According to the passage, why is adding trees to an already-paved neighborhood slow and expensive?

On a July afternoon in a California city, two neighborhoods four miles apart can differ in air temperature by as much as fifteen degrees. The difference is not weather. It is surface.

Dark asphalt and dark rooftops absorb sunlight and release it slowly as heat, which keeps a paved block warm long after sunset. Researchers call this effect the urban heat island. Trees work against it in two ways. Their canopy blocks sunlight before it reaches the pavement, and the water they release through their leaves cools the surrounding air, much as sweat cools skin.

Canopy is not spread evenly across a city. In most large American cities, the neighborhoods with the least shade are also the ones with the lowest household incomes, a pattern that traces back to how those blocks were built rather than to anything the residents chose. Streets laid out for warehouses and freeways were paved wide and planted thin. Once the concrete is down, adding trees becomes slow and expensive work: sidewalks have to be cut, roots need far more soil than a narrow planting strip holds, and a young tree has to be watered for three to five years before it can survive on its own.

The consequences are measurable. Emergency room visits for heat-related illness rise faster in low-canopy neighborhoods during a heat wave, and summer electricity bills there run higher because air conditioners run longer. Several California cities have responded with canopy targets, which are public commitments to reach a set percentage of shade cover by a given year, and have begun planting on the hottest blocks first instead of spreading trees evenly across every district.

Progress is slower than the targets suggest. A city can plant ten thousand trees in a season and lose a third of them within five years if no one is responsible for watering them. The cities with the best survival rates are the ones that pay for maintenance contracts and hire neighborhood residents to check on the trees, which is less visible than a planting day and considerably more effective.