From The Ground Up
By Mark Webber
One of the greatest misconceptions by property owners and construction managers is that trees can easily withstand injuries from nearby construction activities. The roots of a tree are quite shallow and wide, so it’s important to understand why every root really does matter—and how to avoid damaging them.
The Root Of The Matter
Roots provide structural support to trees and are critical in supplying water and nutrients. While weather can impose great stress upon the canopy of the tree, the roots play a major role in dampening the strain placed on the tree from inclement weather. These loads in the upper canopy create potential tree-damaging energy loads that are transferred through the tree and ultimately affect the tree’s root-anchorage system. Therefore, it is incredibly important that a property owner work to protect and minimize unnecessary root damage by preventing construction-induced trauma to the tree roots.
By working to prevent construction damage, the trees’ fine root systems are also protected. A tree’s fine roots provide water and mineral salts from the soil to a tree’s leaves and/or needles for the life-supporting chemical process called photosynthesis. For tree roots to function properly, most trees require a minimum of 21-percent oxygen space in the soil. Additionally, the fine roots defend the tree against soil-borne pathogens by unique materials that surround each root tip called the root cap.
Tree roots, being sites of carbohydrate storage, play a vital role in a tree’s ability to withstand stress, especially during droughts, floods, construction, and other abiotic (non-living) events. By protecting these roots, the tree will have a long and productive life.
Not All Trees Should Be Preserved
Many trees are capable of surviving the process of construction, and it’s important that the project manager and owner work closely with an ISA-certified arborist before and during a project. An ISA-certified arborist will be able to communicate regarding each tree’s pre-construction condition and how that tree will be able to withstand the stress of construction—or not.
If a tree’s critical root zone (CRZ) is injured by excessive soil compaction, an increase in grade, mechanical injury to tree tops and/or roots, and other causes, the resulting injuries will cause the tree’s vital functions to be disrupted. Depending on the severity of these injuries, the tree will begin to show symptoms of decline as soon as 3 years after the event. Construction injuries can cause trees to lose limbs; or even worse, trees can uproot and fall over (also known as windthrow). In many cases, the company or persons who caused the damage initially cannot be held responsible since the work concluded years prior.
The national standard for building near trees is the ANSI A300 Part 5, which addresses issues related to the conservation of trees and woody shrubs during construction and development activities in the trees’ vicinity. This standard is an excellent resource, and one would be well-advised to work with an arborist when making decisions regarding tree preservation.
If you are considering building, constructing, or reconstructing any facility near trees, be sure to measure the position of each outer tree canopy in relation to the new construction. If the individual tree’s canopy is not at least two times that distance from the work area, then that tree is at risk of injury.
The next step is to lay out the area of construction by installing wooden stakes where the structure will be expanded or rebuilt on the property. Then discuss with the construction crew how much space they need when doing the work. After this discussion, place the stakes in the areas of those construction limits. Then measure and record each tree’s diameter breast height (DBH), which is measured at four and a half feet above the soil line.
Calculating The CRZ
To survive construction, most tree species will require reduced construction activity at least one to two times the individual tree’s DBH in inch measurements, then converted in equal value to feet measurements times one or higher.
Increasing A Tree’s Chances
Limiting construction access to a tree’s CRZ is important. The construction manager or property owner should allow only one access route on and off the property. All contractors must be instructed as to where they are permitted to drive and park their vehicles. Often, this same access drive can later serve as the route for utility wires, water lines, or even a driveway. Additionally, specify storage areas for equipment, soil, and construction materials, and limit areas for burning (if permitted), cement washout pits, and construction work zones. These areas should be located well away from protected trees.
Mulching the area within the CRZ with 4 to 6 inches of wood-chip mulch is a great way to reduce moisture loss from the soil, and as the mulch decomposes, it will create an ideal place for new tree roots to grow, which will help the trees recover. Do not apply mulch any closer than 12 inches from a tree’s trunk. These mulching practices will ensure there is no damage done to the protected trees’ root systems.
Before construction, set up sturdy fencing around each tree that has been predetermined by the CRZ formulation calculation to ensure that the root system, as well as the upper part of the tree, will be protected. These barriers should have weatherproof signs mounted on them to forewarn others not to alter or change the barriers. Some owners may choose to enlist a penalty system for contractors who move or violate these protected CRZs.
Any trees that have been exposed to injury are susceptible to biotic (living) pathogens, such as insects and disease. To help these trees recover, the property owner should provide supplemental irrigation, fertility, deadwood, and other acceptable horticultural practices. Pruning construction-exposed trees should be limited only to deadwood or branches that could cause potential injuries in the construction process. Retaining as many leaves and/or needles on a tree will be the key to encouraging the plant to regenerate as much energy by photosynthesis to replace the losses from construction injuries.
Planning Is Everything
If you choose to retain existing trees on a construction project, proper planning and implementation well before the project starts is required, and well after its completion. Trees provide many important benefits to humans and animals alike, and preservation can be accomplished with a certain degree of horticultural and arboricultural certainty if the trees’ critical root zones are protected and the trees are cared for after the construction work is completed.
Mark Webber is a Board-Certified Master Arborist by the International Society of Arboriculture with nearly 40 years of professional experience. He provides investigations, reports, and testimony in matters related to tree maintenance and removal; management of nursery and landscaping operations; and an assortment of horticultural issues. Mark earned a Bachelor of Science degree in Horticultural Science with a minor in Agricultural Business from The Ohio State University. He is designated as a Master Nursery Technician by the Ohio Nursery & Landscape Association. Mark is also recognized as a TechArborA by the U.K.’s Arboricultural Association.
ANSI A300 Construction Management Standard - Part 5.
Best Management Practices (BMP) - Managing Trees During Construction by Kelby Fite and E. Thomas Smiley.
Up By Roots by James Urban, FASLA, ISA.
Trees and Development: A Technical Guide to Preservation of Trees During Land Development by Nelda Matheny and James R. Clark