Tree
Taproot
A taproot is a single thick root that grows straight down, and while many young trees start with one, most mature landscape trees rely on spreading side roots.
A taproot is the main root that grows downward from a seedling. University of Arkansas Extension says trees often start life with a taproot, but the taproot only grows downward during the first season.
After that, most trees spread sideways with lateral roots. The ISA journal Arboriculture & Urban Forestry says that if a taproot persists, it is usually largest just beneath the trunk and shrinks fast in width as other roots branch from it.
This matters for stumps. People often worry a deep taproot will keep a stump alive after grinding, but most roots lie in the top foot or so of soil. Some oaks, hickories, pecans and black walnuts have stronger taproots than most. Roots left below the cut rot over time. See do roots die after stump grinding.