By Michelle Sutton
Photos courtesy of the authors of Structural Pruning: A Guide for the Green Industry (Urban Tree Foundation 2013).
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,” says Dr. Ed Gilman, Professor of Urban Trees & Landscape Plants for the University of Florida Environmental Horticulture Department.
Gilman does Extension outreach to municipal arborists and urban foresters to teach pruning practices based on his 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 we in New York are experiencing increasingly bizarre weather, including tornados in places where tornados were once unheard of, Gilman’s research has application to us.
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 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 (see video).
Of the eighty live oak trees in the study, twenty trees were unpruned, twenty were thinned, twenty were raised, and twenty 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. Gilman says, “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.”
Where Alex Shigo began showing us how tree cuts should be made, Gilman and colleagues are studying where to make the cuts in the crown. His findings advance the discussion on how commercial arborists prune trees. Gilman says, “We think the arborist should prune the large aspect ratio branches—those that are big compared to the main trunk(s)—to get huge benefits in how trees function in storms.”
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 municipal arborist often faces the need to do the structural pruning on trees in the landscape of various sizes. 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 ISA and just $24.95 for members.
“This guide shows you in a concise way, what to do early to prevent problems later,” Gilman says. “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 around the country. He says, “When I come up to a tree to do a demo, we look for the branches that are going to cause issues. That’s always the big branches, regardless of size or age of tree.” For small to medium sized trees, reducing the length of the large aspect ratio branches is the way to reduce future storm damage. Gilman says that it’s most beneficial to do this pruning when the tree is young in order to prevent risk of failure of mature trees.
When trees are already mature and there are large aspect ratio branches to contend with, Gilman says that research conducted by his team and others shows that reducing the length of these large branches, thereby thinning the edge of the canopy, provides better structure and less risk.
A sustainability-related reason to take care of the largest branches when they’re young: “If you’re serious about managing carbon,” Gilman says, “it’s better to structurally prune a tree when it’s young, because 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 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 U.S.
Gilman says, “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?’ 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,” Gilman says, “by all means—employ reduction pruning of one of the stems.” 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,” Gilman says.
Unrelated to the codominant stems research, Gilman is next going to conduct 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 will be funded by the Tree Fund and is the first pruning study of its kind.