City Trees

This question has been on my mind for a couple of weeks now...

You've probably noticed the large trees in the downtowns of cities that line the sidewalks. For example there are the trees the line the sides of the main branch of the Multnomah County library in downtown Portland. These trees are 40 feet tall and have good-sized trunks. But they're planted in the middle of the city, in a sidewalk, right next to the street. There's very very little bare earth next to these trees, practically none.

So where on earth do they get their water?