To keep up with research based changes in arboriculture and urban forestry we rely on guidance from a number of professional sources. The most important are the American National Standards Institute (ANSI) Practices and Limitations. Because the ANSI A300 standards are written in a very austere style, they are interpreted and expanded upon in the ISA Best Management Practices (BMP’s). Both of these types of publications guide professionals in writing and implementing tree work.
I was interested in the limits of root removal and established acceptable distances and amounts. As arborists we understand “how the roots go is how the tree goes” and the necessity to prevent root loss where possible. Arborists and urban foresters are challenged with root pruning due to infrastructure conflicts involving driveways, sidewalks, curbs, sanitary sewers, utilities, and other obstructions. Many of us also provide guidance for construction and development projects and we are required to create tree protection zones (TPZ) or root protection zones (RPZ) to help conserve trees throughout these processes and beyond. Most publications we rely on are published or available through the International Society of Arboriculture (ISA), the Western Chapter ISA (WCISA), or the Tree Care Industry Association (TCIA).
After reviewing several commonly used publications and Best Management Practices, it is apparent there are some general themes regarding root removal. Although root pruning along with the consequences has not been well studied, some understood principals apply. Roots compartmentalize similar to branches and Compartmentalization of Decay in Trees (CODIT) is applicable. The development of decay in cut roots can take years or potentially decades to develop. There are many factors that differ in root removal versus branch pruning and the consequences can be much more significant if a tree becomes unstable. Just as with branch pruning wounds, cut roots will become courts for wood decaying organisms and pathogens. Some best management practices can help reduce infection, but if the disease complex (sometimes referred to as the disease triangle) is complete, infection will likely occur regardless of our efforts.
There have been countless mitigation recommendations regarding what to do after root removal. For example backfilling within one hour, covering wounds with petroleum jelly or wound paints, fertilizing, mycorrhizal inoculations, nutrient applications, covering with wet carpet or burlap etc. are recommended in various ordinances, guidelines, and specifications. There is limited, if any, scientific evidence or studies indicating any of these remedies do more than make us feel like we are helping. Once the roots are cut, the ingress of air is initiated and the CODIT process begins. Backfilling and mulch may still be the best mitigation strategy as it creates an optimal environment for root growth along with fostering beneficial and antagonistic fungi to help reduce infection.
Within the publications reviewed the general theme was to not remove more than twenty-five percent (25%) of the root system and to make linear cuts beyond six times the trunk diameter distance (a factor of six). Ironically, or not so ironically, these two figures, twenty-five percent of the roots and six times the trunk diameter distance on one side, yield the same amount of projected root loss in a symmetrical root system. The distance of six times the trunk diameter is under the premiss the entire root zone is calculated at twelve times the trunk diameter in radius. Similarly, if the root zone is calculated at a factor of eighteen then nine times the trunk diameter would be the maximum encroachment yielding a twenty-five percent loss. In the most recent publications I found no information about cutting roots at say four or five times the trunk diameter distance. Best Management Practices: Root Management (2017) does state cutting roots at three times the diameter can result in loss of stability and anything closer would result in “severe loss of stability.”
Both the ANSI Standard Part 8 and the ISA Best Management Practices: Root Management (2017) suggest “selective" root removal is preferred over “non-selective” removal. Non-selective removal of roots is a “shoot first ask questions later” approach where roots are cut at an arbitrary distance. Selective root removal requires pre-excavation, typically by hand or with a pneumatic excavating equipment such as an Air Spade®, Air Knife®, or similar tools. Selective removal allows for the roots to be exposed prior to cutting at the appropriate locations. It is suggested roots greater than one inch in diameter should be pruned rather than left torn or crushed (Costello, L., Watson, G., Smiley, E. T.. 2017). However, it should be noted that most municipalities have their own criteria and the location, size range, amount that can be removed, and timing vary in the local ordinances. Non-selective root removal is likely more common and roots should still be “cut clean to a flat surface with all surrounding bark intact.”
If you are a municipal arborist, the distances recommended for root pruning rarely apply in the real world. Most municipal situations require root removal at the root collar where sidewalk and curb damage has occurred. Understanding the species characteristics and evaluating the trees and site prior to and after root removal is critical. The ISA Municipal Study Guide (2008) (Pg. 121) states “there are no clear standards for either the amount of root tissue that can safely be removed or the minimum distance of pruning from the trunk.” In this instance having good written specifications, policies, or ordinances may help guide the process. The ANSI A300 Part 8 section 84.5.5 states the following: “when roots are damaged within six times the trunk diameter, mitigation shall be recommended.” Similarly ANSI A300 Part 5 section 126.96.36.199 states “When the distance of 6 to 18 times the trunk diameter cannot be met, appropriate mitigation or determination that the work will not impact tree health and stability shall be performed.” These statements allow for the arborist specifying the work to provide a written recommendation for mitigation when typical guidelines cannot be met to achieve the desired objective. For municipal arborists this may be helpful in guiding contractors or workers when root removal at the trunk is required or unavoidable to retain a tree.
Root cutting is never advised but may be necessary under certain circumstances. Shaving sapwood off the top is preferred to severing the entire offending root. This allows for maintaining some structural integrity and root function. Municipal arborists and urban foresters evaluating potential root loss and cutting should perform a systematic and proper evaluation. To help evaluate the likelihood of failure and survival after root loss, an assessment of following factors is a good starting point (Table 1).
It may be appropriate to perform a Level 3: Advanced Risk Assessment (ISA. Best Management Practices: Tree Risk Assessment. 2017) by performing a “pull test” to help identify if the root pruned tree is stable or the use of “decay detection devices” can be employed to gain greater information about potential for failure at this critical part of a tree.
Nearly all the publications reviewed, stated root removal on one side should be performed no closer than a distance equal to six times the trunk diameter (Table 2). The rationale behind this distance is likely because it results in a maximum of about twenty-five percent total root loss (based on an assumed root area radius of twelve times the trunk diameter) and is not too close to the trunk to likely cause structural instability or health concerns. There is language in the standards and BMP’s to allow for cutting within this maximum encroachment distance if warranted and mitigation is recommended.
Root cutting is never advised but may be necessary. If it is, a systematic evaluation of the tree and site should be performed. Roots should always be cut with sharp tools so as to leave “a clean flat surface with intact surrounding bark.” Once root removal occurs, a management plan with required mitigation measures and periodic inspection intervals should be established and implemented. Regardless of checklists, formulae, and arbitrary distances, the most valuable tool is our own experience, observations, and judgement. If it were just about the numbers, who needs the arborist?
ANSI A300 Part 5: American national standard for tree care operations: tree, shrub and other woody plant management: standard practices (Management of Trees and Shrubs During Site Planning, Site Development, and Construction) 2012.
ANSI A300 Part 8: American national standard for tree care operations: tree, shrub and other woody plant management: standard practices (Root Management) 2013.
ANSI A300 Part 9: American national standard for tree care operations: tree, shrub and other woody plant management: standard practices (Tree Risk Assessment a. Structural Failure) 2017.
Costello, Laurence R., Bruce W. Hagen, and Katherine S. Jones. 2011 Oaks in the urban landscape: selection, care, and preservation. Oakland, CA: University of California, Agriculture and Natural Resources, Print.
Costello, Lawrence, Gary Watson, E. Thomas Smiley, 2017. Best Management Practices: Root Management International Society of Arboriculture, Print.
Costello, Lawrence and Catherine Jones, 2003, Reducing Infrastructure Damage by Tree Roots: A Compendium of Strategies. Boston
Fite, Kelby, and E. Thomas Smiley. 2016 Managing trees during construction, second edition. Champaign, IL: International Society of Arboriculture.
Clark, James R., and Nelda P. Matheny. 2008 ISA Municipal Specialist Certification Study Guide.Champaign: International Society of Arboriculture,.
Smiley, E. Thomas, Nelda Matheny, and Sharon Lilly, 2017, ISA. Best Management Practices: Tree Risk Assessment: International Society of Arboriculture. Print