ISA Tree Pruning Practices 2 — Questions and Answers

Question 1: What is the branch bark ridge and how does it guide proper pruning cut placement?

It is a raised line of bark on the upper side of a branch union indicating where to make a flush cut
It is a ridge of compressed bark at the upper junction of a branch and trunk that helps identify the branch collar and the correct angle for the final pruning cut — cuts should be made just outside it (Correct answer)
It is damaged bark that should be removed during pruning
It is the ridge at the base of the trunk where roots begin

Correct answer: It is a ridge of compressed bark at the upper junction of a branch and trunk that helps identify the branch collar and the correct angle for the final pruning cut — cuts should be made just outside it

The branch bark ridge (BBR) is a raised line of bark pushed up at the junction where a branch meets the trunk. It indicates the top of the branch attachment zone and helps the arborist angle the final pruning cut to preserve the branch collar.

The branch bark ridge is formed as the trunk and branch grow and push bark up at their junction. It is a critical visual guide for proper pruning cut placement. The final pruning cut should be made from just outside the BBR on the upper side to just outside the branch collar on the lower side. This angle preserves the branch collar — the swollen area at the branch base that contains the tree's chemical and physical defense systems (CODIT Wall 3). Cutting inside the BBR (a flush cut) removes the branch collar and its defenses, allowing decay to enter the trunk directly. Leaving too much of a stub beyond the collar prevents wound closure. In some species, the BBR is clearly visible; in others, it may be subtle or embedded in the bark. Learning to identify the BBR in various species and branch angles is a fundamental arboricultural skill.

Question 2: How should pruning priorities differ between a young, recently planted tree and a mature established tree?

Both should receive the same type and amount of pruning
Young trees should receive structural pruning to develop good form and strong attachments; mature trees primarily need crown cleaning, hazard reduction, and maintenance of existing structure (Correct answer)
Young trees should never be pruned; mature trees should be heavily pruned annually
Young trees need crown reduction; mature trees need crown raising

Correct answer: Young trees should receive structural pruning to develop good form and strong attachments; mature trees primarily need crown cleaning, hazard reduction, and maintenance of existing structure

Young tree pruning focuses on establishing a strong, well-spaced scaffold branch structure and a dominant leader to prevent future problems. Mature tree pruning shifts to maintaining health and safety through removal of dead, damaged, and hazardous branches.

The pruning objectives for trees shift dramatically with age and development stage. For young trees (typically the first 10-25 years), structural (training) pruning is the priority: establishing a single dominant leader, selecting well-spaced scaffold branches with wide attachment angles, removing codominant stems and branches with included bark, and eliminating crossing or rubbing branches. This is the most cost-effective arboricultural investment because defects are corrected while branches are small (small wounds, fast closure). For mature trees, the structure is established and the priorities shift: crown cleaning removes dead, dying, diseased, and broken branches; reduction cuts address clearance needs or reduce end-weight on extended limbs; thinning improves light penetration and reduces wind load if needed; and hazard mitigation addresses any structural defects that have developed. Heavy structural pruning on mature trees creates large wounds and should be avoided — prevention through young tree training is far more effective.

Question 3: What is the significance of the 'aspect ratio' in evaluating the safety of a branch attachment?

It measures the length of the branch relative to its diameter
It compares the diameter of the branch to the diameter of the trunk at the point of attachment — ratios approaching 1:1 (codominant) indicate weaker attachments (Correct answer)
It measures the angle of the branch relative to horizontal
It calculates the weight of the branch relative to the whole tree

Correct answer: It compares the diameter of the branch to the diameter of the trunk at the point of attachment — ratios approaching 1:1 (codominant) indicate weaker attachments

The aspect ratio compares branch diameter to trunk diameter at the attachment. When this ratio approaches 1:1 (the branch is nearly as large as the trunk), the attachment is structurally weaker, with a higher likelihood of included bark and failure.

The aspect ratio (also called the diameter ratio) is the diameter of a lateral branch divided by the diameter of the parent stem at the point of attachment. Research has shown that as this ratio increases toward 1:1, the probability of included bark and weak attachment increases dramatically. At low ratios (e.g., 0.3:1 — a branch one-third the parent's diameter), the branch collar forms normally with strong interlocking wood grain. As the branch approaches the parent's diameter, the branch collar cannot form properly, bark becomes included between the two stems, and the union relies increasingly on the weaker bark-to-bark contact rather than wood-to-wood fiber connection. Aspect ratios above 0.7:1 are generally considered high-risk, and ratios approaching 1:0 (codominant stems) represent the highest failure risk. This is why structural pruning in young trees focuses on subordinating or removing competing leaders before they grow large enough to create high aspect ratios.

Question 4: When is it appropriate to use wound paint or wound sealant on a pruning cut?

On every pruning cut to prevent decay
Generally never, except on fresh wounds on oaks in regions with active oak wilt during the high-risk transmission season (Correct answer)
Only on cuts larger than 4 inches in diameter
Only on conifer species to prevent pitch flow

Correct answer: Generally never, except on fresh wounds on oaks in regions with active oak wilt during the high-risk transmission season

Research has shown wound dressings do not prevent decay and may hinder healing. The one recognized exception is applying wound paint immediately to fresh oak pruning wounds during the oak wilt transmission season to deter spore-carrying beetles.

The practice of applying wound dressings (tree paint, pruning sealant, wound tar) to pruning cuts was standard for decades until research — particularly by Dr. Alex Shigo and the US Forest Service — demonstrated that these products do not prevent decay, do not speed wound closure, and can actually trap moisture and create favorable conditions for fungal growth beneath the coating. The tree's own CODIT process is more effective than any external treatment. The one widely recognized exception is in regions where oak wilt (Bretziella fagacearum) is present. During the high-risk transmission season (approximately April through July, when nitidulid beetles are active), fresh wounds on oaks should be immediately sealed with wound paint or latex paint to prevent beetles from visiting the wound and depositing fungal spores. Outside of this specific scenario, wound dressings are unnecessary and potentially counterproductive.

Question 5: What factors should an arborist evaluate when deciding the pruning cycle for a specific tree?

Only the species of the tree
Growth rate, structural condition, proximity to targets, site conditions, age class, previous pruning history, and client objectives (Correct answer)
Only the time since the last pruning
Only the height and diameter of the tree

Correct answer: Growth rate, structural condition, proximity to targets, site conditions, age class, previous pruning history, and client objectives

The appropriate pruning cycle depends on multiple factors: species growth rate, existing structural defects, risk exposure, site-specific growth conditions, the tree's age and development stage, what was done previously, and the client's goals.

Determining the optimal pruning cycle for a specific tree requires evaluating numerous factors. Growth rate varies by species and site conditions — fast-growing trees like silver maples may need attention every 3-5 years, while slow-growing species like oaks may be fine at 7-10 year intervals. Structural condition matters: trees with identified defects or a history of branch failure need more frequent monitoring and maintenance. Proximity to high-value targets (buildings, parking areas, playgrounds) increases the consequences of failure and justifies shorter cycles. Age class affects objectives: young trees benefit from frequent structural training; mature trees need less frequent but focused maintenance. Previous pruning history determines current condition — a tree with good structural pruning history may need less frequent attention than one that was neglected or improperly maintained. Finally, client objectives (aesthetics, clearance, fruit production) may influence the cycle. Arborists should recommend site-specific cycles rather than applying a one-size-fits-all schedule.

Question 6: What is 'drop crotching' and how does it relate to proper crown reduction?

Removing the main crotch of the tree to prevent splitting
Making reduction cuts back to lateral branches large enough to assume the terminal role, effectively lowering the crown while maintaining branch structure (Correct answer)
Dropping cut branches into the crotch of the tree for later removal
A technique for splitting logs using a wedge

Correct answer: Making reduction cuts back to lateral branches large enough to assume the terminal role, effectively lowering the crown while maintaining branch structure

Drop crotching (crown reduction through reduction cuts) involves cutting branches back to lateral branches that are at least one-third the diameter of the removed portion. This lowers the crown while maintaining proper branch structure and avoiding the harmful effects of topping.

Drop crotching is the colloquial term for crown reduction using proper reduction cuts. When a tree must be reduced in height or spread — for utility clearance, structural risk mitigation, or view restoration — each branch is cut back to a lateral branch (the 'crotch' where two branches meet) large enough to assume the terminal role. The ANSI A300 standard specifies the remaining lateral should be at least one-third the diameter of the branch being cut. This ratio ensures the lateral has sufficient leaf area and auxin production to suppress excessive sprouting and maintain normal vascular function. Unlike topping (which makes indiscriminate heading cuts to stubs), crown reduction through drop crotching maintains the tree's natural form, preserves functional branch architecture, minimizes stress and decay risk, and produces less problematic regrowth. The technique requires more skill and time than topping, which partially explains why topping persists despite being universally condemned by arboricultural professionals.

ISA Tree Pruning Practices 2

What is the branch bark ridge and how does it guide proper pruning cut placement?