Structural Pruning is one of the specialized practices we perform at Harrison McPhee.
Structural pruning is a process where the goal is to create a stronger tree, (large or small) through the reduction of mass within a tree. This entails the judicious pruning back, or shortening of long limbs (also know as subordinating) to reduce weight, leverage, and surface area. This is where heavy wet snow, or even rain, can accumulate and cause a structural failure of a limb. There will also be less motion, or "wind whipping," as a result of shortening the limbs. The limbs are pruned back to smaller, secondary limbs that are large enough to continue growing from where the reduction cuts were made. This practice creates limbs that are less prone to a structural failure, while also re-distributing the tree’s growth hormones, to encourage growth and foliar mass in other areas of the tree. Meanwhile, over time the remaining portion of the formerly long limbs put on girth and therefore add stability.
Another part of this process is the pruning and removal of dead limbs from within the tree. This improves safety and the pruning and removal of crossing, conflicting, or redundant limbs. Depending on the situation, size and species of tree, further thinning may be recommended. All of these pruning activities combine to provide less wind resistance to lessen the “sail effect”.
Additionally, limbs that compete with a dominant trunk would be subordinated, or removed, as would those limbs that have a poor structural branch angle of attachment to its parent limb, or main trunk. In many cases, when poor branch angles of attachment are established in large limbs or trunks, a through bolted, high tensile steel support cable may be recommended to minimize the motion of the tree trunks in windstorms and other severe weather.
Followup pruning is recommended 2 to 4 years after the initial structural pruning has been performed. Ideally, steel support cables should be inspected on a yearly basis.
In a recent case, we engaged plant pathologist Dr. Nick Brazee, of University of Massachusetts, to perform a tomography scan on a large black oak tree in the city of Newton, MA, to assess its internal decay. The tree owner had reported that fruiting bodies/mushrooms, now identified as Niveoporofomes spraguei , a known pathogen of oak trees, causing root and lower trunk rot, had appeared regularly in recent years around the base of the tree. The tree was scanned in two locations: low, about 35 cm from the soil line, and again, at about 140cm so above the soil.
Many mature trees, oaks especially, harbor interior decay, and are still structurally sound. This particular tree had some internal decay at both sampling heights, but it was less than half of the cross-sectional area, and was surrounded by a ring of intact, healthy wood. The outermost wood fibers appeared to be intact and to resist bending.
In discussion with the owner of the tree regarding other potential risks the tree could pose, and also the possible removal of the tree altogether, the decision was made to structurally prune the tree to increase its general level of safety. The photos below document the structural pruning process as described above: