It's an old standard by now that healthy street trees are critical to most any good street. I've written about the basics and their relation to zoning codes here. Samuel Geer has an excellent ongoing set of posts about how to properly plant and care for street trees at Streets.Mn. I'm enjoying this set of posts because Geer clearly starts from the premise that cities are a good thing for humans and it's important to figure out how to have successful trees in cities. Quite a few folks in the arborist and landscape design world don't share those values, and don't understand the balance between the needs of humans and the needs of nature. If, for example, you run across someone that won't ever allow you to plant a tree in less than 8 feet of tree lawn, you might be encountering such a person.
From the current post about the importance of soil and water:
Assuming a typical spacing range for boulevard trees, you are looking at about 200 – 500 ft3per tree of oxygenated growing media for the roots, which is not enough to support most large canopy trees. Here is a rule of thumb, taken from the excellent book Trees in the Urban Landscape: Site Assessment, Design, and Installation - you want 2 ft3 of soil for every ft2 of canopy area. So for a tough urban tree like a honeylocust with a 30 foot wide canopy, you would want about 1,413 ft3 of soil. Naturally, this volume isn’t absolutely necessary, but the lack of oxygenated soil is one of the main reason why the average life expectancy for an American street tree has been estimated to be between 7 and 15 years.
Another factor in the short shelf life of trees is the challenging establishment period, when trees are most vulnerable to drought and disturbance. This is particularly true for trees that are planted in highly disturbed urban sites, where compacted soil acts as a barrier to growth and limits access to water and oxygen, immediately stunting the tree and leading to decline and death. This condition is common in undersized tree pits typically specified in parking lots or commercial areas, where grading, filling, and construction activity create soil conditions that cannot sustain long-term growth. Compound this with shoddy nursery stock and low-bid planting methods and you have created a disposable landscape, where every year or two you pull one tree out of its coffin and put a new one back in without fixing the underlying problem.