Does this tree have deep roots?

In my initial interview with a prospective client, I frequently am asked to generalize about "this kind of tree."  One of the most common questions of this type is whether the tree has deep roots.

In fact, few trees have any deep roots at all, and those that do don't have many of them.  The reason is primarily the availability of oxygen in the soil.

Roots require oxygen for cellular metabolism and growth, which has to pass through the soil from the atmosphere to reach them.  Leaves produce oxygen, but except for a few water-tolerant species like bald cypress, trees have no vascular system to deliver oxygen to the roots.  And trees are certainly not the only organisms requiring oxygen in the soil!

The farther down you go in the soil, the less oxygen is available.  For most tree species, levels below 10-15% suppress growth, and 3-5% brings it to a halt.  (Atmospheric oxygen is about 21%.)  As a result, 80-90 percent of any tree's roots will occur in the top two feet of soil.  This is a broad generalization, but fairly reliable.

(See "Tree Root Systems.")

This takes us back to why the client asked whether a particular tree has deep roots.  As I understand the question, it has to do with the safety of the tree.  But if trees generally don't have deep roots, then what is the correct question regarding safety?

There are many factors influencing tree safety, and thus many questions to be asked.  While there is no way to make a tree completely safe, short of removing it entirely, there are prudent steps to take.  An arborist's job is to provide information and offer choices and recommendations.  Ultimately, the decision rests with the tree owner.  But the condition of the roots is indeed one of the right questions, and one that is too often overlooked.

Disturbances in the root area need to be taken into account.  The closer to the trunk, the greater the influence of the disturbance.  Contrary to popular misconception, the root zone typically extends far beyond the reach of the branches (commonly called the "drip line").  Trenching, compaction, construction and adding soil all have negative impacts on existing roots.  Additionally, drought and inadequate drainage frequently stress or kill trees.  A tree's age, condition, and species have great bearing on the severity of impact from such disturbances.  But some species just will not do well in a given soil.

I have a client who wanted deciduous native trees in her front yard, and selected bigleaf maple (Acer macrophyllum).  Unfortunately, "native" does not mean "tolerant of heavy clay soil" that's common in its native environment.  We finally substituted sweetgum, an exotic species, after two attempts to establish bigleaf maple ended in death.

If a tree is not well suited to the soil where it is planted, the effect will be systemic as well as structural.  The roots need to venture into the surrounding soil to support the tree and supply it with water and nutrients.  This is why we usually recommend NOT amending the soil in a tree planting hole.  Otherwise the roots would tend to remain where they can more easily get the nutrients they need, as well as air and water, and this would destabilize the tree.  (There are exceptions and ways to mitigate this tendency where amendment is necessary.)

A lawn will generally inhibit the growth of tree roots.  In the case of trees adapted to summer drought, such as coast live oak (Quercus agrifolia) or valley oak (Q. lobata), lawn irrigation can be fatal.  But all tree roots are out-competed by lawns for air, water, and nutrients.  Under a lawn, tree roots have to grow closer to the surface to have any chance of meeting their needs, which results in a shallower root system and a less well-anchored tree.

I have touched on some of the important issues for trees relative to their roots systems.  No analysis would be complete without taking into account the condition of the canopy, changes in the tree's exposure to wind and sun, the location and value of what we call "targets," or things that can be damaged by falling trees or parts, and other factors.

Copyright 2008 T. Gray Shaw