The capital city of the Sultanate of Oman is Muscat, which is located at 58˚32'43.02''E and 23˚36'51.58''N. It is classified as a hot arid climate (BWh) environment based on Köppen climate nomenclature (Al-Rasbi & Gadi, 2021). Rainfall in the country is often less than 100 mm per year, well below the global average of 1123 mm (Al-Ghafri et al., 2014). The observed annual mean air temperature (˚C) and rainfall (mm) levels as of 2016-2019 in the study area are 28.6˚C and 86.0 mm respectively (CAA, 2020). The city of Muscat is home to a number of parks and gardens, in addition to having trees lining its roads and streets. The Directorate General of Landscaping and Public Parks, which falls under the purview of the Muscat Municipality, is in charge of maintaining all different types of vegetation. This research was done in Muscat city covering different parks and roads distributed across the city with a total of 100 plots and 41 transects distributed in different locations (Figure 1).

To more accurately reflect the urban vegetation under review, stratified sampling was employed (Zhang et al., 2015; Bijalwan et al., 2010). The fieldwork for the survey started on December 1, 2020 and lasted for a full four months, finishing on March 28, 2021. A 30 m by 30 m plot was set up in each of the parks used for the samples. The exact coordinates of each plot were determined using a Global Positioning System (GPS-Garmin −62s) to within three meters. The GPS coordinates were used to determine the focal point, and a plot was drawn in the surrounding area. For roadside plantation, a systematic sampling using line transect was conducted (Adeyemi, 2016; Fan et al., 2007) and tree trunks were measured every 100 m with 50 m intervals in every 1 km transact.

During the fieldwork, the scientific names of various tree species were documented. Diameter at breast height (DBH) is a biometric feature (Getnet & Negash, 2021), that is generally measured 1.30 meters off the ground (Ali et al., 2020; Miah et al., 2020). At this height above the ground, a measuring tape was utilized to get the value of the circumference at breast height (CBH) of each tree. When there was more than one stem with a height of fewer than 1.30 meters, separate measurements were collected for each of the stems (Yilma & Derero, 2020). In addition, small saplings were included by measuring the circumference at base height (also known as CBH), at 10 centimeters above the ground. Then, the diameter of the trunk was calculated by using the following equation (Snehlata et al., 2021).

D = C π (1)

where D = Diameter, C = Circumference, π = pi = 3.14.

To determine the biomass and, hence, the amounts of carbon stored above and below ground in trees, non-destructive allometric approach was used. Site- or species-specific allometric equations are preferred because they yield more

accurate estimates of biomass and carbon. When such specialized equations are unavailable, however, general ones are often substituted (Ekoungoulou et al., 2018). Since species-specific biomass equations are not available, generic biomass equations are used from different models (Brown, 1997; Cairns et al., 1997; Yao et al., 2015; Snehlata et al., 2021).

AGB = 42.69 − 12.800 ( DBH ) + 1.242 × ( DBH ) 2 (2)

BGB = exp [ − 1.0587 + 0.8836 × ln ( AGB ) ] (3)

AGB = 0.182 × D 2.487 (4)

BGB = AGB × 0.24 (5)

Since forest trees often have a higher biomass than trees planted in urban environments, these allometric equations are typically developed from forest trees. As urban trees have different surroundings in terms of space, light and nutrients that natural forest, variation in biomass is multiplied by a conversion factor of 0.8 (Dangulla et al., 2021; Lahoti et al., 2020). Tree carbon storage capacity was evaluated using biomass values and a carbon default value of 0.50 (Mngadi et al., 2021; Shen et al., 2020). Afterwards, in order to produce CO₂ equivalent (CO₂eq), the amount of carbon present was multiplied by 3.67, the ratio of the molecular weight of carbon dioxide to the atomic weight of carbon (Amiri et al., 2020).

CO 2 e q = 0.5 × Biomass × 3.67 × 0.001 (6)

where; 0.001 = factor for converting CO₂eq from kg to tonne.

The following chart illustrates the methodology used in this study (Figure 2).

A total of 2860 trees belonging to 36 species were recorded at this sampling area. Azadirachta indica (25.5%), Conocarpus erectus (19.2%), Ficus spp. [Ficus benjamina, Ficus religiosa, Ficus benghalensis, Ficus macrocarpa, Ficus elastica, Ficus nitida] (15.5%), Tabebuia rosea (9.2%), Peitophorum pterocarpum (9.0%) were the dominant species among the sampling sites (Figure 3). The DBH categories ranging from <50.0 cm, 50.0 - 99.9 cm, 100.0 - 149.9 cm, 150.0 - 199.9, and >200.0 cm each accounted for 74.0, 22.8, 2.9, 0.2, and 0.1 percent of the total, respectively (Figure 4), with total number of trees falling in each category as 2118, 651, 83, 6 and 2 respectively. The mean DBH of all sampled trees was 40.8 cm, while maximum value was 233.4 cm. However, the maximum mean DBH values recorded were 54.6 cm, 57.4 cm, 44.6 cm, 43.8 cm, 51.5 cm, 42.5 cm, 41.9 cm and 40.8 cm for Ficus spp., Triadica sebifera, Albizia lebbeck, Azadirachta indica, Syzygium cumini, Peltophorum pterocarpum, Conocarpus erectus, and Casuarina equisetifolia respectively (Figure 5). Table 1 shows the percentage of DBH classes in parks and roadside.

In general, mean DBH of the trees that were measured ranged from 3.2 to 57.4

cm, which was a smaller than the DBH range of trees (11.0 to 77.1 cm) that was found in the urban regions of Delhi, India (Snehlata et al., 2021); and in Pakistan (11.93 - 50.96 cm) (Zubair et al., 2022). Sharma et al. (2020) found a maximum of 298.7 cm at Amity University campus, India. Such differences are attributable to the fact that climatic circumstances vary from place to place, in addition to the relative abundance of mature and young trees.

The most dominant species in terms of carbon sequestration potential are Ficus

spp., Azadirachta indica and Conocarpus erectus with a total of 3399.3 tCO₂eq, 2845.2 tCO₂eq, 2286.9 tCO₂eq, which represents 76.1% of the carbon sequestered by trees in the study area. The next set of species Albizia lebbeck, Tabebuia rosea, Syzygium cumini, Pongamia pinnata, Delonix regia contributed by 13.3%, and the remaining 22 species only by 3.0% (Table 2). However, the total number of trees that belong to the same species can have a significant impact on this

percentage contribution. In addition to that, the age of trees and other factors that affect the growth of the tree could influence the total biomass. Table 3 shows the contribution of the dominant species. Research on similar species done in Pakistan by Ajani and Shams (2016), found that Azadirachta indica can store 662.3 kg C, and Conocarpus erectus 192.7 kg C and in India by Sharma et al. (2020), estimated that Ficus benjamina can sequester 30.53 tons of carbon. These studies measure less number of trees compared to this work. In contrast, a study in Nigeria conducted by Eneji et al. (2014), found that Azadirachta indica can store 5448.8 kg C, and Albizia lebbeck 1040.4 kg C.

The total carbon storage which represents above-ground carbon and below-ground carbon varied among the locations of sampling with a significant difference (P ≤ 0.05), ranging between 1425.9 tonne and 1632.6 tonne for Parks and roadside respectively. The total amount of CO₂ equivalent (CO₂eq) sequestered by the trees in the parks is about 5227.6 tonne, with an average of 3.2 ± 0.1 tonne per tree. In contrast, plantation along roadside has the capability to sequester 5985.7 tonne with an average of 4.9 ± 0.2 tonne per tree (Figure 6). It

seems that roadside plantation is sequestering more CO₂eq during their life span up until this study. In general, all parks and roadside plantation covered in this study have the capability to sequester 11,213.3 tCO₂eq with an average of 3.9 ± 0.1 tonne per tree. A study conducted by Amoatey and Sulaiman (2019), estimated that urban green spaces in Muscat can sequester around 11,100 tCO₂/ha. Another study with similar objective to measure carbon storage in the same city but in the aquatic ecosystem (a piece of mangrove site within the city), found that it has the capability to sequester around 9512 tCO₂eq (Al-Nadabi & Sulaiman, 2021).