City Trees

May/June 2015

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Getting the Word Out 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 struc- ture and less risk. A sustainability-related reason to take care of the larg- est 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 In 2013, Gilman et al wrote a guide to structural pruning, avail- able through ISA, geared toward commercial arborists but equally useful to municipal arborists seeking to train their staff. Edward F. Gilman Brian Kempf Nelda Matheny Jim Clark A GUIDE FOR T HE GR EEN INDUST R Y A GUIDE FOR T HE GR EEN INDUST R Y A GUIDE FOR T HE GR EEN INDUST R Y Pruning Structural Simulated hurricane winds of 110 mph in Gilman study 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 codom- inant 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 fre- quently 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 resis- tant to damage from simulated storm winds than the unpruned trees. "If you're concerned about storm-re- lated branch failure in a tree with codominant stems," Gilman says, "by all means—employ reduction pruning of one of the stems." 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. 13

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