In projects that require digging or grading, trees are too often regarded simply as objects or obstacles. Roots that are inconveniently located  in relation to the project tend to get cut, crushed, buried and suffocated. The above-ground portion of trees typically receive more careful attention, simply because that portion is visible for all to see.

Roots, on the other hand, are hidden from view. When they are traumatized by trenching or grade changes, the ugly truth is often swept under the carpet.

Here are a few typical scenarios I have witnessed over the years:

 A property owner plans to build a lovely inn, retaining a grand old tree at one side of the location. Tree protection is written into the plans. A backhoe contractor disregards root protection specs, severing numerous large woody roots on one side of the tree. The old tree looks fine for about two years, then the upper branches start to die. Within about five years, the tree has severe dieback and is taken down. 

 A developer plans a small subdivision. The site has a few large old trees, which are retained to grace the nice new homes. A miniscule root protection zone (rpz) is established for the trees. Just outside of the tiny rpz, soil is graded, compacted and paved. The homes are sold and the trees look OK for a year or two. Then, a large old specimen, of a species that is not very tolerant of change, declines, slowly dries up and dies.

  In a project requiring trenching near old trees, the excavator digs a trench through a tree root zone within a few inches of the base of a tree trunk, severing all of the anchoring roots on one side of a tall, mature tree. The tree is now structurally unstable as well as mortally wounded in terms of its ability to absorb water and minerals from the soil.

Construction protection is a challenging aspect of an arborist’s field of work, especially when trees are obstacles for contractors on a tight schedules. Without getting into the legal, political and contracting aspects of this, let’s look at a few simple guidelines that can help to avert tree disasters.

Protection zone/critical root zone

In general, trees live on a system of wide-spreading, very shallow roots. Large woody buttress roots near the trunk are most critical for anchorage, while smaller woody and non-woody roots absorb water and minerals. Root protection zones should be determined with respect to those functions.

In a helpful article, “Root Pruning” (Western Arborist, summer 2007) Bruce Hagen summarized a large body of information from various sources  to provide us with guidelines. At the risk of over-simplifying, I’ll say that the gist of it is this: The number and size of roots cut and the distance from the trunk are most important. 

Here are two  rules-of-thumb he provided regarding “Critical Root Zone:”

In terms of structural stability, major buttress roots severed within a distance equal to three times the trunk diameter provide little or no structural support.

In terms of tree health, the critical distance is five times the trunk diameter for healthy, vigorous trees if the root pruning is only on one side; seven times if more than one side, and ten times the trunk diameter if all sides are impacted.

He goes on to provide numerous guidelines regarding decay in roots, wind effects, trenching techniques, compaction effects, timing, irrigation and monitoring root-pruned trees.

Tree species and age

Different tree species have inherently different abilities to generate new roots when their roots are severed. Likewise older trees tend to have less capacity to recover from damaging changes.

For example, coast redwoods can sprout new roots or shoots from just about any point on the tree, and their wood is resistant to decay. So, a vigorous, young coast redwood has relatively good potential to withstand root zone disturbances.

In contrast old valley oaks tend to survive on a long-established equilibrium of  foliage  and roots supporting and sustaining a huge mass. When stressed, they become increasingly susceptible to disease and decay. They need greater attention to care and protection. A plan should be in place long before the graders and trenchers start their engines!

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