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www.arborage.com Arbor Age / July 2014 11 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 fi ndings advance the discussion on how commercial arborists prune trees. "We think the arborist should prune (not remove) the large aspect ratio branches — those that are big compared to the main trunk(s) — to get huge benefi ts in how trees function in storms," said Gilman. Pruning the large aspect ratio branches is the foundation of sound structural pruning; it suppresses the growth of, and me- chanical 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 specifi cally 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 is- sues," he said. "That's always the big branches, regardless of size or age of tree." For young to medium sized trees, reduc- ing 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 benefi cial 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 Ideally, structural pruning starts in the nursery. A bark inclusion in the union indicates there is very little, if any, wood of one stem wrapping around the other. This is one of the conditions that make unions very weak and can be avoided by appropriate structural pruning when trees are young. Photos courtesy of the authors of Structural Pruning: A Guide for the Green Industry (Urban Tree Foundation 2013).