I was again saddened to see the state of the sidewalk trees of Toronto when I visited last week. I don't understand why the City can't plant trees in a way that will give them a fighting chance! It's an eyesore and a complete waste of money to plant a tree in a concrete box inlaid in the sidewalk, with about 20cm by 20cm of space for rainwater to enter. Yet they do it over and over, replacing the dead sticks with new ones that will die in 1 summer. Take this photo as an example. At left, the trees at the new MARS Discovery building were planted in the lawn. The trees along the sidewalk (the same species, I believe) were planted in concrete inlays. They are the same age and are only 5m apart, yet one set thrives and the other is completely dead. For anyone who lives in Toronto, this is predictable. Why it isn't predictable to the city's arborists is a good question I would love to have an answer for. It can't be more economical to use concrete when you know you will have to replace the trees every few years!
Incidentally, here in Cambridge, MA, they plant the trees with narrow metal gratings around the roots, so water can enter. They seem to be doing very well, and don't cause any disruption to the available walking space on the busy sidewalks, while still allowing for rainwater irrigation and some air exchange in the soil.