Last month we travelled to Orvieto for the GreeninUrbs Nature Based Solutions Conference. It was great to hear about the variety of research that is being carried out into green infrastructure. Here is a short blog on the what we learnt about tree design in urban environments. 

Air pollution is responsible for 3.7 million deaths each year and is the biggest environmental killer. Green infrastructure including trees and natural planting help to tackle this problem, providing a multitude of benefits that help to overcome the negative effects of air pollution and other urban problems. Trees filter dangerous VOC’s (Volatile Organic Compounds), making it cleaner and safer to breathe whilst capturing and storing carbon which reduces urban emissions in the air.

The Urban Heat Island Effect is defined as an urban area/city that is warmer than the surrounding areas because of the increased human activity in that area, such as transport and building energy use. The Urban Heat Island Effect is increased during warmer months, especially during heat waves which means cities become an uncomfortable place to live and work. This puts a strain on energy use due to an increase in demand for air conditioning, and puts pressure on health services as a result of the dangerous conditions endured by the older generation. Trees provide vital shade for cities which helps to cool during summer months, reducing the urban heat island whilst also reducing noise pollution in highly congested areas.

However, though trees provide a multitude of benefits, if landscape design is not well considered, the resulting green infrastructure could cause more harm whilst trying to do good. Surprisingly, vegetation such as trees and grasses are only a temporary solution for capturing most atmospheric particulates. Leaves only act as a surface to capture particles, relying on rainfall to transfer them to the soil. If high winds occur, particles can be misplaced back into the air. Furthermore, plants also produce VOC’s themselves, and the levels are heightened during warmer temperatures. Therefore, considerations should be made to planting not only for enhancing biodiversity but the levels of VOC’s that the species emits.  

The level of benefit that the tree provides is largely linked to where the trees are placed. If trees are planted too densely, they can slow the flow of wind. Wind is important in highly polluted environments, as it helps to disperse the particles, meaning the air is less densely polluted and thus less dangerous to breathe. Similarly, trees are often planted along roadside verges, to provide habitats for misplaced wildlife and capture car fumes. However, if these trees are planted too densely, they create a canopy effect which traps the pollution in the area, preventing dispersion of pollution particles. As a result, pedestrian walkways may be more polluted alongside these roads, than those with no trees, as the air is more densely polluted. To mitigate this problem, trees should be planted between the road and pedestrian walkway, acting as a barrier both for safety and pollution. Considerations should also be made to the canopy size of the tree, and its leaf density so as to let the wind flow through the trees for dispersion.

If designed properly, trees can be crucial in steps to prevent climate change. However as discussed, implementing green infrastructure that creates the highest value for urban environments is not simple, and often results in wasted money and efforts if not adequately designed. Trees that will live the longest and grow to their fullest potential are an investment and will provide the greatest benefits. Consideration should always be made to the situation of trees in relation to traffic, buildings and how people use the space.