The Tree of Heaven in its typical habitat. Yep, that's heaven. Not quite what you expected, is it? Creative Commons photo by spike55151 on Flickr.
Ailanthus is almost perfectly adapted to urban living. A mature tree can produce hundreds of thousands of seeds, which scatter in the wind and lodge themselves in precisely the same nooks and crannies where loose bits of dirt collect. The tree's powerful root growth is strong enough to break up concrete and pavement. Thus, Ailanthus trees are important pioneer species in the transformation of vacant lots: their roots break up impervious surfaces and loosen compacted soils so that other, less hardy plants can follow afterward. In Williamsburg, there are dozens of old parking lots that Ailanthus trees have helped to transform into grassy savannas.
In spite of all this, some people think that Ailanthus trees are "weeds." They call Ailanthus "invasive" (it's native to China), they moan about cracks in their precious concrete infrastructure, and they claim that the trees have a "strong, offensive odor" (a quote from the National Parks Service Alien Plant Working Group, which has a desperado-style outlaw poster about Ailanthus here). Like it or not, though, Ailanthus is here to stay. So save yourself some hassle and appreciate this tree.