I realize that mature trees typically have shallow roots that often go beyond the drip line of the tree. Very true. Roots of some trees have been traced back to a tree as far as a mile or more away.
In a situation where there is not enough water going to a tree from its shallow root system, can you make up for that by just adding water close to the trunk and allowing the tap root of the tree to get access to that water? Many experiments have been done to determine how water supply affects roots. One thing we do know is roots are incapable of following or seeking out water and nutrients, including oxygen. They simply grow where there is an ample supply of resources and die back where the supply is inadequate. It was found that with regard to a particular red maple, >90% of the tree's fine roots were in the top 6" of soil in early spring, with fine roots disappearing in the top 6" and growing deeper and deeper as summer progressed such that by August's end more than 90% of fine roots were located at depths below 18". That shouldn't be read to mean that all trees are capable of being so nimble when it comes to adjusting to droughty conditions.
Many trees and shrubs don't have a tap root (root radical). Anything created by a cutting or layering won't have a taproot. Too, in many cases long taproots are severed when seedlings are potted individually, and will develop several anchor roots growing downward from horizontal roots radiating from the basal wound on the cutting.
Roots of a seedling:
Roots on cuttings or layered plants are normally much more tidy and balanced than those seen on seedlings:
Al
