Old pictures of tree roots growing as a mirror image of the above ground structure may be beautiful but not really accurate.
This image of deeply rooted trees comes from the 1930’s research on trees growing in loose soils. In more recent years scientists found tree roots growing horizontally in the top few feet of the soil and extending as far as 2 to 4 times the diameter of the tree’s canopy. This looks more like a plate attached to the bottom of the tree trunk.
Soil conditions impact the depth and structure of tree roots. They grow where water, minerals and oxygen are available. This is usually in the top two feet of soil.
The fine feeder roots responsible for absorbing water and nutrients grow upward from larger roots closer to the soil surface where water, nutrients and oxygen are most abundant.
A bit more information: Understanding how roots grow can help us mange our lawns and landscapes in a more tree-friendly manner. Mulch the area under trees to eliminate competition from lawn grass and prevent mower damage to surface roots and trunks of the tree.