| Deep-rooted trees use a lot of water, which causes some gardeners to advise cutting back on them to preserve the water table. However, there are many other issues, depending on the region. For example, when water use is an issue in backyard gardens, many deep-rooted trees can be trained through underwatering to take their water from the soil rather than the hose. Alternatively, some people - especially forest gardeners - suggest planting more around deep-rooted trees to take advantage of the water, minerals, and nutrients they bring up through the soil.
In Australia, where the native vegetation is often clear-cut and the salt from the ocean gets blown into the soil, planting deep-rooted trees and other deep-rooted plants is one ecological way to de-salinate the earth.
Deep-rooted trees also prevent erosion, by holding the ground together as well as by holding rainwater and returning it to underground water tables. According to Trees for the Future, one ten-year-old Guanacoste tree holds over eight tons of water.
Some deep-rooted trees include:
Most oak trees
Black walnut
Hackberry
Hickories
Butternut
Tuliptree
Apple and stone fruit trees are variously described as having deep and intermediate roots. Intermediate-rooting trees include:
Apple
Sweet cherry
Sour cherry
Peach
Pear
Plum
Grape, wine and fresh
American chestnut
Chinese chestnut
Sugar maple
Trees with shallow roots can be planted in between deep-rooted trees to take advantage of their water and natural mulch.
Sources - and for further reading:
http://www.clima.uwa.edu.au/beanfiles/salinitysp.htm explores the question of salinated and de-salinizing soils;
http://www.treesftf.org/Newspg6.htm is one issue of the Trees for the Future newsletter;
http://www.users.on.net/arachne/plants.html discusses permaculture gardening using mostly tree crops;
http://www.gov.on.ca/OMAFRA/english/crops/facts/info_tiledrains.htm#Since offers some good lists of Canadian trees with different root systems, from the Ontario Ministry of Agriculture and Food. | 
