Aaron Schulz
ISA Board Certified Master Arborist
Prescription Pruning Qualified - ISA Florida Chapter
Trees have A LOT of branches but not all of them require pruning. However, having more branches doesn’t always mean it’s better if these branches aren’t in the correct orientation on a tree. Trees can, and will, break under the right conditions: when the load exerted on a tree part exceeds the tree part’s strength. Oak Bros Certified Arborists aren’t fortune tellers that can predict tree part failure but we can identify tree defects, what I’ll refer to as “conditions of concern”, which commonly contribute to tree part failure potential. Taking a proactive approach to pruning that is based on tree structure means that Oak Bros understands how to manage your trees whether they’re newly planted, 50 years old, or over 100 years old. Being able to identify how conditions of concern affect tree growth and structure is paramount in increasing tree longevity and maintaining a tree that is compatible with our urban environment. On a daily basis I count hundreds of trees which are pruned improperly. Remember from the previous blog that our arboricultural pruning standard, ANSI A300 Part 1, states that if a pruning objective doesn’t exist then no pruning is to be done. What this boils down to is: if there is no defined goal for the pruning then that tree shouldn’t be pruned. This is where it takes the skilled Certified Arborists at Oak Bros to identify exactly what needs to be pruned and how much to remove.
What Needs to be Pruned?
Identifying what to prune in a tree is a key aspect for a Certified Arborist to develop a pruning specification that fits the needs of your tree and property. Careful consideration is given to the tree species, structure, conditions of concern, branch size, tree health, available soil, and age of tree. Not all trees or growing environments are the same and each tree requires different considerations. Let’s walk through these considerations and see how they impact how Oak Bros decides what needs to be pruned and how much to remove.
Tree Species
Not all trees are created equally and most can’t tolerate aggressive pruning or large branch removal. Some species, such as Hackberry and Silver Maple, are poor at managing pruning wounds and decay readily. When trees which are more sensitive are assessed for pruning, we will recommend smaller pruning cuts because we understand how these trees are going to respond.
Tree Structure
Every tree has an optimal structure whether its mature form is decurrent (wider than tall), like a Bur Oak, or excurrent (strong central leader that’s taller than wide), like Baldcypress. The general rule of thumb for tree structure is that all trees should start with an excurrent form that has a dominant, central leader. Yes, that giant Bur Oak pictured below should be grown with a single stem when young. Eventually, genetics will take over and the horizontal branches will begin to outgrow the leader, giving a Bur Oak its picturesque form at maturity.
|
|
I’ve spent the majority of my career studying tree structure and implementing structural management on trees of all sizes. The Bur Oak to the left is being trained with a dominant leader. What most tree owners don’t understand is that when a shade tree is planted, let’s say 2” caliper and 10 feet tall, nearly every branch on that tree, aside from its leader, is temporary. What that means is that these branches should be removed over time to encourage optimal structure as the tree ages. Structural pruning requires a methodical approach and a significant understanding of how trees respond to pruning decisions. Our Certified Arborists and production staff are trained to implement structural pruning strategies. While the thought of paying for structural pruning seems like a high cost, what most tree owners don’t understand is that maintenance costs actually decrease over time because, as the tree ages, less structural issues are able to form. |
|
|
Structural pruning isn’t always aesthetically pleasing. It’s goal-driven and a level of trust by the tree owner is required. Pruning cuts may seem oddly placed but they are very strategic and I guarantee you that the end result is worth it. Remember, having more branches isn’t always better. |
The harsh reality we have to face in our community is that not every tree care business understands how to correctly prune trees. Pruning decisions made by other tree care businesses can inhibit optimal structure, and it’s something we encounter on a daily basis. When those large, low branches begin to hang too low while you mow, the natural tendency is to only remove the branches that are in your way. In most cases, this happens from the inside-out. This is what we refer to as “lion-tailing”. In some cases, such as the Honeylocust directly below, tree care businesses go to the extreme of removing all of the interior branches.
|
|
In the case of lion-tailing, it’s addition by subtraction. Let me explain: trees are just levers with more levers attached to them. Every branch (or lever) that grows is there for several reasons, but primarily weight distribution in response to light, gravity, and loading (wind, ice, snow, and rain). During the growing season when trees experience wind, drag is created. The denser a canopy, the more drag the tree experiences. The more branches that are equally spaced, the more equal the drag is dispersed across a branch. Except in the case of lion-tailed branches. Following lion-tailing, trees experience more loading due to the shift in weight distribution towards the ends of branches. In addition, lion-tailed trees experience injury on interior branches and have an increased susceptibility to drought injury due to having few interior leaves. These are the trees that we see fail on a regular basis during wind, snow, and ice events. Misinformed pruning decisions in the name of “aesthetics” actually increase the potential for failure and are much more costly to repair and maintain. |
Conditions of Concern
Conditions of concern are attributes of a tree or site that contribute to a tree owner’s or arborist’s concern. Unfortunately, there is a lot of fearmongering with some tree care “professionals” in the Bloomington-Normal area. Just because a tree doesn’t make perfect structure doesn’t mean its failure is imminent. We have to coexist with trees in the urban environment; there is no way to eliminate that without removing every tree. Understanding how to identify and address conditions of concern is what separates Oak Bros Certified Arborists from every other tree care business in our area. Let’s go through some of the common conditions of concern that Oak Bros Certified Arborists encounter:
Aspect Ratio
|
|
Aspect ratio is the relationship between branch diameter and trunk or leader diameter. As the relationship, or size, of a branch and trunk/leader become more equal (> 50%), the unions become weaker because the aspect ratio is higher. For example, a 10” diameter branch growing from a 12” stem would have an aspect ratio of 83% (or 5:6). This branch is much weaker than if it were a 6” diameter branch growing from a 12” stem. When high aspect ratio branches exist, they become branches to prune. The key here is knowing how much to prune. Several factors need to be considered: tree species, age, condition, branch size, severity of aspect ratio, and existence of other conditions of concern associated with branch structure. In the photo to the left, you can see a Freeman Maple with poor structure. The leader is on the right side with a nearly identical diameter stem attached to it. There is also 2” of included bark. As this tree ages, the consequences of high aspect ratio and included bark become more severe and increase the potential for failure. The pictured Freeman Maple is 100% correctable in the right hands. |
Bark Inclusion
Speaking of bark inclusions…….Trees with branches that have included bark are structurally weaker than those without. Bark inclusions are stems which have bark embedded between them, which prevents or reduces the connective tissues between branch and trunk wood.
|
|
In the picture to the left, you can see a Honeylocust that is actively failing from decades of developing included bark. Bark inclusions are best mitigated as the tree ages, as the condition is smaller. Unfortunately, tree owners don’t know what they don’t know so a condition such as bark inclusions continue to develop until an Arborist diagnoses it. In the Bloomington-Normal area, many tree care businesses view bark inclusions as death sentences for trees. Those that do so are misinformed about how to successfully manage this condition. Bark inclusions certainly contribute to tree part failures but are not death sentences. In a world full of risk, trees certainly can’t be overlooked. At Oak Bros, our Certified Arborists weigh mitigation strategies against a tree owner’s risk tolerance. In my professional opinion, there is too much fearmongering by other tree care businesses in their efforts to generate more work. A bark inclusion could be correctable but because they don’t have the knowledge to do so, immediately recommend a tree be removed. |
The Honeylocust pictured above was beyond management based on the severity of the crack and the tree owner’s risk tolerance. Not pictured on that tree were several other cracks from bark inclusions, over-extended branches, and poor structure from having never been structurally pruned during its 40-year life.
|
|
The Honeylocust pictured to the left was a victim of the 2021 ice storm. We can actually see a lot of the reasons why this branch failed: bark inclusion (dark area at the union), lion-tailing, and over-extension (which we’ll cover shortly).
As it turns out, trees can, and will, fail when the load exceeds tree part strength. One condition of concern exacerbates the effects of others if they exist. Managing tree structure is a powerful tool when in the right hands. Not all tree part failures can be prevented but a significant amount of potential can be reduced with the right pruning strategy. |
Branch Size
Every growing season trees age and as tree parts age they become larger and start to have nonconductive tissue, which we refer to as heartwood. Because heartwood is nonconductive, it does not have the ability to respond to injury and compartmentalize wounds (yes, pruning is a wound). All trees begin to produce heartwood at different ages and branch sizes. The goal of pruning is to complete the objective using pruning cuts that are the smallest possible diameter and fewest number required. As branch size increases, so does the ratio of sapwood to heartwood. When we think about branch size removal, we always consider whether we can achieve the same objective by removing smaller branches. To formulate a plan we have to understand the surface area of pruning cuts. For example, let’s use a 10” diameter branch as a baseline to see how other pruning cut options will have nearly the same amount of surface area exposed, but with smaller pruning cuts and a reduced potential for decay:
|
Branch Diameter |
Number of Branches Removed |
Surface Area of Cut |
|
10” |
1 |
78.54 |
|
7” |
2 |
76.96 |
|
4” |
5 |
75.42 |
|
2” |
25 |
78.5 |
|
|
I could make 1x 10” diameter pruning cut, or I could make 25x 2” diameter pruning cuts and expose nearly identical surface area.
The reason we focus so much on these smaller pruning cuts is because of heartwood exposure. When more heartwood (dark area in the pruning cut pictured to the left) is exposed in a pruning cut, the likelihood of a wood decay fungi establishing increases. Remember, heartwood can’t fight back and pruning wounds take time to seal so the larger the pruning cut is, the longer the exposure time.
|
Branch Length
As previously mentioned, trees are made up of several levers. The longer the branch, the more torque that branch experiences. Trees have their own “body” of crown with branches that work as one system to slow wind forces. When branches become too long and stick outside of the crown’s body they become what we refer to as “over-extended”. When over-extended branches occur, they are more exposed and susceptible to more forces, particularly wind, which increases failure potential. This is considered a condition of concern and, thus, a branch to prune. Reducing branch length from the outside-in, while not lion-tailing the branch, is highly effective at reducing weight and something that Oak Bros Certified Arborists are trained to take note of.
|
|
The Sycamore pictured to the left has two large 1st order branches which extend outside of the crown’s body and over a busy street, as well as another over-extended 1st order branch parallel to the sidewalk. These over-extended branches would greatly benefit from weight reduction from the outside-in to make them more stable during wind, ice, or snow events.
The pruning specification for this tree would include pruning cuts near ends of 1st and 2nd order branches, with a maximum cut size of 3” in diameter. The reasoning for smaller pruning cuts is because Sycamore has poor decay resistance.
|
Condition & Age
When Oak Bros Certified Arborists assess your trees for pruning recommendations, we also take into account the tree’s condition and age. Healthy young trees can tolerate a larger pruning dose. Mature trees tolerate smaller pruning doses because they are less efficient with resource preservation and allocation. A misunderstood fact of trees is that they continuously lose energy through respiration. There’s no reason to remove a higher amount of tissue than is required to complete the objective.
Available Soil
You may be thinking to yourself, “what does soil have to do with pruning?”. When a tree has less available soil, it manufactures less resources. When trees have less resources, they don’t function optimally. The condition of the crown is a direct reflection of the condition of the root system and soil environment so when a poor growing environment exists, leaves are less efficient at manufacturing energy through photosynthesis. In cases like this, our pruning amount decreases to retain as many leaves as possible.
What’s Next?
The final blog in this pruning series will be focused on how the Certified Arborists at Oak Bros develop pruning specifications based on the information discussed in pruning blogs 1 and 2.
