.; Dubayah, R.; Costedoat, S.; Enquist, B.J.; Fatoyinbo, L.; Goestz, S.J. The effectiveness of global protected areas for climate change mitigation. Nature communications 2023, 14, 2908. https://doi.org/10.1038/s41467-023-38073-9

Enisan, G.; Alabi, M.O.; Omole, F.K. Tree planting as a means for urban aesthetics and development in Nigeria. Journal of the Institute of Town Planners 2020, 26(1), 55-68.

Esperon-Rodriguez, M.; Rymer, P.D.; Power, S.A.; Barton, D.N.; Tjoelker, M.G. Assessing climate risk to support urban forests in a changing climate. Plants, People, Planet 2022, 4(3), 201-213. https://doi.org/10.1002/ppp3.10240

FAO. Global Forest Resources Assessment, Rome. https://doi.org/10.4060/cd6709en (accessed on 21 January 2025).

FAO &UNEP. The State of the World’s Forests 2020: Forests, Biodiversity and People. FAO and UNEP, Rome, Italy, 2020, pp. 214. https://doi.org/10.4060/ca8642en

Fuwape, J.A.; Onyekwelu, J.C. Urban forest development in West Africa: benefts and challenges. Journal of Biodiversity and Ecological Sciences 2011, 1(1), 77-94.

FRIN. Meteorological section report. Department of Environmental Modeling and Biometrics, Forestry Research Institute of Nigeria, Ibadan, Nigeria. https://frin.gov.ng/contact-us/ (accessed on 12 May 2025).

Filho, W.L.; Luetz, J.M.; Dinis, M.A.P. University forests and carbon sequestration: an untapped potential. Discover Sustainability 2024, 5, 362. https://doi.org/10.1007/s43621-024-00590-y

FDA. Forest Declaration Assessment. Forests under fire: Tracking progress on 2030 forest goals, 2024.  https://forestdeclaration.org/wp-content/uploads/2024/10/2024ForestDeclarationAssessment.pdf

Gonçalves-Souza, D.; Vilela, B.; Phalan, B.; Dobrovolski, R. The role of protected areas in maintaining natural vegetation in Brazil. Science Advances 2021, 7(38). https://doi.org/10.1126/sciadv.abh2932

Husch, B.; Beers, T.W.; Keenshaw Jr., J.A. Forest mensuration. 4^th Edition. John Wiley & Sons Inc., New York, U.S.A., 2003, 443 pp.

Ige, P.O. Relationship between the Slenderness Coefficient and tree or stand growth characteristics for Triplochiton scleroxylon (K. Schum) stands in Onigambari forest reserve, Nigeria. Journal of Forest Research and Management 2017, 14(2), 166-180.

Isola, J.O.; Fawole, O.A.; Oluwaponle, I.A.; Ojedokun, R.O.; Owoade, A.D. Suitability assessment of soils around Forestry Research Institute of Nigeria (FRIN), Ibadan for maize production: A parametric analyses 11221. Nigerian Agricultural Journal 2021, 52(2), 85-92.

Jumawan, J.H.; Sinogbuhan, A.J.; Atienza, D.D.; Cadavez., R. Aboveground biomass estimation and tree vegetation assessment of Bood Promontory and Eco-Park in Butuan City, Philippines after 20 years of establishment. Thailand Natural History Museum Journal 2024, 18(1), 17-40.

Keay, R.W.J. Trees of Nigeria. A revised version of “Nigerian Trees” Clarendon Press, Oxford University Press, 1989, 476 pp.

Lawal, M.O.; Adeyanju, T.A.; Ogundimu, O.A.; Fadimu, B.O.; Odiaka, I.E.; Eniola, O.; Ganiyu O.A. Wildbird Abundance and Richness in Forestry Research Institute of Nigeria (FRIN), Jericho, Ibadan, Oyo State. Journal of Research in Forestry, Wildlife and Environment 2020, 12(3), 266-278.

Liu, X.; Huang, Y.; Xu, X.; Li, X.; Xia, L.; Ciais, P. High-Spatiotemporal-Resolution Mapping of Global Urban Change from 1985 to 2015. Nature Sustainability 2020, 3(7), 564-70. https://doi.org/10.1038/s41893-020-0521-x

Livesley, S.J.; McPherson, E.G.; Calfapietra, C. The Urban Forest and Ecosystem Services: Impacts on urban water, heat, and pollution cycles at the tree, street, and city scale. Journal of Environmental Quality 2016, 45(1), 119-124. https://doi.org/10.2134/jeq2015.11.0567

Lowe, R.G. Volume increment of natural moist tropical forest in Nigeria. Commonwealth Forestry Review 1997, 76(2), 109-113.

Mi, C.; Ma, L.; Yang, M.; Li, X.; Meiri, S.; Roll, U. (2023). Global protected areas arefuges for amphibians and reptiles under climate change. Nature Communications 2023, 14(1), 1389. https://doi.org/10.1038/s41467-023-36987-y

Moreno, R.; Nery, A.M.; Zamora, R.; Lora, A.; Galán, C. Contribution of urban trees to carbon sequestration and reduction of air pollutants in Lima, Peru. Ecosystem Services 2024, 67, 101618. https://doi.org/10.1016/j.ecoser.2024.101618

Moshood, F.J.; Muhali, M.O.; Ngwuli, C.P. Species diversity and public perceptions of urban trees in Ilorin metropolis, Kwara State, Nigeria. Forest and Forest Product Society 2022a, 24-32.

Moshood, F.J.; Ibrahim, T.M.; Salami, K.D. Stability assessment of tree species in the University of Ilorin permanent site. TUW Trends in Science and Technology Journal 2022b, 7(1), 370-378.

Naidoo, R.; Gerkey, D.; Hole, D.; Pfaff, A.; Ellis, A.M.; Golden, C.D. Evaluating the impacts of protected areas on human well-being across the developing world. Science Advances 2019, 5(4). https://doi.org/10.1126/sciadv.aav3006

Nero, B.F.; Kuusaana, E.D.; Ahmed, A.; Campio, B.B. Carbon storage and tree species diversity of urban parks in Kumasi, Ghana. City and Environment Interactions 2024, 24, 100156. https://doi.org/10.1016/j.cacint.2024.100156

Olajiire-Ajayi, B.L.; Ogundana, O.A.; Adenuga, D.A. Assessment of stand growth and slenderness coefficient of Nauclea diderrichii A. Chev and Terminalia ivorensis De Wild and Thur in Forestry Research Institute of Nigeria, (FRIN) arboretum, Oyo State, Nigeria. FUDMA Journal of Sciences 2024, 8(4), 55-6. https://doi.org/10.33003/fjs-2024-0804-2522 

Olalude, G.A.; Esiegbe, E.O.; Alabi, P.A. Modelling and Forecasting Urban Population Growth in Nigeria Using Autoregressive Integrated Moving Average (ARIMA) Models. European Journal of Science, Innovation and Technology 2024, 4(4), 311-327.

Onemayin, J.J.; Olayiwola, V.A.; Abiodun, F.O.; Musa, F.; Idris, R.S. Land use influence on some soil physical and chemical properties of an alfisol at Forestry Research Institute of Nigeria. International Journal of Plant & Soil Science 2020, 32(4), 1-8. https://doi.org/10.9734/IJPSS/2020/v32i430263

Onilude, Q.A.; Adesoye, P.O. Relationship between slenderness coefficient and tree growth characteristics of Triplochiton Scleroxylon (K. Schum) stands in Ibadan metropolis. Obeche Journal 2007, 25(2), 16-23.

Onyekwelu, J.C.; Mosandl, R.; Stimm, B. Tree species diversity and soil status of primary and degraded tropical rainforest ecosystems in South-Western Nigeria. Journal of Tropical Forest Science 2008, 20 (3), 193-204.

Onyekwelu, J.C. Urbanization and the Challenges of Urban Forestry. In Green economy: balancing environmental sustainability and livelihoods in an emerging economy. Proceedings of the 36^th annual conference of the Forestry Association of Nigeria, Uyo, Akwa Ibom state, November 5-8, 2013, pp. 402 – 419.

Onyekwelu, J.C.; Olusola, J.A. Role of sacred grove in in-situ biodiversity conservation in rainforest zone of south-western Nigeria. Journal of tropical forest science 2014, 26(1), 5-15.

Onyekwelu, J.C.; Lawal, A.; Mosandl, R.; Stimm, B.; Agbelade, A.D. Understory species diversity, regeneration and recruitment potential of sacred groves in south west Nigeria. Tropical Ecology 2021, 62, 427-442. https://doi.org/10.1007/s42965-021-00157-2

Onyekwelu, J.C. Forestry Research and Development. In Forest for international job and wealth creation. Proceedings of the 45^th annual conference of the Forestry Association of Nigeria, Katsina, Katsina State, May 6-11, 2024, pp 497-518.

Onyekwelu, J.C.; Agbelade, A.D.; Stimm, B.; Mosandl, R. Role of sacred groves in southwestern Nigeria in biodiversity conservation, biomass and carbon storage. Environmental Monitoring and Assessment 2024, 196-269. https://doi.org/10.1007/s10661-024-12407-6

Oyerinde, V.; Olusola, J.; Adeoye, S. Assessment of Avenue trees species diversity in two selected tertiary educational institutions in Ondo State, Nigeria. Journal of Forestry Research and Management 2018, 15(2), 149-167. https://doi.org/10.13140/RG.2.2.16126.64325

Pradhan, R.; Sarkar, B.; Manohar, K.; Shukla, G.; Tamang, M.; Bhat, J.; Kumar, M.; Chakravarty S. Biomass carbon and soil nutrient status in urban green sites at foothills of eastern Himalayas: Implication for carbon management. Current Research in Environmental Sustainability 2022, 4. https://doi.org/10.1016/j.crsust.2022.100168

Pyšek, P.; Hulme, P.E.; Simberloff, D.; Bacher, S.; Blackburn, T.M.; Carlton, J.T.; Dawson, W.; Essl, F.; Foxcroft, L.C.; Genovesi, P.; Jeschke, J.M. Scientists’ warning on invasive alien species. Trends in Ecology & Evolution 2020, 35(8), 685-694. https://doi.org/10.1016/j.tree.2020.03.007

Şeren, E.; Çelekli, A. Biodiversity Loss: A Global Issue Threatening Ecological Balance. 7th International Conference on Health, Engineering and Applied Sciences Sarajevo, 2024, pp. 84-98.

Sharma, K.P.; Bhatta, S.P.; Khatri, G.B.; Pajiyar, A.; Joshi, D.K. Estimation of Carbon Stock in the Chir Pine (Pinus roxburghii Sarg.) Plantation Forest of Kathmandu Valley, Central Nepal. Journal of Forest Science and Environment 2020, 36, 37-46.

https://doi.org/10.7747/JFES.2020.36.1.37

Ubaekwe, R.E. Carbon Stock and Storage Capacity of Tree Species in Strict Nature Reserve of Omo Biosphere Reserve, Ogun State, Nigeria. Asian Journal of Environment & Ecology 2020, 13(3), 26–36. https://doi.org/10.9734/ajee/2020/v13i330184

UNCCD. Global Land Outlook Thematic Report on Rangelands and Pastoralism. Global Land Outlook, Bonn, Germany. https://www.unccd.int/resources/global-land-outlook/glo-rangelands-report (accessed on 10 April 2025).

UN DESA. World Economic Situation and Prospects. United Nations, 2022. https://doi.org/10.18356/9789210011839

UN DESA. Department of Economic and Social Affairs, Population Division. World Population Prospects: The 2012 Revision, Volume I: Comprehensive Tables ST/ESA/SER.A/336. United Nations, New York, 2013.

UN DESA. World Urbanization Prospects: The 2018 Revision. United Nations, New York, 2018. https://www.un.org/development/desa/pd/content/world-urbanization-prospects-2018-revision-key-facts

UNEP. Making peace with nature: A scientific blueprint to tackle the climate, biodiversity and pollution emergencies, 2021. United Nations Environment Programme. Nairobi, Kenya. https://www.unep.org/resources/making-peace-nature

UNEP. Emissons Gap Report 2024: No more hot air … please!. United Nations Environment Programme Nairobi. https://doi.org/10.59117/20.500.11822/46404

UNESCO. Greening Education Partnership: getting every learner climate-ready, United Nations Educational, Scientific and Cultural Organization. https://www.unesco.org/en/articles/greening-education-partnership-getting-every-learner-clmate-ready (accessed on 10 July 2025)

Wang, S.; Peng, J.; Lin, Y.; Hu, T. Revisiting the role of china’s protected areas in Carbon storage. Earth’s Future 2025, 13(9), 1-14. https://doi.org/10.1029/2025EF006202

Weiskopf, S.R.; Isbell, F.; Arce-Plata, M.I.; Di Marco, M.; Harfoot, M.; Johnson, J.; Lerman, S.B.; Miller, B.W.; Morelli, T.L.; Mori, A.S. Biodiversity loss reduces global terrestrial carbon storage. Nature Communications 2024, 15(1) 4354. https://doi.org/10.1038/s41467-024-47872-7

Zanne, A.E.; Lopez-Gonzalez, G.; Coomes, D.A.A.; Ilic, J.; Jansen, S.; Lewis, S.L.; Miller, R.B.B.; Swenson, N.G.G.; Wiemann, M.C.C.; Chave, J. Data from: Towards a worldwide wood economics spectrum. Dryad 2009, 235, 33. https://doi.org/10.5061/dryad.234

 

Academic Editor: Dr. Iuliana MOTRESCU

Publisher’s Note: Regarding jurisdictional claims in published maps and institutional affiliations, ALSE maintains neutrality.