EXPLORING URBAN TRANSITIONAL SPACES AS CONTRIBUTORS TO URBAN GREENING – BEIRUT CASE STUDY

Abstract

Trees are considered as the lungs of the city as they help mitigate air pollution. In dense urban areas, however, trees are non-existent and there is a need to explore the potential contribution of plants in alternative spaces such as vegetated transitional spaces, i.e. areas linking the inner and outer urban spaces. Most studies that report the beneficial impact of plants on urban air quality focus on assessing tree canopies while very few studies have attempted to explore the contribution of vegetated balconies to improved urban air quality. This study aims to assess the extent to which transitional spaces, specifically private balconies in residential areas, can mitigate air pollution. The study was performed in Beirut, Lebanon. A total of 150 vegetated balconies were selected following purposeful sampling to capture examples of successfully maintained green balconies in the city of Beirut. The balconies were photographed from the street, and the number and types of plant species were deduced from the images. The mature size of each recorded plant species was estimated based on local expertise of typical container grown plant sizes in Beirut assuming that herbaceous plants are grown in 30 cm containers and woody plants in 50 cm containers. The canopy volume contributed by each plant species was then calculated following the formula by Thorne et.al, 2002 which is based on height and width of the plant. The small tree canopy equivalent (150,000,000 cm3) of the 150 case study vegetated balconies was then calculated by assessing the potential contribution of 20 such balconies in a 10 floor apartment building. In addition, the small tree canopy equivalent of each recorded species was calculated assuming 10 plants per balcony and assessing the potential contribution of 20 such balconies in a 10 floor apartment building. The results revealed 10 vegetated balcony assemblages that contributed the equivalent of 2-5 small trees per building. With respect to individual plant species, maximum canopy volume contribution was obtained from the following woody plants species, Trachelospermum jasminoides (2,617,994 cm3), Jasminum officinale (2,356,194 cm3), Olea europaea L. (2,120,575 cm3) and herbaceous plant species Strelitzia sp. (2,144,661 cm3), Araucaria heterophylla (1,047,198 cm3), Musa acuminata Colla (1,769,764 cm3). Results of this study provide quantitative estimates that shed light on the potential role that residents of urban dense neighborhoods can play in contributing to urban greening and improving urban air quality by planting their balconies.