“A leafy neighbourhood feels 15°C cooler than areas with no trees.” TU Delft now has a test set up with 75 tree species to check what is the coolest tree.

Trees provide all kinds of ecosystem services, such as converting CO2 into carbohydrates, producing oxygen, and purifying the air by trapping particulates. They provide habitats for fungi, mosses, insects, mammals and birds and so form the basis of all ecosystems, including in urban areas. But trees also help to keep city squares, streets and buildings cooler. Their leaves absorb solar radiation, transpire water and create shade, so lowering the temperature in their immediate environment. During a heat wave, a dense foliage provides much-needed shelter and a park or public garden with lots of trees quickly feels like an oasis in the urban desert. On a hot, windless day, a leafy neighbourhood may well feel 10 to 15°C cooler than an urban area that is more exposed to the sun, which can be enough to prevent heat stress. Trees thus play a key role in the design of climate-proof and healthier cities.

To determine the cooling performance of various species of trees, in 2018 TU Delft’s Faculty of Architecture and the Built Environment and the Dutch Association of Landscape Gardeners (VHG) formed the Urban Forestry joint research programme to support fundamental research into the relationship between tree architecture and the urban microclimate. Urban landscape specialist René van der Velde, who is affiliated with the Faculty’s Chair of Landscape Architecture, is leading the programme. Lotte Dijkstra joined the programme as a researcher. “Although gardeners and other ‘green’ professionals already understand a great deal about the performance of tree species in the built environment, there is very little empirical knowledge about their cooling capacity.”

In order to determine how physical properties such as the trunk and crown shape, the branch structure, and the leaf characteristics influence the local air temperature and the apparent temperature, this spring the research team designed and built special ‘climate arboreta’, experimental plots containing tree species that are common in Dutch parks and gardens.

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