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 resulting in substantial economic, social, and environmental challenges. One of the major risks is the deterioration of living environment in urban areas due to the high soil consumption and pollution of environmental components. For this reason, cities are required to adopt measures to reduce air pollution concentration and CO2 emissions, preserve biodiversity and mitigate the urban heat island effect. In this context, tree planting has been suggested as one cost-effective strategy as green infrastructures can provide important environmental and social functions which contribute to the quality of life and health of city dwellers. Tbilisi is the largest city in Georgia, with a population of over 1,100,000 inhabitants (about 30% of total population of Georgia). Due to its size and physical and geographical shape, the air pollution in Tbilisi is considerable, caused by the large number and percentage of outdated vehicle fleet and the massive construction activities that produce various polluting gases and particulates. The green space availability in Tbilisi (5-6 m2 per inhabitant) is low compared to other European cities, and in recent years the need to increase the amount of urban vegetation has been underlined at planning level. In our study, we implemented for the first time in an Southern Caucasian city the i-Tree Eco model to quantify the main ecosystem services from common tree species in Tbilisi urban parks. Two parks, Expo Park (694 trees) and Red Park (1027 trees) in the city of Tbilisi have been measured and a model simulation was performed for the year 2018. These urban forests store large amounts of carbon in their woody 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 Expo park and Red park, respectively. They also positively contribute to the air quality by removing 119.6 and 90.3 kg of pollutants (CO, NO2, O3, PM2.5, SO2), and reducing water runoff of 269.5 and 200.5 m3, respectively. This analysis highlights the key role of urban forests in improving the environmental sustainability of the city of Tbilisi and provides important decision support for the tree species selection in this geographic area with the aim of maximizing the benefits trees can supply to cities.