The role of tree species in cooling the urban climate – application in tree planting and landscape architecture

2015 | Dr. Henrik Sjöman, Swedish University of Agricultural Science / Gothenburg Botanic Garden

Towns and cities worldwide are facing rapid challenges in terms of sustainable living and investment. With an ever-increasing urban population and predicted changes in climate and weather, the way we choose to plan, design and manage the everyday urban landscape will be of highest importance in securing an inhabitable environment for future generations. Recent years have seen a number of research studies on how the urban tree canopy helps mitigate the adverse effects of urban runoff (storm water), the urban heat island effect, and high uncomfortable temperatures during hot summer days. Trees are as such considered important to reducing the impact and extent of urban runoff and pluvial flooding, heat exhaustion (illness and mortality) and increased energy use in buildings due to air conditioning.

The chief goal of this project is to study how different species of trees differ in projected leaf and branch cover, i.e. leaf area index and branch area index. Another goal is to investigate how the different species vary in terms of climate regulation by focusing on mean radiant temperature, surface temperature and air temperature. Consequently the objective is to discuss the results in terms of sustainable urban development, i.e. where, why and how the species studied could be integrated in the urban fabric with regards to public places (thermal comfort) and buildings (reduced energy use).

Study Results

Today we know how significant our urban tree population is in delivering important ecosystem services to achieve sustainable urban development. However, less is known about how different species in solitary plantings (i.e. in residential areas, pocket parks, parking spaces and along streets, etc.) differ with regards to shading and cooling due to their architectural make-up. The aim of this project is to study how different species of solitary trees may influence the mean radiant temperature and physiological equivalent temperature in complex urban environments depending on summer or winter. This study contrasted tree qualities and benefits in summer vs. winter as determined by the individual characteristics of each tree. The leaf and branch area index of 62 different species and genotypes from solitary individuals were studied. The different indexes were subsequently incorporated into the microclimate model ENVI-met, a software program with a three-dimensional interface designed to simulate surface-air interactions in complex urban environments. The results showed how different tree species, with regards to architectural make up, differed significantly when it came to mitigating urban temperatures and thermal stress.

Year: 2015

Funding Duration: 1-3 years

Grant Program: Jack Kimmel International Grant

Grant Title: The role of tree species in cooling the urban climate – application in tree planting and landscape architecture

Researcher: Henrik Sjöman

Publications from Grant:

• Hirons, A., Sjöman, H. 2018. Evidence based tree species selection guide. TDAG UK: https://www.myerscough.ac.uk/media/4932/hirons-and-sjoman-2019-tree-species-selection-for-green-infrastructure-v13.pdf
• Sjöman, H., Deak Sjöman, J. 2018. Urban Trees as Prodigy. TOPOS no. 103 2018.
• Sjöman, H. Hirons, A., Deak Sjöman, J. 2017. Criteria in the selection of urban trees for temperate urban environments. In Konijnendijk van den Bosch, C., Ferrini, F., Fini, A. (Eds.) Routledge Handbook of Urban Forestry. Pp 339-362.

Presentations:

• Sjöman, H. 2018. Selection of urban trees. Royal Botanical Garden Kew, London, England. 13 Mars. Invited speaker.
• Sjöman, H. 2018. Selection of urban trees. Birmingham University, Birmingham, England. 15 Mars. Invited speaker. 
• Sjöman, H. 2018. Discovering Successful Urban Trees – 21st Century Plant Exploration.  The Urban Forest Tomorrow Conference, University of Toronto Mississauga, Canada. 14-15 June. Invited speaker.
• Sjöman, H. 2018. Tree collections and their impact for sustainable urban environments. Yorkshire Arboretum, England. 11 October. Invited speaker.
• Sjöman, H., 2017. Selection of urban trees – presentation of ongoing tree research in Sweden. Tbilisi Botanical Garden, 17 Mars, Tbilisi, Georgia. Invited speaker.
• Sjöman, H. 2017. Selection of urban trees – perspective that needs to be included in the selection process. Barcham Trees Yearly Seminar 26 October. Invited speaker.
• Deak Sjöman, J., Johansson, E., Sjöman, H. 2017. Microclimatic qualities of strategically placed trees in Lomma Harbour, Sweden. Green Cities Conference 2017, Bologna Italy. 12-14 September.

For more information on this project, contact the researcher via TREE Fund at treefund@treefund.org.