Deep roots: fact or fiction?

I keep reading about strategies to encourage trees to grow deep roots, in the hope that they become more drought-resistant and are able to access nutrients from deeper soil layers. Examples of such strategies are watering sparingly; injecting water deep into the soil instead of surface watering; or growing trees from seed to create a tap root.
But other knowledgeable folks claim that all the vital feeder roots are within 6-12" of the surface, so these strategies would be doomed to failure. I'm confused.
No doubt this varies by type of tree, so let's just focus on fruit trees.

Patrick, Only nut trees make taproots, & only if grown from seed. Any other transplanting usually destroys the taproot. Fruit trees do only have most of their moisture & nutrients absorbed by lateral roots in the top 12-18" of the soil. See the drawings in my Roots book. So mulch, mulch, mulch. (and read my book on drip irrigation to see how best to supply the water in the top 12-18" of soil. ) Deeper roots help stabilize the tree, anchor it and survive during extreme drought. Most of the nutrition is gathered by the lateral roots in the upper zones.

If one were to grow something like alfalfa under the fruit trees, might it do a good enough job of pulling up deep water and storing it in the top 18" that the fruit trees could benefit from it?

Jonathan

Jonathan, Todd Dawson has done a lot of work on hydraulic lifting - where trees gather moisture at night, bank it near the surface of the soil so it's readily available to the canopy. In the process some shrubs get some moisture to further there growth. (It's described in detail in my Roots Demystified book.) But this is not an agricultural system. I have no studies on alfalfa and trees. Might trace down Dawson and ask him. Google his name along with hydraulic lifting.