1 Introduction

Understanding how carbon stock and biomass of woody species contribute to or mitigate CO2 emissions is crucial in assessing their role in climate change dynamics (Shiferaw et al. 2022). CO2 is one of the most abundant greenhouse gases (GHGs), which traps surface heat in the atmosphere and prevents it from being released into space, and increasing the global temperature. Over the past 250 years, the amount of CO2 emission in the atmosphere has increased from roughly 280 to > 412 ppm, which is what causes global warming (Maryo et al. 2023). To mitigate climate change impacts and manage forests sustainably, it is crucial to protect existing carbon pools and increase carbon sinks. In this particular situation, the United Nations Framework Convention on Climate Change (UNFCCC) has introduced the Reducing Emissions from Deforestation and Forest Degradation (REDD+) initiative, which aims to maintain carbon storage in tropical forests of developing nations as an economic incentive (Ganamé et al. 2021; Salunkhe et al. 2023). The IPCC recommends precise quantification of carbon storage potential through monitoring, reporting, and verification (MRV) strategies for the effective execution of REDD+ (Ganamé et al. 2021).

It is widely recognized that tropical forests including reserve forests play a vital role in mitigating global climate change issues by acting as carbon sinks due to their 50% global carbon stock and one-third of global primary productivity (Daba et al. 2022; Huang et al. 2020; Salunkhe et al. 2023). Globally, forest vegetation shares approximately 80% of terrestrial above-ground, and 40% of terrestrial below-ground biomass carbon storage (Panagiotopoulos et al. 2020). Carbon is stored in carbon pools like standing forests, understory plants, leaf litter, soils, rocks, and sediments making the forest function as both a carbon source and carbon sink (Odiwe et al. 2017; Olorunfemi et al. 2019). For the sustainable management of this tropical forest, estimation of carbon stock, biomass (above- and below-ground), and CO2 emission are a crucial biophysical constraint, that provides information about the growth, health, and productivity of the forest ecosystem (Salunkhe et al. 2023).

Anthropogenic activities such as deforestation, logging, and agricultural expansion significantly contribute to carbon emissions and disrupt the natural balance of carbon cycles in ecosystems (Omotoso and Omotayo 2024). Between 19% and 20% of greenhouse gas emissions worldwide are attributed to agricultural land use, which also includes nitrous oxide application, intensive tillage, and land preparation (Jantke et al. 2020; Rahman et al. 2021). The structure of forest stands is altered by anthropogenic activities and natural disturbances, which affect carbon stocks, biomass, CO2 emissions, and fluxes at the site level (Nave et al. 2010; Bohara et al. 2021). In many of these systems, disturbances change the overall community structure (Shaforth et al. 2002) which in turn ultimately affects community and population dynamics. Clements (1936) viewed disturbance as a negative force that destroys climax assemblages and brings instability to the system, while Bohara et al. (2021) considered it as a positive force that might increase species diversity in the community by preventing competitive exclusion by dominant species.

The biomass, carbon stock of the forest, and the potential of the forest to sequester carbon depends on various factors such as forest type, the age of the forest, management regimes, the size of trees, the density of trees, biomass decomposition, stand condition, degree, and history of disturbance (Sharma et al. 2020; Barrette et al. 2021; Czerepko et al. 2021; Su et al. 2021). The magnitude of carbon change due to tropical forest deforestation is difficult to predict (Bohara et al. 2021) as tropical forests contain more species than any other ecosystems (Gibson et al. 2011) and are large carbon sinks (Kauppi et al. 2015).

It is essential to know the stock of carbon as biomass per unit area for different forest types to assess the impact of deforestation and re-growth rates on the global carbon cycle as about 89% of stored carbon gets lost due to the loss of living biomass in the ecosystems (Keith et al. 2014; Ghimire et al. 2018). Above-ground biomass represents 60% of total tree biomass (Turchetto 2020) and this makes biomass measurement more important when calculating plant carbon pool (Ketterings et al. 2021). Also, below-ground biomass, deadwood biomass, and litter biomass measurements are required to determine the total carbon stock by plants over a specific time (Banik et al. 2018). The measurement also helps to determine the effects of land-use change and deforestation on net carbon fluxes. Therefore, both the above-ground biomass (AGB) and below-ground biomass (BGB) are determined for better calculation of total forest carbon (Bohara et al. 2021).

The nexus between carbon stock, biomass, and CO2 emissions of woody species provides insights into sustainable management practices, aiding in the development of strategies for balancing economic activities, like logging, with the preservation of carbon stocks. Findings from such studies contribute to the formulation of policies addressing deforestation, emphasizing the importance of maintaining healthy woody ecosystems for carbon sequestration (Raihan et al. 2023). Some species diversity studies such as chimpanzee habitat structure community and butterfly species diversity have been carried out in Ise-Ekiti forest reserve Nigeria following its establishment (Jacob et al. 2016), however, little information on the nexus between carbon stock, biomass, and CO2 emissions of woody species in disturbed and undisturbed areas in Ise-Ekiti forest reserve is available, and hence this study was conducted.

2 Methodology

2.1 Description of study area

The study was conducted in the Ise-Ekiti Forest Reserve, which is located in Southwestern, Nigeria. The reserve covers an area of 56.77 km2 and it is on latitude 7∘ 22 60 N and longitude 5∘ 220 E. The forest can be accessed from Ise Ekiti town, which is approximately 6 km to the north and 9 km to the south of the reserve via the Akure-Benin expressway from Uso community in Ondo State (Olaniyi et al. 2014). The forest reserve falls within the tropical rainforest hemisphere and the annual temperature ranges between 25°C and 28°C, with a minimum temperature of 19°C and a maximum temperature of 33°C. The annual rainfall is between 1200 mm and 1380 mm. Ise-Ekiti forest reserve is blessed with a diversity of plant species, some of which include Gmelina arborea, Mansonia altissima, Tectona grandis, Alstonia boonei, Ceiba pentandra, Entandrophragma cylindricum, Terminalia ivorensis, Khaya ivorensis, and Milicia excelsa. (Greengrass, 2006; Ogunjemite et al. 2005; Fig. 1).

Fig. 1
figure 1

Description and location of Ise-Ekiti Forest Reserve, Southwestern, Nigeria

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2.2 Vegetation sampling and collection of data

Ten plots (25 m x 25 m), five in each of the disturbed and undisturbed plots were randomly established. In each plot, the woody plants were identified and enumerated. Evaluating the distribution and composition of woody species within a specific area by observing and sampling plants of varying sizes is very crucial. Additionally, the size keeps fieldwork at a manageable scale. The girth sizes of the woody species (tree and shrubs) were measured at a height of 1.3 m above the ground (DBH) for plants ≥ 3.0 m in height, and the mid-point for those ≤ 3.0 m in height, using a diameter tape. For the woody species with a height of ≥ 3.0 meters, the diameter at Breast Height (DBH) was measured at 1.3 meters above the ground level and a diameter tape was wrapped around the trunk, and the measurement was taken. For woody species with a height ≤ 3.0 meters, the mid-point was determined by locating the center of the woody speciess' height, and the diameter tape was then wrapped around the trunk at this mid-point for the measurement. The above-ground biomass, below-ground biomass, carbon stock, and CO2 emission values within the disturbed and undisturbed plots were determined using allometric equations.

For all trees taller than three meters, the vertical length between the lowest and tallest living part was measured in meters. The tree species' height was determined using the Spiegel Relaskop.

2.3 Wood density

The wood density values reported by Zanne et al. (2009) and Agboola (2017) for woody species in tropical moist forests in Osun State and global wood density data were used as the wood density values in the study.

2.4 Total biomass of woody species calculation

After calculating the above-ground or below-ground biomass (kg), the total biomass of tree or shrub is calculated in megagrams per hectare (Mg/ha), and this value is extrapolated to the hectare by Salas et al. (2017) as follows:

$${\text{TB}}= \Big(\sum {\text{AU}}/1000\Big) \times (\mathrm{10,000}/\mathrm{plot area})$$
(1)

where TB: total biomass (Mg /ha); ∑ AU: sum of the tree or shrub biomass of all trees or shrubs in the plot (kg/plot area); Factor 1000: sample’s conversion units of kg/Mg; DM Factor 10,000: conversion of the area (m2) to hectare.

2.5 Above-ground biomass of woody species calculation

The above-ground biomass of woody species within the sampling plots was determined using the following parameters: (1) height, (2) wood density, and (3) diameter at breast height. The allometric equation was used to determine the above-ground biomass of woody species with DBH ≥ 10 cm within the sample plots.

2.6 Allometric equation for above-ground biomass of tree species

The allometric equation described by Henry et al. (2010) was used. The equation is shown below:

$${\text{Biomass}}=0.0347\times {\left({\text{SG}}\right)^{\wedge}}_{2}\times {\left({\text{D}}\right)^{\wedge}}_{2}\times {\text{H}}$$
(2)

where SG: Wood density; H: woody species height; D: woody species diameter at breast height.

2.7 Belowground biomass of tree species calculation

The below-ground biomass was determined using the equation by Mokany et al. (2006) as appropriate.

$$\mathrm{BGB of tree}=0.205\times \mathrm{AGB if AGB }\le 125 {{\text{mgha}}}^{-1}$$
(3)
$$\mathrm{OR BGB }= 0.235\times \mathrm{AGB if AGB }>125 {{\text{mgha}}}^{-1}$$
(4)

where AGB: above -ground biomass; BGB: below-ground biomass.

2.8 Allometric equation for shrub species

The shrub above-ground biomass was estimated using an allometric equation developed by Ali et al (2015):

$$\mathrm{In }\left({\text{AGB}}\right)=-3.50+1.65\times \mathrm{In }\left({\text{D}}\right)+0.842\times \mathrm{In }({\text{H}})$$
(5)

where ABG: above-ground biomass (KG); In: natural logarithm; H: total height (m); D: diameter of the stem (cm).

2.9 Carbon stocks estimation

Above-ground carbon content was estimated using an allometric equation developed by Kauffman and Donato (2012).

$${\text{Above}}-\mathrm{ground\ carbon\ content }=\mathrm{ AGB}\times 0.50$$
(6)

Below-ground carbon content was estimated using an allometric equation developed by Kauffman and Donato (2012)

$$\mathrm{Below-ground\ carbon\ content }=\mathrm{ BGB }\times 0.39$$
(7)

2.10 CO2 emission estimation

Estimated C stocks were converted into CO2 equivalents (quantity of C × 44/12) for calculating CO2 stock by biomass of trees (Aryal et al. 2013; Bhatta et al. 2018)

$${{\text{CO}}}_{2}{\text{e}}=\mathrm{C }\times 3.67$$
(8)

Figure 2 below indicates the flowsheet of the methodology used in the study.

Fig. 2
figure 2

Flowsheet of the Methodology

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3 Results

3.1 Biomass, carbon stock, and CO2 emission values of tree species in the study areas

Biomass of tree species in disturbed plots ranged from 0.001 Mg ha-1 to 7.850 Mg ha-1 in disturbed plots while it ranged from 0.001 to 16.631 Mg ha-1 in undisturbed plots. The results showed that Chrysophyllum albidum had the lowest total biomass value of 0.001 Mg ha-1 and Albizia zygia had the highest total biomass value of 7.850 Mg ha-1 in disturbed plots, while Spathodea campanulata had the lowest total biomass value of 0.001 Mg ha-1 and Ceitis zenkeri had the highest total biomass value of 16.631 Mg ha-1 in undisturbed plots (Table 1).

Table 1 Biomass (Mg ha-1), carbon stock (Mg C ha-1), and CO2 emission (Mg C ha-1) of tree species in the study areas
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The total carbon stock in disturbed plots was found to be the lowest in Discoglypremna caloneura and Chryophyllum albidum (0.001 Mg C ha-1) and the highest was found in Albizia zygia (3.925 Mg C ha-1), while the total carbon stock in undisturbed plots was found to be the lowest in Spathodea campanulata (0.001 Mg C ha-1) and highest in Ceitis zenkeri (8.316 C Mg ha-1). The CO2 content in disturbed plots ranged from 0.002 to 14.405 Mg C ha-1, with Chryophyllum albidum having the lowest value (0.002 Mg ha-1) and Albizia zygia having the highest value (14.405 Mg C ha-1), while, the CO2 content in undisturbed areas ranged from 0.002 Mg C ha-1 to 30.518 Mg C ha-1, with Spathodea campanulata having the lowest value (0.002 Mg C ha-1) and Ceitis zenkeri having the highest value (30.518 C Mg ha-1) (Table 1).

Figure 3 shows the total value of biomass, carbon stock, and CO2 content of all the trees in both disturbed (70.768 Mg ha-1, 35.384 Mg C ha-1, and 129.859 Mg C ha-1 ) and undisturbed plots (102.645 Mg ha-1, 51.323 Mg C ha-1, and 188.354 Mg C ha-1).

Fig. 3
figure 3

Total Carbon Stock, CO2 Emission, and Biomass Values of all Trees in the Study Areas

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3.2 Biomass, carbon stock, and CO2 emission values of shrub species in the study areas

The biomass values of shrubs in disturbed plots varied from 0.001 to 0.056 Mg ha-1, with Alchornea cordifolia species having the lowest value of 0.001 Mg ha-1 and the Solanum erianthum species having the highest value of 0.056 Mg ha-1, whereas the biomass value of shrub species in undisturbed plots varied, with Alchornea cordifolia species having the lowest value (0.001 Mg ha-1) and Baphia nitida species having the highest value (0.015 Mg ha-1) (Table 2).

Table 2 Biomass, carbon stock, and co2 emission of shrub species in the study areas
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The carbon stock values for the shrub species were found to range from 0.003 to 0.028 Mg C ha-1, with Alchornea cordifolia species having the lowest value of 0.003 Mg C ha-1 and the Solanum erianthum species having the highest value of 0.028 Mg C ha-1 in disturbed plot, while in undisturbed plots, the carbon stock of shrub species was found to be lowest in Alchornea cordifolia species (0.001 Mg C ha-1) and highest in Baphia nitida species (0.008 Mg C ha-1) (Table 2). The CO2 emission values in disturbed plots vary from 0.001 Mg C ha-1 for the Alchornea cordifolia species to 0.103 Mg C ha-1 for Solanum erianthum species, while the CO2 emission of shrub species encountered in undisturbed plots varied from 0.2 to 4.64 Mg C ha-1, with Chytrathus macrobotryx species having the lowest value (0.001 Mg C ha-1) and Baphia nitida species having the highest value (0.028 Mg C ha-1) (Table 2). Figure 4 indicates the total values of biomass, carbon stock, and CO2 emission of all the shrubs in both disturbed (0.123 Mg ha-1, 0.061 Mg C ha-1 , 0.225 Mg C ha-1, and undisturbed plots (0.067 Mg ha-1, 0.034 Mg C ha-1, 0.124 Mg C ha-1).

Fig. 4
figure 4

Total Biomass, Carbon Stock, and CO2 Emission of all Shrubs in the Study Areas

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3.3 Biomass (Mg ha-1) and carbon stock (Mg C ha-1) of woody species (tree and shrub) in disturbed and undisturbed plots in the study areas

The total biomass (sum of tree above-ground, tree below-ground, and shrub above-ground biomass) in disturbed plots ranged from 6.862 to 8.131 Mg ha-1 while it ranged from 13.212 to 29.1412 Mg ha-1 in undisturbed plots. The overall total of biomass woody species (trees and shrubs) in disturbed plots was recorded as 52.888 Mg ha-1 whereas it was recorded as 103.861 Mg ha-1 in undisturbed plots (Table 3). The results showed that total carbon stock (sum of tree above-ground, tree below-ground, and shrub above-ground biomass) in disturbed plots ranges from 3.423 to 4.067 Mg C ha-1 whereas it ranged from 6.598 to 14.566 Mg C ha-1 in undisturbed plots. The overall total carbon stock in trees and shrubs in disturbed plots was recorded as 26.425 Mg C ha-1 while it was recorded as 53.324 Mg C ha-1 in undisturbed plots (Table 4).

Table 3 Biomass (Mg ha-1) of tree and shrub species in disturbed and undisturbed plots in the study areas
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Table 4 Carbon stock (Mg C ha-1 ) of tree and shrub species in disturbed and undisturbed plots in the study areas
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3.4 Correlation analysis of biomass, carbon stock, and CO2 emission values of shrub and tree species in the study areas

In undisturbed plots of shrubs, biomass, carbon and CO2 emissions have a strong positive correlation of 1.000**. The correlation analysis indicates that there is a very strong positive linear relationship between biomass, carbon stock, and CO2 emission. When the biomass of an undisturbed plot (shrubs) increases, both the carbon stock and CO2 emission levels also increase significantly. Similarly, when the carbon stock increases, there is a corresponding increase in both biomass and CO2 levels. Furthermore, an increase in CO2 emission levels is associated with higher biomass and carbon stock. In disturbed plots of shrubs, just like in undisturbed plots, the same occurrence was observed (Fig. 5).

Fig. 5
figure 5

Correlation Analysis in Disturbed and Undisturbed Plots of the Tree

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However, in undisturbed plots of trees, biomass, and carbon have a very strong positive correlation of 0.999**. Biomass and CO2 emissions have a very strong positive correlation of 0.999**. Carbon and CO2 emissions have a very strong positive correlation of 1.000**. In undisturbed tree plots, there is a strong positive linear relationship betweenbiomass, carbon, and CO2 emission levels. This implies that as the biomass of trees increases, both the carbon stock and CO2 emission levels also increase significantly. Similarly, an increase in carbon stock is associated with higher biomass and CO2 emission levels. Moreover, higher CO2 emission levels are correlated with greater biomass and carbon stock content. In disturbed tree plots, there is a moderate to strong positive linear relationship between biomass, carbon, and CO2 emission levels. This indicates that as the biomass of trees increases, there is a tendency for both the carbon stock content and CO2 emission levels to increase, though the relationship is stronger between carbon stock content and CO2 emission. Biomass and carbon stock content have a moderate positive correlation of 0.458**. Biomass and CO2 emissions have a moderate positive correlation of 0.469**. Carbon stock content and CO2 emission have a very strong positive correlation of 0.974** (Fig. 6).

Fig. 6
figure 6

Correlation Analysis in Disturbed and Undisturbed Plots of the Shrubs

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4 Discussion

The total mass of living things within a certain region or ecosystem is known as its biomass. Biomass, as it relates to trees, is the live plant material, which includes the roots, stems, branches, leaves, and other plant parts (Ben-Iwo et al. 2016). One essential element of the carbon cycle is the amount of carbon in biomass. About half of the dry weight of plant biomass is made up of carbon (Houghton et al. 2009). In the current study, the total biomass in undisturbed plots of the tree was higher than that of disturbed plots. The higher total biomass in tree species recorded in undisturbed plots (0.001 to 7.850 Mg ha-1) compared to the disturbed plots (0.001 to 16.631 Mg ha-1) could be a result of the stand structural metrics (tree height, DBH, and number of individuals) in the forest reserve that are substantially higher than those in the disturbed plots (Adeyekun et al. 2020; Nwobi and Williams 2021). The variation in the value of biomass obtained in different plant species is consistent with the findings of Odiwe et al. (2017), who obtained the highest biomass accumulation for Celts zenkeri in their study on secondary forests in Ile-Ife, Nigeria.

The higher total biomass in tree species recorded, compared to the disturbed, could also be probably due to age and less disturbance since minimal anthropogenic activities were observed in the undisturbed plots. The differences in biomass observed in the plots can also be attributed to the diversity of plant species, growth forms and stages, basal area, densities, ages, and diversities encountered in the disturbed and undisturbed plots. These findings are consistent with the observation of Sharma et al. (2016), who reported that above-ground biomass levels can vary depending on location. It has been documented that variations in tropical forests can be attributed to a range of factors including disturbance history, species diversity, stem density, canopy height, stem size distribution, soil conditions, the potential for sequestration, ecological adaptation, and management strategies of woody species (Conti and Díaz 2013; Maryo et al. 2023). This suggests that in order to diversify land use types with a varied composition of woody species, extensive management methods must be put in place.

Determining the effect of disturbances on carbon sequestration requires knowledge of the carbon stock values of tree species in both disturbed and non-disturbed environments (Enkossa et al. 2023). When compared to their undisturbed counterparts, landscapes that are disturbed due to human activity or deforestation have lower carbon reserves (Gebeyehu et al. 2019). Disturbances cause carbon stores to reduce, which has an impact on tree species' capacity to sequester carbon overall (Muluneh and Worku 2022). The total carbon stock values of trees in disturbed plots in this study vary from 0.001 to 3.925 Mg C ha-1 with Discoglypremna caloneura and Chryophyllum albidum having the lowest (0.001 Mg C ha-1) and Albizia zygia having the highest (3.925 Mg C ha-1) while the total carbon stock value of the tree in undisturbed plots was found to be lowest in Spathodea campanulata (0.001 Mg ha-1) and highest in Celtis zenkeri (8.316 Mg ha-1). The result of higher carbon stock recorded for undisturbed plots compared to disturbed plots is consistent with the report of Daba et al. (2022) who worked on the impact of forest management on carbon stock of tropical moist Afromontane Forests in disturbed and undisturbed forests.

The highest total carbon stock value recorded in Celtis zenkeri (627.79 Mg ha-1) in undisturbed plots is consistent with the findings of Odiwe et al. (2017), who obtained the highest carbon stock accumulation for Celts zenkeri in their study in secondary forest in Ile-Ife, Nigeria. The highest carbon sequestration per tree recorded in an undisturbed plot could be a function of tree species' wood density (Redondo-Brenes 2007). Variations in the overall ecosystem carbon stores across different regions may be related to ecological distinctions, management activity levels, and the model used to estimate carbon stocks (Tadese et al. 2023). Generally, forest disturbance has been pointed out to alter the forest stand structure thereby influencing site-level carbon stocks (Nave et al. 2010).

Deforestation, logging, and changes in land use are examples of human activities that can release stored carbon as CO2. A measurement of the carbon stock in woody vegetation helps to reduce atmospheric CO2 emissions because when trees are cut down or burned, the carbon stored in their biomass is released as CO2 (McNicol et al. 2018). The CO2 emission value of the tree recorded in disturbed plots in this study varied from 0.002 to 14.405 Mg C ha-1 while the CO2 emission value of the tree in undisturbed plots ranged from 0.012 Mg C ha-1 to 30.518 Mg C ha-1 with Sterculia gigantea having the lowest value (0.01 C Mg ha-1) and Celtis zenkeri having the highest value (4887.52 Mg C ha-1). Generally, the higher CO2 emission of trees in undisturbed plots compared to the values obtained in disturbed plots might be due to the higher density of the tree (Ghimire et al. 2018) and basal area (Amir et al. 2018). Estimating the biomass of the forest ecosystem allows us to estimate the amount of CO2 emission that the forest can sequester from the atmosphere. Forests cover over half of the world's tropical forest area (FAO 2010), sequester more carbon, and thus play an important role in the global carbon cycle.

As regards shrub species, the results show that the disturbed plots have higher biomass, carbon stock, and CO2 emission values than the undisturbed plots. The higher values recorded in disturbed plots could be a result of more availability of light since the place is opened, which allows a higher number of shrubs to grow faster and accumulate more carbon. However, it is important to note that this increase in biomass and carbon stock may not be sustainable in the long term, as continued disturbance can lead to soil degradation and loss of biodiversity (Fearnside and Guimarães 1996; Chazdon 2014; Abdelrahman 2023).

Woody species conservation has varying potentials depending on the type of land used. Undisturbed Plots were more important for the conservation of woody species than disturbed plots (Maryo et al. 2023). The results of this study indicated that the total biomass of woody species in disturbed plots was 52.888 Mg ha-1, while in undisturbed plots it was 103.861 Mg ha-1. Similarly, the total carbon stock of woody species in disturbed plots was 28.425 Mg C ha-1, while in undisturbed plots it was 53.324 Mg C ha-1. The higher carbon storage observed in undisturbed plots may be due to factors such as higher species density, basal area, and DBH (Brown 1997; Kothandaraman et al. 2020; Chaudhary and Aryal 2023). It is known that tree density can affect biomass levels, with greater tree density resulting in higher biomass (Gautam and Mandal 2016). The distribution of biomass and carbon stocks in a forest can vary due to a range of factors including tree species, soil nutrient availability, climate, and disturbance regimes (Houghton 2005; Joshi and Dhyani 2019).

Woody species in disturbed and undisturbed areas are correlated in a complicated way that includes differences in diversity, structural alterations, and specialized distribution patterns. Natural or man-made disturbances are important in determining the distribution, variety, and composition of woody species in ecosystems (Xi et al. 2021). Comprehending the complex relationships between disturbed and undisturbed shrub and plant areas offers a comprehensive perspective on ecosystem performance and adaptability to external disruptions. The strong positive correlations observed in undisturbed shrub plots suggest that undisturbed shrub plots with higher biomass are contributing to increased carbon storage and CO2 emission levels. This implies that healthy and thriving shrub ecosystems play a crucial role in carbon sequestration, which is important for mitigating climate change. The correlation analysis in disturbed shrub plots indicates that despite disturbances, the relationships between biomass, carbon, and CO2 remain significant and areas affected by disturbances continue to play a role in the global carbon cycle and climate regulation. This finding agrees with Pugh et al. (2019), who reported the important role of forest disturbances in global biomass turnover and carbon sinks.

In the same vein, the strong positive correlations observed in undisturbed tree plots suggest that as trees grow and accumulate biomass, they store more carbon, which contributes to mitigating the levels of CO2 emission in the atmosphere. This implies that undisturbed tree plots play a crucial role in carbon sequestration and also underscores the importance of undisturbed forests in mitigating climate change. This finding aligns with the work of Joshi and Singh (2020), who reported the carbon sequestration potential of disturbed and non-disturbed forest ecosystems. The fact that there is a positive correlation between biomass and CO2 emission level, despite disturbances, suggests that the disturbed plots still contribute to absorbing CO2 emission from the atmosphere and this underscores the resilience of ecosystems to maintain their capacity to sequester carbon even when subjected to disturbances. The moderate correlation between biomass and carbon implies that disturbances in these plots can lead to changes in carbon storage, either through biomass loss or regrowth. This finding is consistent with the results of Harris et al. (2016), who reported the attribution of net carbon change by disturbance type across forest lands. The limitations of this work include lack of funding, time frame, site-specific nature, and management history.

5 Conclusion

Knowledge about variation in biomass, carbon stock, and CO2 emission of woody species in disturbed and undisturbed areas is pertinent for a clear understanding of forest ecosystems and causes of climate change. To reduce this problem, the urgent need to focus on studies on forest biomass and carbon stock estimation of tropical forests is crucial, as they contribute 50% of global carbon stocks and one-third of global primary productivity (Salunkhe et al. 2023). The objective of this study focuses on the nexus between carbon stock, biomass, and CO2 emissions of woody species in the Southwestern forest reserve, Nigeria. Here, the girth sizes of the woody species in disturbed and undisturbed plots in this study were measured at a height of 1.3 m above the ground (DBH) for plants ≥ 3.0 m in height and at the midpoint for those ≤ 3.0 m in height, using a diameter tape.

The results showed that total biomass, total carbon stock, and total CO2 emission values of tree species in undisturbed plots were higher than the values of total biomass, total carbon stock, and total CO2 emission value recorded in disturbed plots. The results also revealed that the total biomass total carbon stock, and total CO2 emission values of shrub species recorded in disturbed plots were higher than the values of total biomass, total carbon stock, and total CO2 emission recorded in undisturbed plots, respectively. The study concluded that disturbances, such as human impact or land-use changes, can significantly affect the levels of biomass, carbon stock, and CO2 emission in both shrubs and trees. The study also concluded that the main purpose of establishing reserve forests seems not to be totally achieved as human activities exploitation in the forest reserve are still increasing climate change. Finally, it concluded that the values of biomass, carbon stock, and CO2 emission in both disturbed and undisturbed plots follow this trend: CO2 emission>biomass>carbon stock.

Further study should consider the economic value of different woody species and the cost-effectiveness of conservation strategies which could be essential for practical conservation management.