A tree’s fate is largely tied to the health of its roots and the state of the surrounding soil. Unfortunately for many homeowners and arborists, the bulk of the tree’s roots are out of view. Without taking drastic or invasive investigative steps, the health of the tree’s roots must be inferred, rather than directly observed.
Accordingly, the best way to protect your tree is to protect enough of the tree’s roots to sustain it through any damage. With luck, the tree will survive the trauma, generate new roots and thrive for years to come.
Roots perform four key functions for trees. Drawing water from the ground is among their most important duties, but roots also absorb essential minerals from the soil. Roots support the weighty trunk, branches and canopy; serve as a conduit for the transport of different substances; and store energy for future use.
When roots sustain damage, the leaves often wilt or drop prematurely. This reduces the amount of water and minerals that can travel up the tree, and it reduces the amount of food that the tree can produce. If enough of a tree’s roots suffer damage or disease, death is all but certain.
Just as a tree’s branches do not all perform the same functions; a tree’s roots delegate different tasks to different portions of the root system. While the large, woody roots at the base of the trunk help stabilize the tree and serve as a shared conduit for the transfer of water and minerals, the fine, white roots absorb most of the tree’s water and minerals.
The Damage Done
A variety of traumatic events can compromise a tree’s roots, but construction damage is one of the most common culprits. Heavy vehicles and foundation work cause soil compaction, which damages roots and alters the soil structure. Grade changes are also damaging — raising the grade buries the tree’s roots too deeply, while lowering the grade exposes the roots, which can lead to further damage and decay.
Trenches dug for utility lines may cut large swaths of a tree’s root system, and landscaping equipment may slice through surface roots. Even if the tree is capable of coping with the reduced water and mineral uptake caused by such events, it is more likely to fail in high winds.
Because few treatments can help repair damaged roots, preemptive strategies are of paramount importance.
Critical Root Zone
Not all root damage is deadly. Trees can survive with some portion of their existing roots – they do not need the entire network. Over time, trees recover from root damage by generating more roots. However, some portion – a critical portion – of these roots is necessary for the tree’s survival.
Tree care professionals call this portion of roots the tree’s critical root zone. Although the tree’s entire root system may extend two to three times farther from the tree than the critical root zone does, this area must be protected at all costs.
Different authorities calculate the critical root zone of a tree in different ways. A 1991 study by Patricia Lindsey and Nina Bassuk, published in the Journal of Arboriculture, recommends allowing 2 cubic feet of root zone space for each 1 square foot of canopy spread. (Bassuk, 1991) This usually translates to a circle with a radius of about 1.5 times the radius of the drip line.
Another formula in common use requires 1-foot of critical root radius for every 1-inch of diameter at breast height (4.5 feet above the ground). In other words, a 6-inch thick tree requires a critical root radius of 6 feet. Other tree professionals recommend allowing 1.25 feet for each inch of trunk diameter.
The critical root zone varies with species, site and soil conditions, so determining the critical root radius of a tree accurately requires extensive knowledge and experience. However, by using the formulae above, you can determine the general size of a given tree’s critical root zone.
Bassuk, P. L. (1991). Specifying Soil Volumes to Meet the Water Needs of Mature Urban Street Trees and Trees in Containers. Journal of Arboriculture.