Studies Find Small Urban Forests Can Help Cool Cities

Heat waves are no joke. Temperatures can increase to a dangerous level, exacerbating existing health issues, induce heat stroke, and can even result in death. Increased temperatures do not affect all communities evenly, however.

Studies have found that the highest temperatures and the most heat-related deaths occur in urban areas. Concrete, asphalt, and other paved surfaces often readily absorb and release heat, causing a phenomenon known as an urban heat island effect.

Urban land managers have the potential to mitigate some of this heat island effect with the types of foliage they incorporate into their landscape designs.  Studies find that tree canopy cover can provide shade that reduce the heat absorbed in paved surfaces and lower surface temperature in general. However, many policies that try to implement urban forests as cooling effects focus on large green spaces, which are often not feasible in urban areas.

The Conversation posted a piece by Lingshan Li titled “Small green spaces can help keep cities cool during heat waves”. This article explores various studies that show how smaller green spaces, such as yard, rooftops, and small parcels of undeveloped land, make important contributions to urban cooling effects.

The article cites a study in Adelaide, Australia, which quantified the effect smaller parcels of urban spaces had on surface temperature. They found that tree canopy cover, and to a lesser extent of grass cover, could lower local surface temperatures up to 6˚C (10.8˚F). For suburban yards and gardens, local surface temperatures decreased up to 5˚C (9˚F). 

The Li article references another study that looked at how the cooling effect can be magnified by selecting a diverse array tree species that have larger leaves and high transpiration rates (the level of water evaporation occurring from leaves). The tree canopies made of these diverse species have a larger maximum temperature drop in the summer compared to those that are less diverse.

The closeness of trees and green areas also play a part, as green spaces that are spread apart or unevenly distributed have a lower cooling effect. The proportion shrubs, smaller trees or fragmented tree sections also have lower transpiration rates and less shade potential. While they may have a lower cooling effect, the smaller species may be more readily available and affordable, which are trade-off important for land managers to consider. Planting trees in groups instead of individually or in lines is also recommended for their greater cooling effect.

When planning for your next urban project, please consider incorporating more green spaces and consulting where you get your stock for species with larger leaves and higher transpiration rates.

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