"How? It's surprisingly simple. Turns out, demand for trees in neighborhoods behaves a lot like a luxury item, as opposed to a basic necessity.
Tim De Chant at The Per Square Mile blog wrote about research on this a couple of weeks ago. Then, he went out and found examples, using images from Google Earth.
Research published a few years ago shows a tight relationship between per capita income and forest cover....They found that for every 1 percent increase in per capita income, demand for forest cover increased by 1.76 percent. But when income dropped by the same amount, demand decreased by 1.26 percent. That’s a pretty tight correlation. The researchers reason that wealthier cities can afford more trees, both on private and public property. The well-to-do can afford larger lots, which in turn can support more trees. On the public side, cities with larger tax bases can afford to plant and maintain more trees." (Thanks Toby)