What make trees cool the environment? Two methodologies investigating the heat-stress reduction potential in relation to tree traits for the Western European climate

Urban trees are key in heat resilient design strategies in cities. However, it remains unclear which trees are more effective in relation to human thermal comfort and what tree characteristics contribute most to cooling. These questions have been addressed with two measurement approaches in a Western European context. The first method consisted of three-hour long simultaneous measurements on multiple days on the same location. While the second method is based on short ten-minute serial measurements at varying times, days and locations. The cooling effect of the trees is expressed by the human thermal comfort indicator known as the Physiological Equivalent Temperature index (PET).

From the research we can draw two main conclusions: on the effect of trees and on the measuring methodology. The cooling effect of a tree is related to the amount of shade a tree provides, with crown density showing a significant relationship with cooling potential. Crown diameter, trunk circumference and tree height were of less importance. Even trees with a low crown density already show a significant reduction in PET values reducing the heat stress level by one class, meaning a 6 °C PET reduction. Whereas dense crowns can reduce heat stress by even three classes, up to 19 °C PET.

The main conclusion on the methodology is that simultaneous measurements are  needed to compare heat stress reduction by different tree traits because in serial measurements variation in meteorological and local conditions causes too much additional variation in the measurements. The serial measurements however do specify and amplify the cooling effectiveness of trees in order of magnitude.

The relationship of crown density with cooling potential shows the importance of healthy mature trees. A selection of trees for climate resilience should be focused on the most suitable species for the local growing conditions, such as drought resistance.