Wordwind's scathing indictment of the Ailanthus tree prompted me to look it up. I found some interesting facts about it. It was "The Tree That Grew In Brooklyn." It was introduced into this country over two hundred years ago. The reason it is becoming a pest is that it is resistant to diseases, which can also be a virtue. It is dioecious meaning there are male and female trees. (Yes, Virginia, trees do have sex but it is not very exciting)(And forests have climaxes, but it takes them a couple hundred years.) Both sexes have a downside about either flowers or leaves having a bad odor when crushed. There is a dictionary of botanical terms with lots of dandy words to learn.
The one thing I could not find out is how valuable they are for shade. Here is one URL (others can be found by searching "Botany ailanthus").http://www.botanyworld.com/16.di.htm