Ecosytem Services Assessment of Urban Forests of Adama City, Ethiopia

Background: The recent urban challenges due to climate change and urban environment deterioration requires proper planning and inventories of urban forests. In this paper, trees and shrub information were used to estimate leaf area/biomass, carbon storage, carbon sequestration, pollution removal, and volatile organic compound (VOC) emissions, hydrological and functional values of Adama city urban forest. This study was conducted to assess and quantify the ecosystem services of urban forests of Adama city, Central Ethiopia. Results: The result of i-tree Eco model has indicated that the tree species such as Azadirachta indica, Eucalyptus globulus, Carica papaya and Delonix regia sequester high percentage of carbon which is approximately 14.7%, 7.4%, 7.3% and 6.2% of all annually sequestered carbon respectively. Besides, urban forests of the city was estimated to store 116,000 tons of carbon; the most carbons were stored by the species such as Eucalyptus globulus, Azadirachta indica, Carica papaya and Delonix regia that stores approximately 22.1%, 12.3%, 9.5% and 4.2% of all stored carbon respectively. Trees in Adama urban forests were estimated to produce 19.93 thousand tons of oxygen per year. It was estimated that trees and shrubs remove 188.3 thousand tons of air pollution due to O 3 , CO, NO 2 , PM2.5 and SO 2 per year. In the city, 35 percent of the urban forest's VOC emissions were from Eucalyptus cinerea and Eucalyptus globulus. Besides, the monetary value of Adama urban forest in terms of carbon storage, carbon sequestration, and pollution removal was estimated to 16,588,470 ETB/yr, 118,283 ETB /yr and 12,162,701,080. 9 ETB /yr respectively. Conclusion:


Introduction
In our world, human population growth and urbanization have adverse environmental impacts such as elevated temperatures, increases in air pollution and stormwater quantity, and decreases in stormwater quality, which pose major environmental and public health problems in cities (Rydin et al. 2012;Seto and Shepherd, 2009). In this regard, urban forest ecosystem plays an important role in providing multiple service and environmental bene ts to urban environment (Forrest et al. 1999 ;Strohbach & Haase, 2012).
Ethiopia has one of the largest urbanization rates (about 4-5%) in the world, and its urban population is expected to increase from time to time. Also, urbanization at a rapid pace is a reality at present (Rama, 2013). The current phenomenon in Ethiopia has been associated with environmental problems in most cities. The major problems are urban sprawl, solid and liquid waste management; water, air, and noise pollution; illegal settlements and the degradation of open green areas (Thomas, 2013). 15% in 2000 to almost 30% in 2030 (UN Population Division, 2004). Ethiopia is experiencing the effects of climate change such as an increase in average temperature and change in rainfall patterns.
There are several techniques and models that have been developed to help quantify ecosystem services, such as i-Tree Eco and i-Tree Streets (i-Tree, 2010a). In this work, i-Tree Eco is a software suite were used for the analysis. i-Tree Eco was designed to use standardized eld data from randomly located plots, as well as local hourly air pollution and meteorological data, to quantify urban forest structure, ecological function, and the associated value (Nowak et al. 2008a, McPherson 2010b. The main aim of this study is assessing the ecosystem service of urban forest of Adama city interms of climate change mitigation; speci cally, the study was intended i) to assess carbon storage and sequestiration potential of adama city trees ii) to estimate the oxiygen production and pollution removal by different species of adama city trees and iii) assess the hydrological and functional values of trees in Adama city.

Study Area
This study was conducted in Adama city of Oromia Regional State, Central Ethiopia. Adama city is geougraphically situated between 8° 32′ 24″ N, latitude and 39° 16′ 12″ E longitude within the altitudinal range of 1,712 meter a.s.l. (Fig. 1). The total area of the city is about 13,366.5 hectar and 99 km far from Addis Ababa the capital city of Ethiopia. The annual average minimum and maximum temperature of the study area is 13 0 c and 27 0 c, respectively. The annual average rainfall is 837-1005.7 mm and climate varies due to the great variation in altitude (BoFED, 2012). The total population of Adama is about 303,569 of which 150,228 are males and 153,341 are females. Currently, the city contains 18 kebele administrations.

Research Design and Sampling
The reconessance survey was conducted (from October to December, 2018) by a team of 5 people. The site assessment has done to observe the general plot information used to identify the plots and its general characteristics. In this work, trees and shrub information were used to estimate trees and shrubs leaf area/biomass, pollution removal, and volatile organic compound (VOC) emissions. Finally, tree informations used to estimate forest ecosystem value, carbon storage, carbon sequestration and hydrological functions of Adama city urban forest.
In this study, a total of 214 sample plots (27 percent of the city) have established by using a simple random sampling method. As a general rule, 200 plots (one-tenth acre each) will yield a standard error of about 10% for an estimate of the entire city. As the number of plots increases, the standard error will be decrease; and therefore we were more con dent to estimate for the population. With regard to the sample plot size, the standard plot size for an Eco analysis is a 0.1-acre circular plot with a radius of 11.16 m or 0.0407 hectares. The samples of plots were created directly in the Eco application using the random plots generator via the Google Maps function (Fig. 2).
The diameters of all identi ed trees and shrubs were measured at breast height (1.3 m above ground) using a diameter tape (5 m length). Diameter of individual trees were recorded to calculate basal area and relative basal area of plant species. Height of all sampling trees and shrubs were measured by silva hypsometer.
The eld data collection crews were typically located eld plots using maps to indicate plot location. Aerial photographs and digital maps were used in order to locate plots and features. During random plots distribution in the city, the researchers faced a challenge of miss place placement of some plots; for example, some plot center has fallen in buildings, private land and the border of different land ownerships and land-use types; as a result the researchers professional skills were used to shift the plot center into appropriate locations.

Data collection and analysis
In this study, the data was collected from sample plots which have an area of 0.0407 ha (1/10 ac) that randomly laid in city areas of states and data was analyzed using the i-Tree Eco (formerly Urban Forest Effects (UFORE)) model (Nowak et al., 2008). The state plots were based on Forest Inventory Analysis national program plot design and data were collected as part of pilot projects testing FIA data collection in urban areas (Cumming et al., 2008). For each tree fournd in the sample plots carbon storage, annual sequestration, oxygen production, pollutant removal and hydrological functions were estimated using biomass and growth equations. Inorder to carryout in national estimates of carbon storage and sequestration, the carbon data was standardized per unit of tree cover.

Results
The results of this study were from a complete tree inventory and i-Tree Eco analysis of the 214 plots from Adama city, Central Ethiopia. In this section, the structure, carbon storage, carbon sequestration, volatile organic compound (VOC) emissions, air pollution removal and hydrological functions of Adama city urban forest were analyzed and presented in detail.

Structure of tree species of Adama tree
During data collection, trees were identi ed to the most speci c taxonomic classi cation possible. In this work, eld data were collected during the leaf-on season to properly assess tree canopies. Typical data collection includes land use, ground and tree cover, individual tree attributes of species, stem diameter, height, crown width. In this work a total of 86 woody speiceis have identi ed and the height, crown area, DBH of 806 trees and shrubs were measured at eld level.
Leaf area of trees were assessed using measurements of crown dimensions and percentage of crown canopy missing. In the event that these data variables were not collected, they are estimated by the model. Many tree bene ts equate directly to the amount of healthy leaf surface area of the plant. Trees cover about 20 percent of Adama city trees and provide 8.871 square miles of leaf area. Indeed, total leaf area is greatest in urban areas. In Adama urban trees, the most dominant species in terms of canopy cover and leaf area are Acacia albida, Casimiroa edulis, and Eucalyptus cinerea. The attributes of 20 species were presented in (Table 1).

Carbon Storage and Sequestration
Trees reduce the amount of carbon in the atmosphere by sequestering carbon in new growth every year. The amount of carbon annually sequestered is increased with the size and health of the trees. The gross sequestration of Adama city trees is about 8,291 thousand tons of carbon per year with an associated value of Eth. ETB 1.18 million. Net carbon sequestration in the urban forest is about 7,474 thousand tons. The most common species that are known for the greater share of carbon sequestration in adama urban forest are listed in (Table 2). In particular, the tree species such as Azadirachta indica, Eucalyptus globulus, Carica papaya and Delonix regia sequester the most perecentage of carbon which is approximately 14.7%, 7.4%, 7.3% and 6.2% of all annually sequestered carbn respectively (Fig. 3).

Air Pollution Removal by Urban Trees
Pollution removalby trees and shrubs in Adama city trees was estimated using eld data and recent pollution and weather data available. removal was greatest for sulfur dioxide (Fig. 4). It is estimated that trees and shrubs remove 188.3 thousand tons of air pollution (ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), particulate matter less than 2.5 microns (PM2.5), and sulfur dioxide (SO2)) per year with an associated value of Eth. ETB. 26.2 billion.

Volatile Organic Compaound Emisson
In 2018, trees in Adama city emitted an estimated 51.44 tons of volatile organic compounds (VOCs) per year (33.81 tons of isoprene and 17.63 tons of monoterpenes). The emissions vary among species based on species characteristics (e.g. some genera such as Grevellia robusta was high isoprene emitter) and amount of leaf biomass. In Adama city, 35 percent of the urban forest's VOC emissions were by Eucalyptus cinerea and Eucalyptus globulus. These VOCs are precursor chemicals to ozone formation.

Eco bene t of Adama urban forest
The summary of Ecosystem value that include number of trees, carbon storage and sequestration, pollution removal, and structural value of woody species of Adama urban forest were estimated and summarized in (Table 5).

Discussions
This study provided a quantity of the C stored and sequestered by urban trees in Adama city of Central Ethiopia. The result of carbon sequestration and storage of Adama city was appeared higher than carbon assessment work conducted in cities such as Padua, Bolzano and Florence, Lisbon,Portugal, Zurich Switzerland (Crema 2008;Paoletti et al. 2011;Wälchli 2012). In the results current study the amount of carbon stored and sequestered in Adama urban trees was higher than result indicated in the study of Pace Rocco et al. (2018) regarding ecosystem services modeling for urban trees in Munich city of Germany; which was estimated to be 6225 ton and 214 tons per year respectively. Further more, the carbon storage and sequestration indicated in the current study were also compared with the study results presented for three cities of North America. Accordingly, the carbon storage and sequestration estimates of cities such as New York, Chicago and Jersey City were 1,225,200 & 38,400 tonn C − yr , 854,800 & 40,100 tonn C − yr and 19,300 & 800 tonn C − yr respectively (Nowak and Crane, 2002).
This comparison showed that the annual carbon storage and sequestration of the cites were higher than that of Adama city of Ethiopia except the annual carbon sequestration of Jersey City which was less than Adama city.
The C storage and sequestration results from this study were di cult to assess in terms of accuracy and to compare with other studies because of the use of different estimation methodologies, climatic condition, different species composition, and urban forest structures (Jo & McPherson 1995;Strohbach & Haase 2012).
The pollution removal indicated in this study was lower than the result reported form City of Baton Rouge which was 860 tons/year. In the work of Nowak et al. (2014) recently analyzed the effects of urban forests on air quality and human health in the United States, they found that in highly vegetated areas, trees can improve air quality by as much as 16% (Kroeger et.al 2014). Baumgardner et al. (2012) pointed out that around 2% of the ambient PM10 in Mexico City is removed from the study area. In a study carried out in the city of Barcelona (Spain), Barò et al. (2014) reported that urban forest services reduce PM10 air pollution by 2.66%. Moreover, in the Mediterranean city of Tel-Aviv, Cohen et al. (2014) observed that an urban park signi cantly mitigated nitrogen oxides (NOx) and PM10 concentrations, with a greater removal rate being observed in winter, and increased tropospheric ozone levels during summer.
In this result, the amount of annual Volatile Organic Carbon (VOC) removal was lower than the report of study conducted in Scotlandville's trees which yearly produce 8.91 tons of monoterpene, 125.53 tons of isoprene, and produce 134.43 tons of volatile organic compounds (VOCs); that may contribute to ozone formation. (Nowak & Dwyer 2007).
In Adama urban forest trees such as Acacia tortilis, Azadirachta indica and Ficus elastica have higher potenatial evapotranspirationa and transpiration (Table 4). Similary, Xiao and McPherson (2016) reported that trees in urban areas can increase the return of runoff to the atmosphere through transpiration, providing associated air cooling bene ts. Furthermore, according to the study of Gwynns Falls watershed in Baltimore indicated that heavily forested areas can reduce total runoff by as much as 26% and increase low-ow runoff by up to 13% compared with non-tree areas in existing land cover and land use conditions (Neville, 1996). Studies have also reported that tree cover over pervious surfaces reduced total runoff by as much as 40%; while tree canopy cover over impervious surfaces had a limited effect on runoff.
The Adama urban forest interms of monetory value have presented in the result sction (Table 5). The outcome of current study was compared with the study conducted in city of Baton Rouge the annual monetory value of urban forest service were lower, interms of Carbon storage ($6.2 million/year), Carbon sequestration ($41.0 million) and pollution removal ($ 1.1 million/year).
In general, this work has tried to quantify the ecosystem service value of Adama city of Ethiopia which will help for further urban forest development work and government intervention interms of policy and awareness creation. Further researches should be conducted the assess and evaluate the ecosystem service value of urban trees in several Urban Green Insfrustures (UGI) and comparing with different cities in the country. This will sensitize cities to learn and compute in urban forest development to enhance the ecosystem value of trees.

Conclusions
Urban forests are a signi cant and increasingly vital component of the urban environment that can impact human lives. Trees and forests have a positive effect on human health and well-being by improving air quality and reducing greenhouse gases, mainly through reducing air temperatures and energy use and through direct pollution removal and carbon sequestration. Understanding the value of an urban forest can give decision makers a better understanding of urban tree management.
These results provide baseline information for management recommendations to maximize the ecological bene ts provided by trees. By understanding the effects of trees and forests on the atmospheric environment, urban forest managers and policy makers can decide on the policy and strategic planning of urban greening. Subsequently, it will help for designing appropriate and healthy vegetation structure in cities to improve air quality and consequently human health and well-being for current and future generations. Estimated annual gross carbon sequestration (points) and value (bars) for urban tree species with the greatest sequestration, Adama city Annual pollution removal (points) and value (bars) by urban trees, Adama city