Engineer offers excellent tree + stormwater technical assistance resource; 2/13/09

Using Trees to Manage Stormwater - (Green Street Series ... By Matt Baumgardner, PE
Trees have an integral part in the green street stormwater strategy. Essentially, they help to collect rainwater in the canopy and then direct that water to the ground - allowing the water to infiltrate and recharge the groundwater table in a controlled manner. Water stored in the canopy is transpired back to the atmosphere.
The key factor impacting the tree’s ability to peform these tasks effectively is inadequate soil volume. Trees often have their root movement restricted within tree box installations. Highly compacted soils from pavement installation also impedes the roots from growing to full potential. The compacted soil acts as a deterent to stormwater infiltration, as well. The net effect is that the trees are smaller with reduced canopies.