The Subrerranean Canopy
Roots. They compose a larger portion of trees than many people realize. Sans the mass of the trunk, roots make up approximately half of any given tree’s mass; 40%-60%. They perform many essential functions as the canopy’s counterpart. Water and mineral osmosis, food storage, gas exchange, hormone production, and stabilization are among the tasks that a tree depends on its roots for.
To perform the daunting task of holding itself upright, a delicate balance must be achieved between root and canopy. Whilst aloft in large trees, I am often struck by their bewildering strength as I observe each lead bending, twisting and swaying independently in different directions. All these forces are absorbed and stabilized by the trunk and held firm in the ground by the roots.
Trees respond to these shifting forces with reaction wood. If there is a force that is consistently applied to a tree, that tree will respond with a more vigorous secondary growth (a thickening) in areas that are most adversely affected by that force. For instance if I trim an older, strong scaffold branch from the trunk of a pine tree; at its base that branch will exhibit a pattern of annual growth rings that is thinnest at the top portion of the branch, and much thicker around the bottom area of the branch. The branch grew more strongly around the bottom side to compensate for the constant downward pull of gravity on that branch. This reactive growth occurs throughout the tree, including where it reaches into the earth. Here in the earth, for example, the distribution of roots is affected by forces above ground such as prevailing winds. Consistent winds from one direction will cause a tree to grow its roots more strongly on that side to effectively support the tree.
In an urban setting, a tree’s capacity to sustain itself below ground is often greatly inhibited due to foundations, parking lots, sidewalks, lack of topsoil, heavy foot traffic, etc. Unless a property was specifically designed to accommodate a tree, these limitations often already exist. There are ways, though, that we as stewards of our already existing trees can prevent further root inhibition to promote their health.
Any digging near a tree should take the roots into account. Small holes for planting flowers near the base of a tree should be fine as long as you take care not to indiscriminately cut roots. Work carefully around larger roots, and if you need to cut secondary roots, make clean cuts to allow for shorter healing time. Of greater concern are lateral digging projects that may be shallow, but involve a substantial horizontal section of soil near trees, vertical digging projects that dig deep in an area near trees, and significant grade changes that take place within the dripline (if you were to draw a vertical line from the outer edge of a tree’s canopy all the way around the tree, that line would be it’s dripline.) of a tree.
The first of these occurs, for instance, when a driveway is installed near a tree. In preparation for the driveway subgrade, contractors often over dig the area below the driveway in order to install suitable base on which a solid driveway can be installed. This can be a substantial over dig, or quite shallow depending on the soil and area. At minimum, a very shallow cut is made; 6 inches, for example. Consider now that the vast majority of a tree’s roots grow in the top 18” of soil; the most delicate of which (absorbing roots that are responsible for the majority of water and mineral uptake) grow in the top 6”of soil, reaching up to the surface for water and air (See figure 1 for a rudimentary representation of how shallow a tree’s root system often actually is, compared to the popular belief that roots always grow very deep. Seedlings often develop tap roots early on in efforts to become established, but this taproot in most species becomes secondary to the stronger horizontal roots as a tree ages).
Thus, even with only the top 6” of soil stripped, most of the roots that sustain the tree’s larger supportive roots in that area are killed. Sometimes vigorous trees can react quickly enough to survive and remain healthy. Many times though, the elimination of feeder roots in a large section of a tree leads to the death of those large roots. This in turn, can lead to the gradual decline of the tree over the next several years. Other times, the death of those roots results in structural failure resulting in the tree succumbing to windfall because it no longer can hold itself upright amidst wind and/or gravity. I recently observed a newly installed driveway not 10” from the base of a beautiful ponderosa pine that I estimated to be at least 300 years old. I fervently hope that that amazing tree can withstand the death of a large portion of its roots and remain healthy!
If you are considering a project such as a driveway or sidewalk in proximity to your tree, please consider your tree’s health and longevity; as well as the safety of pedestrians who pass near your tree. Can you divert the path of your driveway or sidewalk to avoid disturbing your tree’s roots? Perhaps a gravel driveway or path would suffice. This would disturb the roots far less and allow for water and air penetration into the soil. Compaction may cause some root loss, albeit far less than would a digging project.
In projects that involve vertical digging, such as a trench for sewer pipe installation or repair near trees; thick primary roots are cut, eliminating the tree’s support and water/mineral absorption in that area. The closer the trench is to the tree, the more damage occurs. A straight trench cut at the dripline of a tree will eliminate approximately 15% of its roots. A trench cut midway between the dripline and the trunk will reduce the tree’s roots by roughly 30% (see figure 2) These are generalizations and can differ from tree to tree, but the important thing to remember is that the closer you get to the trunk the more damage will occur when digging.
Digging a trench near a tree whilst maintaining its roots is no easy task, but it is possible and worth the effort if you have a beloved tree hanging in the balance. Large diameter roots can be dug around, taking care not to damage them and performing clean cuts on those roots which must be cut. This will involve setting aside the convenience of your trencher and digging by hand, but your tree will love you for it!
Significant grade changes near trees are another common stress for tree roots, unless the material added is highly porous. If soil must be added near a tree, less is better. If there is no way around this, and there will be 12” or more added, a tree well is recommended. Leave as wide a circumference as you can spare around the trunk of the tree at its original grade level. Make the grade change, leaving that well untouched. A retaining wall is usually necessary to maintain the well. With this technique, the tree will be left with at least a portion of its roots near enough to the surface to survive as it adjusts to its new grade beyond the well.
Contractors and homeowners, as you embark on your landscaping and/or building projects please consider your trees and their subterranean canopy. If you have questions, please contact an experienced arborist. On construction sites where there will be digging and/or heavy equipment operating near trees, hire an arborist with which to establish plans to properly protect the existing trees. Those trees are an important part of Colorado Springs’ urban forest, and an asset to the value of the property. Beloved Earth LLC is always happy to provide a free consult for any who want to treat their trees right!
1 “Trees: Their Natural History” By Peter Thomas p.72
2 “Trees: Their Natural History” By Peter Thomas p.85