Life isn't easy for city trees. Park trees have it a little better. Despite visits from dogs and vandals, they are able to spread out their roots and branches in a mostly normal fashion. But for those planted on the street, it's a hard go.

I've always heard that the average lifespan for a city street tree is seven years, but I couldn't find the source of this statistic. So I asked Mark Shaw, a master arborist with Bartlett Tree company for 21 years, about the short and ill-fated lives of city trees.

Although he couldn't corroborate the seven-year statistic, he agreed that the lifespan of trees in urban neighborhoods is greatly diminished. "Air pollution, road salt, and compacted soil all gang up on trees. If you get a tree that by some miracle survives, they'll want to cut it down because it gets too big."

Although one of the lovelier sights in a city is a sidewalk lined with magnificent trees, Shaw doesn't think this is a great idea. Planted in small tree pits, the roots tend to wrap around the base of the trunk, and this girdling can eventually strangle the tree to death. The rare tree that does survive outgrows its space and ends up needing to be removed or pruned drastically.

He suggests that if you're going to plant a street tree, avoid those grown in plastic containers, and buy a tree that's been dug up with its root ball intact and wrapped in burlap. Balled and burlap (or B&B) trees are more expensive to plant because they're so heavy, but the roots have a natural spread and are much less likely to girdle.

His other suggestion is to think small. He recommends trees like crape myrtle, which reach a height of only 20-25 feet (although some are much smaller.) The Sargent forms of crabapple have a good shape and are the right size, as long as you don't mind applesauce on the street when the fruit drops. He's also had success with Blackhaw Viburnum (V. prunifolium) which is a large, native shrub that can be coaxed into behaving like a small tree. All these trees can tolerate punishing urban conditions.

Like the story of the country mouse and the city mouse, trees that are most resilient in a natural setting- ironwood, hop hornbeam- usually can't adapt to the urban environment. It takes a certain kind of tree, preferably planted the right way and tended carefully for the first few years, to survive life on the streets.