Tree Planting, Mitigating Heat, and Asphalt Pavement

Urban planners are searching for ways to decrease the solar heat that is absorbed through asphalt. It is suggested that a carefully managed shade plan for trees is the most effective approach.

In the year 1995, around a quarter of a century ago, the City of Chicago discovered in the most negative way what the urban heat islands (UHI) result was. It was during that summer in 1995 when it was observed that the Windy City had three days of temperatures of triple degrees and top at 105 degrees F. One reason was that, unlike the desert and rural cities, the city didn't get much cooler at night. Around 739 people perished from heat-related causes.

The importance of trees in urban areas

The idea behind cutting UHI is to restore the natural world: additional trees and plants generally. Also, Chicago has been working to achieve this through tax-related policies that reward property owners who have gardens on their roofs (i.e., gardens that are placed over buildings). The city is also embarking on a tree-planting initiative with the aim of increasing the amount of trees that can be planted from 17 percent to 20 percent.

A study in the City of Montreal observed significant correlations between tree cover and temperatures in summer, both daytime and night. Tree cover could reduce air temperature at the tree level (4ºC at 20 meters height from the ground), as well as at a higher level (2ºC at 60 meters from the ground).

Trees in concrete and asphalt aren't an easy task.

However, it is important to select the type that will provide the best conditions for good root development, putting the trees in areas that cause the most damage to sidewalks, giving consideration to the signage of retailers, and tackling the problem of pavements that are compacted are all a part of the task. Another aspect to take into consideration is climate change and anticipating the kinds of plants that could live in the years to come when heat or drought, extreme stormwater, and invasive insects are threatening the landscape.

Fortunately, the cultivars of trees are being developed to perform better in urban streets and have natural root systems which are less likely to cause sidewalk and pavement heave. Underground structures designed to limit soil compaction and redirect rainwater to tree roots have become more sophisticated. Landscape architects are now able to plant trees where they can work well and also in relation to the existing structure and signage for retail in an approach that benefits instead of detracting from the community and local business.

Could white asphalt be an option?

Another fairly simple and quick modification to the pavement is to reflect the trend of white roofs that have been seen on buildings that switch from white to black membranes on flat roofs, resulting in an impressive reduction in cooling costs and the heat that is absorbed by the atmosphere. When feasible, white asphalt coatings that increase the reflectivity of light (i.e., an albedo effect) by 5% when using fresh black asphalt to 50% when using a white coating bring the temperatures down considerably (from 90ºF to 123ºF on the day that is sunny). The result is a reduction in temperatures that are in the vicinity of the roadway.

Although appealing, white-coated asphalt is scarce at present. The tests conducted in the Los Angeles area undertaken since 2015 have produced mixed results. The treatment has lowered the temperatures on the road itself; however, the solar radiation reflected by the pavement heated the air over it. The heat is dissipated more easily through the night; however, it can be slightly more uncomfortable for pedestrians walking on sidewalks that are near these streets. In the future, research is needed to define the real benefits and the potential benefits.