By Michelle Sutton
Studying tree pruning and its effects on tree stability is a classic form of applied research — and can be a little lonely. “Only a few researchers are tackling pruning right now, and that can be frustrating,” said Dr. Ed Gilman, professor of Urban Trees & Landscape Plants for the University of Florida Environmental Horticulture Department. Gilman does a great deal of outreach to commercial arborists to teach pruning practices based on his applied research findings.
Because he is Florida-based, Gilman is acutely aware of the need for tree pruning that enhances the tree’s ability to survive wind storms. As many parts of our nation and world continue to contend with severe storms, his research has application well beyond the Sunshine State.
Storms, risk, and where to prune
To find out what method of pruning best equips a tree for resisting storm damage, in 2006, Gilman and his research partners Dr. Jason Grabosky and Dr. Jake Miesbauer subjected young live oak trees that they had grown since 2001 to winds of 110 mph. They employed a wind generator built by wind engineers that was specially designed for hurricane research. (You can see a mesmerizing short video on YouTube if you search for “Effect of Hurricane-Force Winds on Landscape Trees.”)
Of the 80 live oak trees in the study, 20 trees were unpruned, 20 were thinned, 20 were raised, and 20 were given reduction pruning. Those that had been pruned by reduction faired the best. (Reduction pruning Gilman defines as “reducing the length of a branch or stem back to a live lateral branch large enough to assume the terminal role — this is typically at least one-third the diameter of the cut stem.”)
Gilman’s research also focuses on pruning as a means of risk management after a proper risk assessment has been conducted, such as that which the ISA Tree Risk Assessment Qualification course teaches. Said Gilman, “Basically, there are four things you can do after assessing — do nothing, remove the tree, secure it with support, or prune it. Our research is about pruning for risk management.”
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Pruning the large aspect ratio branches is the foundation of sound structural pruning; it suppresses the growth of, and mechanical stress on, big branches. This should begin in the nursery to craft strong branch architecture without creating big wounds — but, in reality, the arborist often faces the need to do the structural pruning on trees of various sizes in the landscape. The sooner the informed arborist gets to the tree, the better.
Getting the word out
Some commercial arborists already possess or are familiar with Gilman’s seminal An Illustrated Guide to Pruning, now in its third edition and 496 pages long. To try to get the word out about structural pruning in a concise way specifically geared toward commercial arborists, Gilman and coauthors Brian Kempf, Nelda Matheny, and Jim Clark wrote an 83-page Structural Pruning: A Guide for the Green Industry, published by the Urban Tree Foundation in 2013, available through ISA ($24.95 for members).
“This guide shows you, in a concise way, what to do early to prevent problems later,” Gilman said. “Our goal in writing this was to get arborists to incorporate the large aspect ratio concept into their everyday pruning work.”
Gilman teaches hands-on structural pruning workshops throughout the country. “When I come up to a tree to do a demo, we look for the branches that are going to cause issues,” he said. “That’s always the big branches, regardless of size or age of tree.” For young to medium sized trees, reducing the length of the large aspect ratio branches is the way to reduce future storm damage. Gilman said that for all arborists, including commercial arborists, it’s most beneficial to do this pruning when the tree is young in order to prevent risk of failure on older trees. “An increasing number of arborists are incorporating structural pruning into their services.”
When trees are already mature and there are large aspect ratio branches with which to contend, he believes research conducted by his team and others shows that reducing the length of these large branches, thereby thinning the edge of the canopy, provides immediate benefit to the customer: better structure, less risk.
Another reason to take care of the largest branches when they’re young may be a new concept for commercial arborists, but one they can use to promote themselves — providing a new service line among environmentally conscious clientele, especially those focused on developing a more sustainable landscape. “If you’re serious about managing carbon, it’s better to structurally prune a tree when it’s young,” said Gilman. “If you don’t, you end up taking large branches off, and with them all that stored carbon goes to the waste pile.”
Current and future research
While previous research by Gilman and cohorts was conducted on trees with one dominant stem, they recently completed a study that looked at what happens when you prune one side of a codominant stem pair. Using Highrise live oaks, they purposefully trained young trees over seven years to have a pair of codominant leaders.
They reduction-pruned one side of the pair at a given “dose” according to a strict experimental design. The researchers then cut the live oaks off at the base, put them in a rented truck, and drove them ten miles down the road to the research site where they had a 20-foot-tall wind machine set up. They secured the lower trunks, and subjected the pruned trees to 50-mph winds to mimic the moderately strong storms that occur in many parts of the United States.
Gilman said, “We sought to address a question I frequently get from arborists, which is ‘If we prune the outer edge of the crown [e.g. via reduction pruning], does that subject the remaining branches to more storm damage?’ We didn’t expect to see more damage after reducing the largest branches, we expected to see less. In this study, we found that reduction pruning of one stem of a codominant pair both reduced strain on the remaining stem and reduced strain on the trunk below the branch union.”
The trees that were pruned were found to be more resistant to damage from simulated storm winds than the unpruned trees. “If you’re concerned about storm-related branch failure in a tree with codominant stems or on a tree with many codominants throughout the crown, by all means employ reduction pruning of one stem of each pair of codominant stems,” said Gilman. With that said, like all good researchers, Gilman would like to see more studies of a similar nature conducted to get additional data. “This study gets a real-world conversation going about how to apply the findings,” he added.
Unrelated to the codominant stems research, Gilman and colleagues are in the middle of conducting a study about how to make removal cuts — aka collar cuts (he says term “collar cuts” is misleading because in many instances there is no actual collar to cut back to … so he prefers the term “removal cuts”). “How do we properly remove a branch when it has no collar?” Gilman’s study will ask. It has been funded by the Tree Fund, and may be the first pruning study of its kind.
Michelle Sutton (michellejudysutton.com) is a horticulturist, writer and editor.