For effective conservation management, it is crucial to comprehend the diversity, abundance, and distribution of the forest as wildlife habitat. The fundamental composition of the forest is significantly influenced by the ecological features of the sites, species diversity, and rate of tree species regeneration. This finding aimed to assess the diversity, abundance, and distribution of tree species as suitable habitat for wildlife species and for sustainable forest management, climate change mitigation, and environmental resilience. The study was conducted in dry season (between 5th September, 2021 and April, 2022). Purposive sampling techniques and systematic line transects were used for data collection and plot delineation. There were laid 8 transects, each 1000 meters long and 500 meters apart. Descriptive statistics such as mean, frequencies, and percentages were used to analyze the data. Correlation analyses and diversity indices were carried out using the R programming software. In the study area, 68 tree species from 34 families were found. Meliacea (6 trees per ha), Caesalpiniceae, and Moracea (5 trees per ha) were the three most common tree species in the study area. Melicia excelsa showed the highest relative frequency (2.256%) and (2.241%). The species Bialonella toxisperma had the highest relative dominance (4.970%). In Melicia excelsa, IVI recorded the highest value (4.970%). The tallest tree and Dbh (Diameter at Breast Height) measured 80.5 cm and 68.3m). The Margelef index was 36.10, the Shannon Wiener index was 5.058, and species richness was 68. However, the high proportion of smaller-diameter trees in the forest reserve implies that, it is strong and healthy. Since natural areas play a critical role in slowing down climate change, strict oversight of these areas should be strongly encouraged.

Keywords: tree species, diversity, distribution, abundance, okpon river

Understanding the forest stands' structure, diversity, abundance, and distribution is essential for conservation management. The ecological characteristics of the sites, species diversity, and the rate of tree species regeneration all play a significant role in the fundamental structure of the forest. Tree species abundance and diversity are essential to the overall biodiversity of forests because trees provide resources such as food, traditional medicine, timber, shade, and habitats for fauna Sushma et al.,¹ Due to numerous anthropogenic factors, the extent of species decline in the second half of the 20th century became a universal or global problem.² The provision and protection of biodiversity services are crucial to describing the pattern of forest structure in order to control or manage the increasing rate of anthropogenic activities within the forest estate.³ The variation in tree species diversity among forest reserves has been attributed to a variety of factors. According to Malhil et al.,⁴ and Lippok et al.,⁵ topography has a significant impact on the local endemism of plant species. Franscico et al.,  reported that disturbance has an impact on diversity and regeneration, including changes in tree growth, tree mortality, understory development in relation to forest reserves, and habitat heterogeneity. Forests are one of the main vegetative elements in India (as well as Nigeria), and they serve as a priceless repository for numerous economically significant species as well as the genetic material for many crop plants and their wild relatives.⁵ Basic understanding of key elements' spatial and temporal ranges as well as the main environmental factors that influence their survival and distribution is necessary for sustainable conservation management.⁶

The study area

Okpon River Forest Reserve was gazetted by  Cross River State in 1930. The reserve occupied a land mass of 31,300 hectares, covering two local government areas, Obubra and Yakurr, respectively. The Reserve lies between Latitudes 5⁰ . 40¹ ,5⁰.50¹ and 6⁰.00¹,  6⁰ .10¹ North of the Equator and Longitude  8⁰. 10¹ ,8⁰. 20¹  and 8⁰.30¹ ,  8⁰. 40¹ East of the Greenwich Meridian   Figure 1. The reserve is bounded in the north by Etung and Ikom LGAs, south by Baise LGA, west by Abi LGA, and east by Eboyi State.

Figure 1 Map of Okpon river forest reserve.

Sampling techniques/procedure

Transects and plots were selected using systematic and purposeful sampling techniques. For the purpose of counting the species of plants, eight (8) transects were set up. At regular intervals of 250 m along the transects, four plots of 50 m x 50 m (0.25 ha) size were systematically placed alternately along each transect. A hundred metres separated each transect. All woody plants with a diameter at breast height (dbh) less than 10 cm were the only ones eligible for measurement and identification.

Data collection and analyses

The study was conducted in dry season (between September, 2021 and April, 2022). Data collected was based on Dbh (≥10 cm). The height of each individual tree was measured using a sunto clinometer, while the diameter of the trees was measured using a diameter tape. The diameter at breast height (> 10 cm dbh), importance value index (IVI), relative frequencies, relative dominance, relative density, and height of the entire individual tree were all calculated.

The 2017 version of the Microsoft Word and Excel package was used to impute the collected data. We used descriptive statistics like means, percentages, tables, and charts. Using diversity indices, the RF, RD, RDO, and IVI of tree species were calculated. The threshold for statistical significance was (P 0.005%). In the "R" software, correlation analysis and diversity indices were both carried out.

Basal areas: of all trees in the samples plots were calculated using the formula:

$BA=\frac{\pi D^2}{4}$   (1)

B.A= Basal Area (m²), D= Diameter at Breast Height (cm) and ᴨ =(3.142).

Species Relative density (RD %):

$RD=\frac{ni\times 100}{N}$   (2)

RD = Relative density of the species, ni = Number of individuals per species and N = Total number of all individual tree of all species in the entire population.  

Species Relative Dominance (%) was estimated using the following equation:

$R{D}_0=\frac{{\displaystyle \sum B{\alpha }_1\times 100}}{{\displaystyle \sum B{\alpha }_n}}$   (3)

Ba₁ = Basal area of individual tree belonging to the i^th species and Ba_n = Stand basal area.

Shannon – wiener diversity index was calculated using the following equation:

$H=-{\displaystyle \sum _{1=1}{P}_1}In(P_1)$   (4)

H’ = Shannon diversity index, S = the total number of species in the community, P₁= Proportion S (species in the family) made u to the i^th spp and In = natural logarithm.  

Important Value Index:

IVI =RF+RD+RD  (5)

RD = Relative density of the species; RD_O = relative dominance

Forest Structure of the Ukpon River Forest Reserve

Table 1 shows the diameter of breast height and the total height of species at the Okpon Forest Reserve. The maximum and minimum diameters at the base were recorded as 80.5 cm and 10.1 cm, with a mean of 25.1 cm and SD of 13.2. The mean total height was 28.6 m, with a standard deviation of 14.1 m. A minimum total height of 5.2 and a maximum of 68.3 m, with a mean of 28.6 m, were recorded in the study area. The diameter distribution in Figure 2 revealed that tree species within the diameter at breast height (dbh) distribution ranged from 10 cm to 70 cm. Fifty-five (55) trees belonged to the Dbh class (10 cm to 20 cm) and were the most frequently occurring in the area, representing 47%. This was followed by trees in the diameter class of 20cm – 40cm with twenty five (25) trees/ha representing 17.10%. the least number of  stem (15tree/ ha) and  (10 trees/ha) falls under diameter class of  40cm – 50cm and 50cm to 60cm representing 11.1% and 5.0% respectively  while the least species within the diameter range of  60 cm > 70 cm represented 11.1 % and 5.0% respectively. Based on the result of this finding Figure 3, the total height of tree species in the study area ranged from 10 to 70 m. Forty trees (40/ha) belonged to the tree height class of 20–30 m, representing 35%, followed by the tree height class of 30–50 m, with 25 trees representing 39.4%. Similarly, 20 trees/ha falls between the height classes of 50 – 60m trees represented 12.8% of the entire tree species in the study location.  Ten trees belonged to the height class between 60 and 70  m, representing 7.6%, while 5 trees per ha belonging to the height classes of 10 to 20 m and > 70 m were in the range of 2.6 each.

Figure 2 Diameter at breast height distribution of tree species in Okpon river forest reserve.

Figure 3 Total height distribution of trees at the okpon river forest reserve.

Tree species diversity and distribution in okpon river forest reserve

The results of this study recorded 248 tree species belonging to 55 families. Caesalpinaceae had the highest number of species (24 trees per ha), followed by Moraceae (20 trees per ha), Euphorbiaceae (15 trees per ha), Apocynaceae and Steculiaceae (12 trees per ha), Leguminocea (10 trees per ha), Annonaceae (9 trees per ha), and Rutaceae (8 trees per ha). These families were regarded as the most dominant in the forest reserve. Sapindaceae and Ebeneceae had 7 species each, Rubiaceae had 6 species, Mimosaceae and Ulmaceae 5 tree Sapotaceae and Anacardiaceae 4 species, Gentianaceae and Lauraceae families 3 species. Eleven families recorded two species, while twenty (20) families recorded one species each (Table 2).

Relative Frequency, Relative Density, Relative Dominance, and Importance Value Index (IVI) of Tree Species at Okpon Forestry Reserve

The result indicates that Brachystegia eurycoma and Melicia excelsa had the highest relative frequency of (2.266) followed by Khaya ivorensis (1.933%), Diospyros mespiliformis, Funtumia elastic, and Lophira alata, which recorded the same relative frequency of (1.826%). The least relative frequency of (0.107 %) were recorded  in 107 tree species Table 2. The results of the relative density of  tree species at the Okpon forestry reserve presented in Table 2 showed that Brahystegia erycoma recorded the highest relative density of (2.241%), closely followed by Khaya ivorensis and Milicia excelsa that recorded the same value (2.028%), Lophira alata (1.814%), and Entandrophragma utile (1.708%), while the least relative density of (0.107%) was recorded in ninety-six (96) tree species.

Milicia excelsa recorded the highest relative dominance (4.970%), followed by Piptadeniastrum africana (4.643%), and Brachystegia erycoma (3.089%). The least relative dominance was obtained in Kigela africana (0.016%). Brachystegia erycoma recorded the highest IVI of 10.66%, followed by Milicia excelsa (9.46%) and Khaya ivorensis (6.865%). The lowest IVI of 0.228% was in Ficus muccuso and Mansonia altissima (Table 2).

Species diversity in the sampled transect of okpon river forest reserve

The results in Table 3, across tree species diversity in the sampled transects indicate that Transect (T₅) had the highest number of individual tree species (145) and had Shannon wiener index of 4.563 with species richness of 106 closely followed by Transect seven (T₇) with one hundred and twenty one (121) number of individual species, Shannon wiener index  and species richness of 4.203 and 75 respectively. Transects one and three (T₁ and T₃) recorded the same number of individual tree species (118); the Shannon-Wiener indices were 4.405 and 4.079, with species richnesses of 88 and 67, respectively. The least number of species, 93, with a Shannon number of 3.925 and a species richness of 57, was recorded in Transects 8 (Table 3). The summary of diversity indices across the sampled transects at the Okpon River Forest Reserve presented in Table 4 shows that the Shannon Weiner index minimum and maximum values were 3.925 and 4.563 with a mean value of 4.287, while species richness had a minimum and maximum of 57 and 106 with an average value of 81.75. Both the Shannon Weiner index and species richness had standard deviations of 0.212 and 15.508, respectively. Table 5 indicates species diversity indices in the study area; Shannon weiner index recorded (5.058), Simpson’s index (0.991), Pielou eveness index (0.917) and Margelef index recorded (36.097). The results of species richness recorded 248 trees in the study sites. Table 5

Forest Structure of Ukpon River Forest Reserve

Diameter and height distributions reflect the horizontal structure and vertical pattern of the forest; these are indications that the forest has the potential for continuous growth. However, the presence of large trees has been reported to be a sign of mature tropical rainforest Zang et al. According to Adekunle et al.,⁷ 222 tree stands were found to be in the lower Dbh class of 10–20 cm, followed by 21–30 cm, which had 78 tree stands, and 72 tree stands in the higher Dbh class of 81 cm. Hence, he reported that the diameter distribution indicated healthy recruitment potentials. The lower-class diameter tree stands could develop into mature trees and replace the old ones in the future if proper conservation efforts are sustained. Thus, this structure is typical of a natural forest.^8,9 The minimum and maximum Dbh of trees in the study area were 10.1 m and 80.5 m, respectively. The forest reserve was characterized by an abundance of trees with small Dbh, which is not unusual for tropical rainforests. Similar results have been reported by previous workers in other tropical rainforests in Nigeria.¹⁰ The reason for relatively fewer number of individual trees with large Dbh values greater than >0.70 (Dbh>70) could be attributed to forest degradation activities which may have removed large individuals as well as the facts that some large-sized trees would have been removed through logging operation for some uses in the past.¹¹

Tree species diversity and distribution in okpon river forest reserve

The results of this study recorded 68 tree species belonging to 34 families. Caesalpinceae, Moracea and Meliacea   were the most abundance families. The area is rich in terms of tree species composition. However, the number is lower when compared with the 99 tree species belonging to 36 families recorded in the Takamanda Rainforest of Southwest Cameroon.¹² Similarly, it is lower than 118 tree species reported by Adeyemi et al.,¹³ for the Oban Division of the Cross River National Park in Nigeria. A higher number of tree species increases the number of ecological niches for fauna and understory flora. Thus, the presence of numerous tree species on a stand conserves not only more trees but other organisms as well. Comparing the results of this study to a similar study by Oluwatosin et al., in Onigambari forest reserve, Ondo State, Nigeria, they obtained a higher number of families (54) of tree species, while Miazu.¹⁴ reported four families in Kuyambana forest reserve, Zamfara State, Nigeria, even lower than the presence study of 34 families recorded in Okpon river forest reserve. He reported the dominance of the Caesalpinaceae, Mimosaceae, and Combretaceae families. This finding corroborated the works of Adekunle (2013), who reported that the tropical rainforest ecosystems of southwest Nigeria are dominated by some specific families such as the Sterculiaceae, Meliaceae, and Moraceae. In this present study, Okpon River Forest Reserve was dominated by the Caesalpiniaceae, Meliaceae, and Moraceae families.

Fabaceae, Melicea, and Caesalpiniacea have been reported as dominant plant families in Nigerian tropical forests.¹⁰ The effect of anthropogenic activities on the growth and distribution of tree species may have played a role in the status of these species in the ecosystem, threatening the occurrence and development of certain species while favouring others. The Caesalpinaceae, Meliaceae, Moraeceae, and Euphorbiaceae were the most prevalent families in the study area. This could be due to their fast regeneration ability associated with symbiotic properties, which may have enabled the species to easily establish within different habitat types. Deka et al. (2012) reported that legumes were the most prominent species recorded in Takamanda forest. This may not be far from the fact that the two forest reserves share some ecosystem characteristics and geographic boundaries.

The value of the Shannon Wiener index for tree species in this present study was (5.05). The value recorded in this study is higher than what was reported in other forest reserves; for instance, Parthasarathy (2001) reported H₁ 3.89, while Adekunle and Olagoke (2010) reported H₁ 4.02 for rainforest in India and Nigeria, respectively. This result is also in line with the findings of Bhat and kaveriappa (2013) who  obtained among the fresh water swamp forest of kulathupazha, Anchal, Shendumeg, Karthakani, Pilarlarkan and Charmady Karnataka, Shannon wiener of (2.53), (3.69), (2.46), (4.04), (3.25) and (4.90) were recorded respectively. The reason for the high Shannon wiener in this present study could probably be due to the existing management practices, enacted law enforcements programs and anthropogenic activities.

Important value index for tree species

The tree with the highest importance value in the reserve was Brachystegia eurycoma (10.6%) followed by Milicia excelsa (9.4%). This is consistent with the works of Soumana et al., who reported that, most tree species could grow best in loamy sandy soils. The tree with the lowest importance index in the study area was Pterocarpus osun (1.1%). This could likely be due to the multi-purpose usefulness (commercial and medicinal uses) of the tree, which is very important to the local economy. According to Curtis and Mcintosh (1951), a high importance value index IVI of a species indicates its dominance and ecological success, its good power of regeneration, and a greater ecological amplitude; these plants also need conservation management, while species that were grouped as having a low importance value therefore need high conservation effort Abdullahi, Abba. The highest regeneration potentials were recorded in Treculia africana (0.025%), which is quite lower in value than the 0.189% Culcacia saxatilis species obtained in the Onigambari forest reserve in Oyo State, Nigeria, by Salami et al. (2016). The differences in value could be attributed to the management practises adopted in the forest reserve. The regeneration potential in the study area was generally very poor. This has a serious implication on the regeneration and conservation of the various species encountered on the renewal of the forest in general. Wale et al. (2012) also noted that lack of adequate regeneration is an issue recognised by foresters and ecologists. Malik and Bhat (2016) also observed limited regeneration and subsequently declining populations of some dominant native species. Jaya Kumar and Nair (2013) observed in their study that only 101 species regenerate well, with one of the dominant species having no seedlings, which is an indication of poor regeneration.^15–21

Based on the results of this finding, a total of 68 tree species from 34 families were encountered in the study area. The results indicate that, Meliaceae had the highest number (6) of tree species, Caesalpiniceae had 5 species. There was low tree species density and high species diversity in the study area. The dominant height in the reserve was 40 m, with a Dbh class of 10–20 cm, which tends to dominate. To ensure suitable habitat for sustainable wildlife management and to prevent the extinction of some tree species in the area, there is a need to restrict logging activities and other illegal activities that have a negative effect on stand density and species distribution and abundance. Also, tight control of natural areas should be highly promoted because it is impossible to overstate how much they contribute to preventing climate change in the Ukpon area of Nigeria's Cross River State.

None.

Authors declare that there are no conflicts of interest.

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