[chris (OP)]

An interesting question came in to the Naked Scientists radio show last night:

A gentleman phoned in to ask how the trees that you see growing along city streets and often encased by the pavement and concrete surrounds nevertheless are able to access sufficient water and nutrients.

We had quite an interesting discussion about it, but I thought I'd enquire what everyone here thinks too?

[CliffordK]

Many trees that are planted in say a 6'x6' box are dwarfs. 

Older larger trees planted along roads frequently have nearby lawns which are often heavily watered.

I believe many of the small squares are also watered, but I could imagine difficulties if a tree had a 20' rootball.

[chiralSPO]

Actually, a common problem for these street trees is too much water (too little air)! If not constructed properly, one ends up with a small box of dirt surrounded on all sides by concrete, and this will not drain very well. Many of the trees that do well in these places are swamp-adapted, like sycamore, swamp oak, and willow.