Conditions of concern may be defined as tree issues that the client and arborist perceive as unacceptable. In my experience, they usually fall into one or more of three categories. Typically, it is about structure, clearance and appearance. (Photo 1) Our task, as arborists, is providing solutions to these issues.

Photo 1: Conditions of concern are usually about structure, clearance and appearance. All photos courtesy of the author.

Part of the solution process is development of tree-based mitigation options, which may include tree support systems, pruning and sometimes removal. Common objectives for pruning often address these conditions of concern. Some of the more common conditions relative to objectives include reducing risk, managing the size of a tree and creating clearance for sightlines or traffic and people.

Photo 2. A dominant leader is the key to a sustainable form.

Photo 3: Trees interfering with line of sight create a high-risk situation.

Reduce risk of failure
One of the more important objectives of pruning is to reduce the risk of failure. This includes removing or reducing defective branches or pruning to improve a structurally weak crown. Risk reduction involves improving tree stability and creating a durable crown with good branch structure. The ideal time to initiate this type of pruning is when the tree is being installed in the landscape, or when the tree is young and newly established in its location. All too often we inherit poor structure, especially on larger, mature trees that require advanced techniques and equipment to remedy these concerns.

Trees form their branch structure genetically when growing in their natural environment. However, this structure may be influenced by nearby trees or by mechanical damage, such as weather events or animal browse. In contrast, trees in the built environment are intentionally pruned with specific goals in mind to create urban-compatible forms. Sustainable, longer-lasting trees are the result of structural pruning when the tree is young; however, a tree of any age may benefit from this type of pruning.

Structural pruning emphasizes manipulating the architecture of the tree, such as branch-aspect ratio – creating strong branch attachments – branch orientation and spacing. The primary goal for trees growing into large shade trees is a strong, dominant leader system. (Photo 2) Crown architecture is dependent on its natural form, which can be either excurrent or decurrent. Excurrent tree species (pyramidal shape), such as most conifers, will develop a central leader naturally. Decurrent species (rounded shape), such as oaks, may not inherently develop a dominant leader, requiring intervention to improve structural conditions.

Structural problems can be created by misguided pruning performed in the nursery or when initially installed. This is where the focus is on symmetry and lush crown development. Often, pruning is necessary to remove dead or dying branches or those affected by insect damage or disease. Damaged or crossing, rubbing branches should be a part of structural considerations as well.

Size management
Trees with nearby conflicts will require size management. This objective requires the arborist to make the overall crown size smaller. This is often the case with utility conflicts that demand branches be removed or reduced to meet clearance requirements. Pruning for size management requires the arborist to pay careful attention to making appropriate reduction cuts. However, this type of pruning situation can easily lead to topping, which is an unsustainable and unhealthy pruning practice.

Topping is the reduction of tree size by heading back many or most large, live branches without regard for tree health or structural integrity. Topping manages height and spread but leaves behind woody stubs, leading to decay, weak branch attachments and increased likelihood of failure.

When trees are not selected properly or placed properly on the site, they often outgrow their intended space and design. Also, new structures placed near trees, such as utility hardware or buildings, require pruning as the tree continues to grow and expand. The objective of clearance pruning is reducing conflicts with buildings, streetlights, signs or even other trees. Utility-
vegetation management is focused heavily on clearance pruning to maintain regulated distances between trees and utility lines.

Graphic 1: Understand and communicate branch order for prescriptive pruning.

Visibility
Pruning to improve or maintain views is another important objective for visibility and safety. Sightlines are a particularly important consideration with street design and urban planning. (Photo 3) Insufficient sight distance is the condition of concern at street intersections for vehicular traffic and pedestrian crossings. Intersection sight distance is typically defined as the distance a motorist can see approaching vehicles before their line of sight is blocked by an obstruction near the intersection. The driver should have an unobstructed view of the intersection, without the interference of any hedges, trees or structures. This is an important consideration in tree selection and placement as well.

Photo 4: Removing second-order interior branches results in lion tailing.

The choice of a tree that will quickly grow and/or outgrow the space also will impact pruning intensity and frequency. This increases maintenance inputs often unavailable to many tree managers, especially for municipalities. Additionally, clients may request pruning in order to maintain the visibility of views, such as a lake or landform in the vista. Maintaining these important vistas requires input from the property owner and collective discussions with the arborist.

Conditions of concern caused by pruning
As might be expected, errant and misinformed pruning can create problems for trees as well. Pruning applied to the wrong branches can create and encourage the development of tree structure poorly suited to many urban spaces. Typically, issues are created when smaller branches are removed from first-order branches by someone standing on the ground with loppers or pole pruners. (Graphic 1) Loppers and pole pruners are useful, but only in combination with reduction cuts at the ends of the pruned first-order branches.

Removing only second-order branches (called lion tailing) leaves foliage in a mass toward the ends of first-order branches. Lion tailing is a substandard pruning practice that can increase likelihood of failure especially in storms, because branches respond by growing too long without enough taper. (Photo 4)
Taper is the thickening of the branch that occurs toward its base. Tapered branches are like flag poles, stronger because they are thicker at the base. Poorly tapered branches lack this expansion or response growth at their attachment point, which can result in failure. Lion tailing also shifts the center of mass farther away from the trunk, resulting in branch weakness and increased failure potential.

Secondly, removing only low, second-order branches shifts future growth into the remaining first-order branch structure, which becomes taller and farther from the trunk. This treatment perpetuates the formation of weak structure and likely increased damage to the tree in storms. Excessive sprouting on the trunk and remaining branches is typical in many species receiving this pruning treatment. These issues can be prevented when structural pruning is applied in conjunction with appropriate crown elevation (vertical clearance).

Unfortunately, the central leader and large-diameter branches are sometimes pruned using only heading cuts to reduce a tree to a desired size or predetermined height. (Photo 5) This practice is referred to by various names, including topping or rounding over. It is not recommended, because it damages trees permanently by compromising structure, health and appearance. Also, significant decay can occur behind large heading cuts.

Photo 5: Inappropriate heading cuts lead to topping.

Conditions of concern caused by lack of pruning
When planted decurrent trees are left to grow without structural pruning, they often develop large-diameter
branches originating low in the crown. Typically, this results in large removal cuts later to maintain clearance over roads or walkways. Other conditions that may form include weak codominant stems and large, overextended branches low on the trunk that extend outside the main body of the crown. (Photo 6) Many of these branches could have been on the tree when it was planted, but an opportunity to prune at or shortly after planting to improve structure was missed.

Photo 6: Low branches and codominant stems create pruning problems for the arborist.

Photo 7: High-quality nursery stock provides for a better structural start.

Tree-based mitigation for conditions of concern
There are several conditions that contribute to weak structure, including codominant stems, bark inclusion, branches clustered together on the trunk, branches with a large aspect ratio, upright branches and poor branch taper. These are considered some of the more critical conditions of concern that require pruning to improve. Often, two or more of these conditions can be present on the same tree. Tree-based mitigation, such as pruning, can reduce the likelihood of failure and result in trees with an acceptable risk that are urban compatible and meet client needs.

Prescriptive pruning is considered a solution to many conditions of concern. Solid, stable structure develops in a manner compatible with urban spaces when appropriate species selection and placement are combined with high-quality nursery stock. (Photo 7)
Trees with urban-compatible architecture allow arborists to prescribe and execute efficient pruning operations to ensure tree structure develops in a manner compatible with their intended spaces.

Stable tree structure challenges failure when exposed to loads such as wind. Prescriptive pruning is consistent with best practices and standards of prescribing pruning objectives encouraging strong structure, which is a component missing from many pruning operations. Improving tree appearance is emphasized as well, because it’s important to tree owners.

The prescription is used as a communication tool between prescribers (sales arborist), producers (climbers) and tree owners. (See Prescriptive Pruning in Graphic 2.)

Graphic 2: Prescriptive Pruning

Conclusion
Trees in our urban forest may present an unacceptable level of risk for the tree owner or manager. This is where an arborist trained in tree-risk assessment can identify the issues and provide mitigation options to reduce the risk to an acceptable level. As arborists, our goal is to provide solutions to conditions of concern with appropriate mitigation options, while avoiding removal when at all possible.

There is a significant gap between removal and retaining a tree, and this gap is filled with arboriculture science and practice. When an arborist applies this science along with best practices, tree issues can be properly mitigated, satisfying the tree owner or manager and creating a longer-lived tree.

Lindsey Purcell is an ISA Board Certified Master Arborist (BCMA), an American Society of Consulting Arborists (ASCA) Registered Consulting Arborist (RCA) and principal with Lp Consulting Group LLC in Cortez, Florida. He spent many years as an urban-forestry specialist and teacher in the Department of Forestry and Natural Resources at Purdue University and serves as the executive director of the Indiana Chapter of the International Society of Arboriculture.

This article is based on a presentation he made on the same topic at TCI EXPO ’23 in St. Louis, Missouri. For a pre-recorded video for that presentation, click here.

Additional information and details on this topic may be found in his book, “Arboricultural Practices: A Science-Based Approach,” available at tcia.org, under Shop, and on Amazon. (Purcell, Lindsey. 2024. Waveland Publishing.)