The cooling power of city trees

On a hot afternoon in summer, the difference between a street lined with mature trees and one without them can be felt immediately. Stepping out of the shade onto bare pavement is like opening an oven door. Scientists have a name for the wider phenomenon: the urban heat island. Because cities are built largely from materials such as concrete, brick and asphalt, which absorb the sun's energy during the day and release it slowly at night, built-up areas are often several degrees warmer than the countryside around them. The effect is usually strongest after sunset, when rural land cools quickly but city streets continue to radiate stored heat.

Trees reduce this heat in two main ways. The first is simple shading. A leafy canopy intercepts sunlight before it reaches the ground, so the surfaces beneath it stay cooler and store less energy. The second is less obvious. Through a process called transpiration, trees draw water up from the soil and release it as vapour through tiny pores in their leaves. Turning liquid water into vapour requires energy, and that energy is taken from the surrounding air, in much the same way that sweat cools the human body. A single large tree can release hundreds of litres of water on a hot day, although the exact amount depends on the species, the size of the tree and how much water is available in the soil.

For many years, the value of urban trees was judged mainly in terms of appearance. Planting schemes were designed to make streets more attractive, and trees were often chosen for their flowers or autumn colour rather than for the amount of shade they provided. This has begun to change as heatwaves have become more frequent in many parts of the world. City planners now increasingly see trees as part of a city's basic infrastructure, as essential as drains or street lighting, and some local governments have set targets for the percentage of land that should be covered by tree canopy.

However, planting a tree is not the same as growing one. Young trees in cities face harsh conditions. Their roots are often confined to small pits surrounded by compacted soil, which limits their access to water and nutrients. They may be damaged by vehicles, vandalism or road salt in winter. As a result, a significant proportion of newly planted street trees die within their first few years. Urban foresters argue that money spent on watering and caring for young trees during this period is at least as important as money spent on planting new ones.

Choosing the right species is another challenge. A tree that thrives in today's climate may struggle in the hotter, drier conditions expected later this century. For this reason, some cities are experimenting with species from warmer regions, hoping they will be better suited to future conditions. This approach is not without risk. Introduced species may not support local insects and birds as well as native trees, and there is a danger that a few popular varieties will be planted so widely that a single pest or disease could destroy a large share of a city's trees. Most experts therefore recommend planting a wide variety of species, so that no single type makes up too large a share of the total.

There is also a question of fairness. Studies in a number of cities have found that wealthier neighbourhoods tend to have more tree cover than poorer ones. Lower-income areas, which often have more paved surfaces and fewer parks, can therefore be noticeably hotter during a heatwave, and their residents are less likely to have air conditioning. Some researchers, such as the urban geographer Helen Marsh, believe that tree-planting programmes should be targeted at these neighbourhoods first. “The benefits of shade are not shared equally,” she says, “and the people who need them most often have the least.”

Trees are not a complete solution to urban heat. On their own, they cannot cool an entire city, and in very narrow streets a dense canopy can sometimes trap vehicle pollution close to the ground. They work best as part of a wider strategy that also includes lighter-coloured roofs and pavements, which reflect more sunlight, and better building design. Nevertheless, few other measures offer such a wide range of benefits at the same time. As well as cooling the air, trees absorb rainwater, provide homes for wildlife and, according to a growing body of research, improve people's mood and sense of well-being.