About Roots and Friends

The roots provide two basic functions to the tree:

• intake of water and water-soluble nutrients;
• structural support - the foundation

Water enters the system through fine, fibrous root "hairs. In most species this phenomenon takes place by means of a symbiotic connection between the root fiber and beneficial fungi, known generally as mycorrhizae. Without mycorrhizae present, roots will not take up water. Any activity that destroys the mycorrhizal community in the tree's root zone will damage or kill the tree. Such activity can include removal of existing topsoil and subsequent lawn or garden installation or major construction.

Structural anchoring foundation is provided by larger, woody roots that spread and branch out laterally from the base of the tree. Normally they extend at least as far as the tree's "dripline" (the outer perimeter of the leaf crown). Often they reach twice as far. Normally they are oriented laterally or obliquely in the top three to four feet of soil. Often a tree's entire root system does not penetrate deeper than 24" below surface grade.

One of the most deceptive, dangerous, and sometimes deadly situations arises when an outwardly healthy, attractive-looking tree fails. Whether by uprooting or by the loss of a large part in the canopy, such failures often indicate serious root problems. Root problems can be divided into four basic categories:

• disease
• mechanical damage
• soil compaction
• damage to pavement and structures

Disease most often occurs in areas of poor drainage - saturated soil, standing water in the "critical root zone" (CRZ). The CRZ is usually defined as that area of ground over-shaded by a tree's foliage canopy (or crown) or bound by the "dripline." In many cases the CRZ is extended to 1.5 times the dripline, or an area that radiates half again as far from the trunk's base to the dripline. This larger CRZ reflects the true extent of most mature tree foot development. On sites scheduled for intensive development, a consulting arborist and engineer may agree on a CRZ actually smaller than the dripline. Depending on tree species and condition and on other site factors, a smaller CRZ works fine and trees can survive and flourish after project completion. Such a result depends on one key requirement: the CRZ must remain inviolable throughout construction.

Even when developers observe CRZ protection measures, other activity nearby can still lead to root problems. For example, excavation and standing trees a hundred feet away can lead to changes in water supply too much or too little which, in turn, will eventually cause root zone problems. Mechanical damage most often results from construction/excavation work. Building foundations, utility ditches, and grade changes wound, tear, break, and rip large roots. Technology now exists to carefully expose large roots and cleanly cut them prior to excavation.

Soil compaction can often lead to poor drainage conditions around standing trees without CRZ protection. Compaction by heavy construction equipment is a common problem on many development sites. The related problems of compaction and subsequent root disease often do not manifest in very large trees until several years after a building project has been completed. On some sites standing trees will appear suddenly and mysteriously die. In other sites they will fail structurally, often with catastrophic loss of life or property. Such failures can and do occur before the tree even looks sick.