My previous two columns have dealt with space in terms of tree selection for a given purpose in a given space, and tree decline caused by the loss of growing space.

Now, let’s look at some guidelines that can help prevent problems with respect to tree health and structural stability.

It is crucial to understand that the root systems of trees spread widely. They do not “mirror” the shape of the above ground portion of the tree or simply grow downward near the base of the trunk. Sure, trees grow deep roots, too. I’ve seen slopes cut for construction projects, revealing how oak roots can follow vertical fissures in bedrock. For a better comparison, and practical purposes, imagine a wine glass sitting on a large platter. The wine glass represents the above-ground portion.  The platter represents the root system.

To carry the analogy one step further, the “platter” portion of the root system keeps the tree healthy and the base of the “wine glass”  — the buttress roots — keeps it standing. How much of the platter or the base can you take away before you have a problem?

In a great article on the subject (“Root Pruning,” Western Arborist, summer 2007), Bruce Hagen outlined construction practices and guidelines for “avoiding or minimizing impacts to tree roots systems when pruning roots, repairing root-damaged pavement and managing construction projects near trees.” He discusses a key aspect of tree protection: the critical root zone (CRZ), a circular area around the tree containing sufficient roots to allow a tree to survive. Drawing on various sources, he gives guidelines based on trunk diameter.

You have probably seen temporary protection fencing set up around trees in construction areas. The typical placement is at the dripline, the edge of the canopy. That is a pretty good distance, but not for leaning or lop-sided trees. Instead, think of the CRZ as a radius from the trunk equal to about 10 times the trunk diameter.  For a tree with a 1-foot trunk diameter, the protected root zone would be a 10-foot radius from the trunk. So, the protected 12-inch diameter tree would sit at the center of a 20-foot diameter circle.

Likewise, a 48-inch DBH tree (4-foot trunk diameter) gets a CRZ radius of 40 feet.

The proportion of CRZ to trunk diameter is variable, depending on the preference of the municipality, the arborist, tree health, species and other factors.

That would be your basic minimum tree protection zone, where no major changes  such as compaction, paving, trenching, grade cuts or fills, should occur. In the real world of construction projects, it is often necessary to make big changes within that area, so we need some guidelines as to what may be done and how it may be done.

First, think about tree stability. As Hagen outlined, “Trees should be considered at high risk (of falling over) if more than one-third of the major buttress roots are severely decayed or severed.” Furthermore, major buttress roots that are severed within a distance equal to three times the trunk diameter should be considered as providing little or no structural support.

So, if that 12-inch DBH tree has six buttress roots, it could be severely destabilized by severing more than two of them within three feet of the trunk.

And that is only with respect to structural stability. 

In terms of tree health, Hagen stated, the loss of roots on one side of the trunk at about five times the trunk diameter is generally considered to be acceptable, depending on tree health, condition and root distribution. The distance should be increased if roots must be cut on more than one side.

So, if a utility trench had to be dug across the root zone of that 12-inch DBH tree, it would be best to locate it no nearer than five feet from the trunk. As an option, if a trench location cannot be changed, it might be possible to tunnel under critical roots and install the required utilities without severing many roots.

Here are a few other important points about root protection:

• Roots anchor trees by tension. Consider the prevailing wind and avoid cutting roots on the windward side. On slopes, don’t sever roots on the uphill side. For leaning trees, don’t sever roots on the side opposite the lean. 

• Where roots must be severed, cut them cleanly with a sharp pruning tool. Don’t tear, rip or split roots. Where possible, prune roots as if they were branches, cutting the root back to its attachment to a good-sized lateral branch root.

• Don’t operate heavy equipment on unprotected root zone surfaces.

Root loss affects a tree’s ability to absorb water and minerals. Consider supplemental irrigation for trees that are to be affected by root zone changes.

Bill Pramuk is a registered consulting arborist. Visit his Web site, www.billpramuk.com, e-mail questions to info@billpramuk.com or call him at 226-2884.