Devon Tree Services - View of an Amsterdam canalDutch researchers have attempted to quantify the role of trees in locally mitigating the nocturnal urban heat island (UHI) effect.

Built up urban environments can be as much as 9°C warmer than the surrounding area with buildings, road traffic and human activities all contributing to the ‘urban heat island’ effect.  Vegetation, especially trees, has a crucial role to play in lowering temperatures by providing shade, reflecting sunlight and by evaporation.  Strategic selection and placement of trees in cities can cool the air by between 2°C and 8°C which could reduce heat-related stress and premature human deaths during heatwaves.

Under the changing climate predicted for the UK there are likely to be an increasing number of incidences where the average daytime temperature is 30°C or above and the night time temperature exceeds 15°C for two consecutive days – the Department of Health threshold value for temperatures that could significantly affect health.   While the South East, London, the East and West Midlands are projected to be the most vulnerable, excess deaths due to heat are forecast to increase in Scotland and Wales.   The risks are greatest in large metropolitan areas such as London, Manchester and Birmingham.

The researchers from the Free University of Amsterdam estimated volumes of trees within the city via a 3D tree model dataset derived from LIDAR data and modelled with geospatial technology.   They then measured air temperature at over 100 different points on a “relatively warm” summer night.   The results indicate that tree volume has the highest impact on UHI within a 40m radius, and that increasing tree canopy volume by 60,000m3 in this area leads to a 1ºC reduction in temperature.   “We tested an empirical model, using multi-linear regression analysis, to explain the contribution of tree volume to UHI while also taking into account urbanization degree and sky view factor at each location,” they explained.

Research like this is vital in showing that trees have an important role to play in preserving and improving our quality of life as the climate changes and temperatures increase.