Trees in the City
A Cities are hot places. Concrete, asphalt and brick take in heat during the day and give it out slowly at night, so the centre of a city can be several degrees warmer than the countryside around it. Planners call this the urban heat island effect. In the past, trees were treated as decoration and were planted in parks and wide avenues for their beauty. Today many cities see them as working equipment that belongs with drains and roads as part of the basic services of a place.
B A mature tree cools a street in two ways. Its leaves block sunlight, so the ground beneath stays much cooler than an open pavement. It also releases water vapour through its leaves, which takes heat from the air in the same way that sweat cools the skin. Measurements in several cities show that a street lined with trees can be noticeably cooler on a summer afternoon than a street without them. Shade also protects asphalt, which cracks more slowly when it is kept out of the sun.
C Trees also help to manage water. When heavy rain falls on paved ground, it runs quickly into the drains, and the drains can overflow. Leaves catch part of the rain before it reaches the ground, and roots open up the soil so that water soaks in more easily. Some cities now build trenches of loose soil beneath the pavement, so that rainwater from the road flows towards the roots of street trees instead of into the drains.
D Air quality is a more complicated matter. Leaves collect dust and some pollutants on their surface, and this can reduce the amount in the air nearby. However, in narrow streets with tall buildings, a thick canopy can trap exhaust fumes at ground level. For this reason, researchers advise planners to choose the species and the spacing with care, and not to assume that more trees always mean cleaner air.
E Planting a tree is the easy part. A young street tree has a hard life. Its roots have little room, the soil is packed hard, and salt, vehicles and careless visitors damage it. Many trees die in their first few years. Cities that report good results usually spend as much on watering and care in the early years as they do on the trees themselves, and they ask residents to take some responsibility for the trees outside their homes.
F Public attitudes are changing the way that cities choose trees. Residents once complained about fallen leaves, roots that lift paving stones, and branches that block the view. Surveys in some cities, however, find that most people would like more trees in their street, and that houses on tree-lined streets are often more valued. Some councils now publish a map of every tree they own and invite the public to report damage or ask for new planting.