Quantifying the cooling effectiveness of urban street trees in relation to their growth

2015 | Dr. Mohammad Asrafur Rahman, Technical University of Munich, Germany

It is often claimed that planting trees can cool our cities and therefore improve the thermal comfort for the city dwellers mainly by shading and evapotranspiration (the process by which plants lose water to the atmosphere). However, urban cooling provided by trees is rarely quantified. The proposed study will investigate 20 trees of two different species and types of urban street canyons in terms of cooling effectiveness and tree growth rate. The study will be carried out in the very center of the Bavarian capital Munich, Germany; a representation of large conurbation. Continuous measurements of water loss from trees using tree stem sap flow gauges, soil moisture potential and stem diameter growth using automatic dendrometers along with the periodical measurements of surface temperature and crown variables as well as tree core data over Summer 2016 will help us to validate the relationship between carbon gain and water relations. Initial setup and instrumentation is mostly available from an ongoing project funded by the Alexander-von-Humboldt Foundation. The research grant from TREE Fund will be used to purchase some more equipment to achieve our objectives of quantifying the cooling effectiveness of street trees in selected sites by incorporating the data into eco-physiological models developed in collaboration with experts at the Technical University of Munich and also to test the proportional relationship of tree growth with their cooling benefits. Moreover, the potential for upscaling this ecosystem service on the basis of micro-meteorological models and data from urban forest inventory will also be explored.

Study Results
It is well known that urban trees can help to mitigate the urban heat island effect through shading the surfaces by their canopies as well as by the process called transpiration (water loss through tree leaves). However, due to high heterogeneity, relative effectiveness of different species and the relationship with the tree growth rate is not properly understood. Our project investigated two contrasting species Tilia cordata and Robinia pseudoacacia at different street canyon conditions in central Munich, Germany. The results showed that there are large differences both in terms of surface and air temperature reduction potential between two species. Results showed that tree shading indeed can cool hard surfaces by 12-25 °C while transpiration can absorb up to 50% of the energy from incoming solar radiation. Moreover, within the canopy, temperature differences can peak to 3-4 ºC and reduce air temperatures underneath by 1-2 ºC depending on the species and coupled well with water fluxes over a spatial and temporal scale. This has direct implications for the radiation energy balance of urban micro-climates. Moreover, trees grown at the open green square (OGS) showed 30% higher transpiration at mid-day compared to the trees at circular paved square (CPS), hence a higher cooling effect (2.3 kW tree−1 at OGS compared to 1.9 kW tree−1 at CPS). Continuous dendrometers (stem fluctuation) and tree-ring dating data also indicated that water use efficiency is more strongly related to cooling potential of urban trees than the growth rate and can be used to upscale the result to stand level.

Year: 2015

Funding Duration: 1-3 years

Grant Program: Jack Kimmel International

Grant Title: Quantifying the cooling effectiveness of urban street trees in relation to their growth

Researcher: Mohammad Asrafur Rahman

Key words: Urban trees, Cooling effect, Tree eco-physiology, Edaphic variables, Shading effect, Growing surface

Peer Reviewed Publications from Grant:

General Audience/Trade Publications:


  • (2016) [Invited] Oral Presentation: Title: ”How useful are our urban trees in mitigating urban heat?” Urban Trees Seminar, Department of Geosciences and Natural Resource Management, University of Copenhagen, Nov 17, 2016.
  • (2016). Oral Presentation. Title: “Comparing the cooling benefits of different urban tree species at contrasting growth conditions”. 46th Annual meeting of the Ecological Society of Germany, Austria and Switzerland, Marburg, Germany, 05-09 Sept., 2016.
  • (2016). Oral Presentation. Title: “Quantifying the cooling effect of trees in relation to their growth conditions, species differences and growth rates”. 2nd International Conference on Urban Tree Diversity, Melbourne, Australia, 22-24, Feb., 2016.

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