درختان و مشکل گرمای شهری

ACities are often warmer than the countryside around them, particularly during summer. Roads, roofs and car parks absorb sunlight during the day and release the stored heat slowly, so densely built neighbourhoods may stay warm long after sunset. Environmental agencies call this pattern an urban heat island. Because extreme heat can be dangerous to health, local governments have become interested in simple ways of cooling streets, and planting trees is one of the approaches most often discussed.

BTrees lower temperatures in two main ways. The first is shade: a canopy stops sunlight before it reaches pavements and walls. According to the US Environmental Protection Agency, shaded surfaces can be more than ten degrees Celsius cooler than unshaded ones at the hottest time of day. The second is evapotranspiration, a process in which a plant draws water up through its roots and releases it as vapour from its leaves. Turning liquid water into vapour uses energy, and this lowers the temperature of the surrounding air.

CThe benefits reach beyond cooling. When trees shade a building directly, less electricity may be needed for air conditioning, which also reduces emissions from power stations. Leaves can trap some airborne pollutants, and the canopy and roots slow down rainwater, so less of it rushes into drains during a storm. For these reasons, the US Forest Service treats street trees, parks and gardens together as a form of green infrastructure that provides practical services to the people who live nearby.

DTrees are not an instant solution, however. A young tree may need many years to develop a wide canopy, and in dry weather it may require regular watering. Roots compete for space with pipes and cables under the pavement, while fallen leaves and branches create maintenance work. Planners therefore have to decide where each tree will do the most good. A single large tree shading a bus stop or a school playground may matter more to residents than several small trees planted where few people walk.