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City planners have expanded urban tree-planting programs in response to research linking tree cover to lower summer temperatures in densely built neighborhoods. Concrete and asphalt absorb and radiate heat throughout the day, creating what researchers call urban heat islands, where temperatures can be several degrees higher than in nearby areas with more greenery. Trees counter this effect by providing shade and releasing moisture through their leaves, a process that cools surrounding air. Planners note, however, that simply planting trees is not sufficient; young trees require years of watering and protection from damage before their canopies grow large enough to produce a measurable cooling effect, meaning the benefits of a planting program are rarely immediate.