Study

On the maximum extent of tree roots

Stone, E. L.; Kalisz, P. J.

Forest Ecology and Management, 1991

Peer-reviewednarrative reviewwild

What they did

A review of excavations, tracer studies and root-grafting observations, assembled to establish how far from the trunk and how deep tree roots have actually been shown to extend.

Studied in: Pinaceae, Fagaceae, Salicaceae
Sample size: published excavation and tracer records for forest trees

Lateral spread commonly reaches several times the radius of the crown, and in open-grown trees roots have been traced far outside the canopy edge. Depth records vary hugely with soil and water table. The convention that roots occupy the area beneath the canopy has no support in the excavation record.

What it means

The authors' reading, and ours. Where they differ, that difference is the point.

How the authors put it

Tree root systems are far more extensive laterally, and more variable in depth, than the canopy-projection rule of thumb implies.

How NatureHQ reads it

This is where the misconception is addressed head-on. The image of roots mirroring the branches is intuitive, memorable and about as wrong as a widely held idea gets: a tree's roots typically extend well past the edge of its crown, and neighbouring trees' root systems overlap thoroughly. That overlap is the physical precondition for everything about shared fungal networks, root grafting and below-ground competition — none of which makes sense if each tree keeps to its own footprint.

Limitations

A synthesis of case studies, many of them opportunistic, rather than a designed survey.
Weighted towards temperate forest species.
Extent is easier to establish than function; that a root reaches somewhere does not establish what it does there.