City Trees

May/June 2015

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Studying tree pruning and its effects on tree sta- bility is a classic form of applied research—and can be a little lonely. "Only a few researchers are tackling prun- ing 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 many parts of the world are experiencing increasingly bizarre weather, including tornados in places where tornados were once unheard of, Gilman's research has application to many of 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 (Quercus virginiana) 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 reduc- tion pruning. Those that had been pruned by reduction faired the best. (A reminder that 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 diam- eter of the cut stem.") 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. Gilman says, "We think the arborist should prune the large aspect ratio branches—those that are big com- pared 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 suppress- es the growth of and mechanical stress on big branches. This should begin in the nursery to craft strong branch architecture without creat- ing big wounds—but in reality, out in the field, the municipal arborist often faces the need to do structur- al pruning on trees of various sizes. Pruning Research Update: Dr. Ed Gilman by Michelle Sutton Photos courtesy of the authors of Structural Pruning: A Guide for the Green Industry (Urban Tree Foundation 2013) Reduction-pruned larger tree 12 City Trees

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