Ecosystem Services Provided By Urban Forests In The Southern Caucasus Region: A Modeling Study In Tbilisi, Georgia

All cities globally are growing considerably as they are experiencing an intensive urbanization process 10 resulting in significant economic, social, and environmental challenges. One of the major risks is the 11 deterioration of living environment in urban areas due to the high soil consumption and pollution of 12 environmental components. For this reason, cities are required to adopt measures to reduce air pollution 13 concentration and CO 2 emissions, preserve biodiversity and mitigate the urban heat island effect. In this 14 context, tree planting has been suggested as one cost-effective strategy because green infrastructures 15 can provide important environmental and social functions which contribute to the quality of life and 16 health of city dwellers. 17 Tbilisi is the largest city in Georgia, with a population of over 1,100,000 inhabitants (about 30% of total 18 population of Georgia). The green space availability in Tbilisi (5-6 m 2 per inhabitant) is low compared to 19 other European cities, and in recent years the need to increase the amount of urban vegetation has been 20 underlined at planning level. the in an Eastern European city the i-Tree Eco model to quantify the main ecosystem services from common tree species in Southern Caucasus. Two parks, Expo 24 Park (694 trees) and Red Park (1027 trees) in the city of Tbilisi have been measured and a model simulation 25 was performed for the year 2018. These urban store large amounts of carbon in their woody 26 tissues (198.4 t for Expo park and 126.5 t for park) and each year they remove 4.6 and 4.7 t of CO 2 for 27 Expo park and Red park. They positively contribute to the air quality by removing 119.6 and 90.3 kg 28 of pollutants (CO, NO 2 , 3 SO ), and reducing water of , respectively. This analysis highlighted the key role of urban forests in improving the environmental sustainability of the city of Tbilisi and provides important decision for the in geographic with the aim of maximizing the benefits can supply This study aims to evaluate the air quality and climate-related ecosystem services of two Tbilisi’s parks. The analysis has been carried out using the i-Tree Eco model, which for the first time has been applied in a scientific study of urban ecosystem services in the South Caucasus. Based on these results, the most common trees species in Southern Caucasus cities were evaluated and compared for more detailed and specific selection in future urban forestation programs.

In our study, we implemented for the first time in an Eastern European city the i-Tree Eco model to 23 quantify the main ecosystem services from common tree species in Southern Caucasus. Two parks, Expo 24 Park (694 trees) and Red Park (1027 trees) in the city of Tbilisi have been measured and a model simulation 25 was performed for the year 2018. These urban forests store large amounts of carbon in their woody 26 tissues (198.4 t for Expo park and 126.5 t for Red park) and each year they remove 4.6 and 4.7 t of CO2 for 27 Expo park and Red park. They also positively contribute to the air quality by removing 119.6 and 90.3 kg 28 of pollutants (CO, NO2, O3, PM2.5, SO2), and reducing water runoff of 269.5 and 200.5 m 3 , respectively. 29 During ongoing process of an accelerated urbanization, an additional emphasis is given to the proper 36 understanding and assessment of interactions between people and nature, particularly in the urban 37 environment (Kaplan 1983) where green spaces play a decisive role in providing essential services for 38 citizens (Unterweger et al. 2017). Improving the sustainability of cities and mitigating their impact on the 39 environment requires a careful assessment of environmental and closely related social, economic, 40 cultural, and ecological issues (Braun, 2005). Urbanization process has enhanced the detachment of 41 humans from nature and drastically reduced natural areas in cities (Cole at al. 2017), so that in major cities 42 green spaces as urban parks and gardens, tree-lined streets, and the vegetation of cemeteries are typically 43 the only chance for citizens to enjoy and get in touch with nature (Beatley 2009). 44 The bigger threat for the urbanized areas comes from the climate change, and cities will be among the 45 most affected ecosystems (Breuste at al. 2013). With more than half of global population domiciled in 46 cities and about 60% of mankind, expected to be urbanized in 2030 (World Cities Report, 2020), the social-47 ecological resilience of cities and ever-growing demand for ecosystem goods and services in the urbanized 48 areas, stay imbalanced and cities produce only a small amount of total ecosystem services (Kampelmann 49 2014). Currently, the urbanized areas cover only a small portion of the total surface of the planet that 50 approximates varies from 1 to 3% (Liu et  to compete with other urban infrastructures (roads, buildings, industrial facilities, general technical 55 infrastructure) for resources and space . 56 Urban areas are characterized by high-level of air pollution caused by anthropogenic sources such as from 57 the stationary (manufacturing, heating from the houses, energy generation factories, other industrial 58 sources) and mobile sources (automobiles and transport) (Shaddick et al. 2020). In addition to polluted 59 air, the Urban Heat Island Effect (UHI) contributes to the increased temperature in the cities ( Tbilisi has experienced several phases of its urban development and expansion, during the last two 115 centuries, transforming from a strategically located trading town into the city of over a million, with its 116 distinctive and diverse culture and important socio-economic role in the Caucasus. Along with the 117 processes of economic downfall, nationalism and the dramatic changes of social fabric, characteristic to 118 post-Soviet transition process (in the states of former USSR), Tbilisi as a modern globalizing metropolis, 119 has brought contradictions, such as undermining the city's heritage, contributing to socio-spatial 120 polarization (Salukvadze and Golubchikov 2016). During the Soviet Era (1921-1991) started the expansion 121 of Tbilisi territory and an intensive urbanization has occurred, when the city increased ten times in terms 122 of territory, and six times as population (Salukvadze and Golubchikov 2016 (Tbilisi Municipality, Green and Recreational Space Standard 2019). However, the major concern, related 129 to the green areas, is that they are located near the city limits, far from the "real city" contour. This study aims to evaluate the air quality and climate-related ecosystem services of two Tbilisi's public 157 parks. The analysis has been carried out using the i-Tree Eco model, which for the first time has been 158 applied in a scientific study of urban ecosystem services in the South Caucasus. Based on these results, 159 the most common trees species in Southern Caucasus cities were evaluated and compared for more 160 detailed and specific selection in future urban forestation programs. 161

Study area 163
The city of Tbilisi presents a stretched (from North to South-East), longish geographical layout, with the 164 major built-up area squeezed between mountains. In 2019, Tbilisi Municipality had a population of 1.

Tree inventory 201
The field work, conducted in August-September of 2019, was designed according the i-Tree Eco v6 field 202 manual (i-Tee Eco, field guide v6.0, 2020). The analyses of collected data are based on fieldwork 203 conducted in the two research sites in Tbilisi, for complete tree inventory of trees in both parks (Vaso 204 Godziashvili Park, aka "RED PARK", and EXPO Georgia Park). Tree sampling was conducted during the 'leaf-205 on' season (July-September) of 2019. 206 The general information collected from the field data included, with other parameters: the identification 207 of species (scientific names), tree diameter at breast height (DBH), height of the tree, height to base of 208 live crown, crown base, crown width, percentage of canopy missing (relative to crown volume), 209 percentage canopy dieback, and light exposure of the crown . Listed data were taken 210 for each tree in the study area with DBH greater than 5 cm. 211

Model settings 212
The model simulation was performed for 2018, the latest year available in i-Tree Eco, using hourly 213 meteorological data (air temperature, radiation, wind speed) registered at the Tbilisi Airport weather 214 station (Tbilisi/Lochini Airport) and precipitation provided by the National Environmental Agency (NEA) of 215 Georgia. 216 The hourly air pollution concentration data (2018) had been provided by NEA, from the operational 217 monitoring station at Kazbegi Avenue in Tbilisi. The station is located in the entrance of the Red park, 218 ensuring accurate data for air pollution concentration, which included: O3, NO2, SO2, PM10 and PM2.5 (fine 219 particulate matter that is 10 microns and with a diameter equal or less than 2.5 microns). After completing 220 data collection, the field data of urban trees, air pollution, and meteorological data were processed using 221 i-Tree Eco software, i-Tree Eco V6. 222 These two sets of data have been used to analyze the effects of park trees in sequestering and storing the 223 carbon, improving the air quality (pollution removal), and avoid rainwater runoff. 224

RESULTS 225
Weather and pollution data 226 In 2018 in Tbilisi the average daily temperature was 15.3 °C with a minimum at the end of December (-0.6 227 °C) and a maximum in July (31.2 °C). The mean Photosynthetically Active Radiation (PAR) was 330.1 W m -228 2 , with a lowest value in December (77.5 W m -2 ) and highest in June (572.5 W m -2 ). Precipitation was 229 distributed relatively evenly (a monthly average of 33 mm) though having seasonal features, with 230 maximums in June, August, and November (73.2, 64.4, 63 mm, respectively), and a lower value in February 231 (6.2 mm) (Fig. 2). 232

Urban forests 249
For the evaluation of ecosystem services provided by urban forests, we have analyzed their structural 250 characteristics. A total of 1,030 trees (tree cover: sempervirens (20%) store more than half of the total carbon (Fig. 4). 275 The gross sequestration is about 4.7 and 4.6 t of carbon per year for Red and Expo park, respectively. As 276 for carbon storage, Cedrus deodara (23.4%), Cupressus sempervirens (19.1%), and Populus alba (11.3%) in 277 Red park, Cupressus sempervirens (29%) and Cedrus deodara (25.6%) in Expo park, are the species that 278 sequester more than half of the total carbon per year (Fig. 4). 279   annual carbon sequestration (4.7 t) is slightly higher than Expo Park (4.6 t). This result is mainly due to a 321 higher The total pollution removal rate was 6.1 and 6.7 g m -2 for Red and Expo Park, respectively. The highest 334 removal rate was for O3 (3.3 g m -2 for Red Park and 3.6 g m -2 for Expo Park), then NO2 (1.7 g m -2 for Red 335 Park and 1.9 g m -2 for Expo Park), SO2 (0.6 g m -2 ), PM2.5 (0.5 g m -2 ), and CO (0.1 g m -2 ). Comparing these 336 values with other modeling studies, the total removal rate per unit tree cover is higher than other 337 European cities, such as Munich list of "recommended tree species, best suited for Tbilisi municipal territory's landscape and the climate" 364 (Tbilisi city hall, 2018). As the City government order states, the Department of Environment and Green 365 spaces, during the planned green infrastructure works, will use the approved species list as a guidance. 366 The list comprises species marked as a "priority" species, and others marked as "recommended" ones. The value and role of urban forests 386 The topics of urban green infrastructure, urban forestry, and ecosystem services, as a conceptual 387 framework for the nature and society relations, is relatively new scientific discipline and it had started its 388 development form early 70-ies of XX century (Gomez-Baggethun, 2009). Despite the popularity, 389 inclusiveness, multifunctional principles, and political appeal of ecosystem services approaches in urban 390 planning, the concept of natural benefits, flowing from nature to society, has very often difficult times of 391 realization and practical applicability in the real-life situations, especially at the stages of spatial urban 392 planning and managerial decisions (Turkelboom et al. 2018). Urban parks, as an important part of the 393 city's infrastructure are generally recognized as an important element of the cityscape. They provide city 394 and its inhabitants with pleasant, livable beautiful environment and act as important biodiversity hotspots 395 in the city space (Nielsen et al. 2014). Urban parks, gardens, other green areas often give the cities their 396 major identity and a recognizable "look" due to the uniqueness and the beauty of the natural, or man-397 made environment, interwoven in urban fabric (as Englischer Garten in Munich, Germany, or the Central 398 Park in New York City, USA). Nevertheless, also very often, the green spaces, urban parks, or other green 399 elements of the city, are not considered as "drivers" and "determinants" of the city's land-use policies, 400 general urban development, or sustainable urban practices. 401 The lack of knowledge is still present in ecology and ecosystem services of urban parks, when small urban 402 green spaces and parks were often regarded as being an attribute, or a decorative element of municipal 403 and urban open space systems due to assumptions that their small size and isolation restricts their 404 capacity of delivering ecosystem services, i.e., making them ecologically less valuable than large city and 405 county parks (Forsyth and Musacchio, 2005). Besides, the studies of ecological benefits and functions of 406 urban green spaces are necessary for putting more focus on the type and quality of park vegetation (Nordh 407 and Olafsson, 2021), its ecosystem functions and benefits than on the share and measure of green space 408 per capita, as it is often presented in general urban literature (Badiu et al. 2016). A small urban forest like 409 Expo park can offset CO2 emitted by more than 80 Georgian citizens (2.14 t per capita, 410 www.worldometers.info) and fine particles of about 40 diesel cars EURO IV with an annual mileage of 411 2000 km (assuming the concentration of PM2.5 is half of the total mass of particles 0.025 g/km). 412 This kind of scientific studies may significantly contribute to the gaps in general knowledge of cities, as 413 ecological systems with its multiple elements and complex interactions between ecosystems, the climate, 414 biotic and abiotic parts of the nature, urban ambient air, soil, human activities and in general, between 415 the social and ecological functions of green spaces in the urban context. It must be emphasized that the 416 topics related to urban parks' contribution to human health and the well-being of the city dwellers is a 417 less studied area though sufficient scientific evidence exists that urban parks directly benefit the physical 418 and mental health of city population (Jasmani, 2013), and number of health and well-being benefits could 419 be well grouped, as increased physical activity, increased life expectancy and reduced health problems 420 and the promotion of psychological health and mental well-being ( Unfortunately, the social-environmental services, provided by the urban parks is poorly studied in Georgia 429 and in future, the research questions related to the equality, equity and just distribution of urban green 430 infrastructure, as a social aspect of sustainable development of Tbilisi, could be of major priority for urban 431 studies in Georgian research institutions. 432

CONCLUSION 433
Urban green infrastructure and park trees contribute to improving air quality and mitigating climate 434 change and its effects in cities. These features of the urban vegetation are well recognized though less 435 implemented in managerial, or other climate-conscious and health safety decisions of the local 436 administrations. In this study, we showed the impact and the environmental importance of trees in 437 Tbilisi's two urban parks using for the first time the i-Tree Eco model in a Southern Caucasus city. The 438 evaluation of ecosystem services provided by urban forests and the selection of suitable and effective tree 439 species for future reforestation programs, are essential for a proper planning and management of urban 440 greening and the sustainable development of rapidly sprawling and growing cities such as Tbilisi. 441 Figure 1 Vaso Godziashvili and Expo Georgia parks, Tbilisi aerial photograph (designed by Giorgi Kirkitadze and Levan Alpaidze, 2020) Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Weather data used for model simulation of Tbilisi in 2018. From top to bottom: daily average photosynthetically active radiation (PAR), daily average air temperature, and total monthly precipitation.  Species that store (carbon storage) and sequester (carbon sequestration) the most carbon in Expo and Red park.