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On a summer afternoon, two nearby streets can have different temperatures. Dark pavement absorbs sunlight and releases some of that energy as heat. Trees, by contrast, shade the ground and release water vapor through their leaves. Both processes can reduce local temperatures.

A city planning team compared three blocks before and after planting trees. The greatest cooling occurred where young trees received regular watering and had enough soil for their roots. Simply counting planted trees, the team concluded, was a poor measure of success. Long-term maintenance mattered as much as the initial planting.