Comments on recent limb failure causing a fatality at Trinity Bellwoods Park (Toronto. June 17 2016)
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On June 20, 2016, Cohen & Master Tree and Shrub Services Ltd. visited the tree that failed at Trinity Bellwoods Park (June 17, 2016). This tree failure resulted in a human fatality. While such an event is extremely rare, it does raise concern around general tree safety. Cohen & Master shares this concern and has investigated further as a means to improve the understanding of the event and to discuss possibilities of improved hazard mitigation.
The tree in question is a Siberian Elm (Ulmus pumila). The tree is located in the south/west portion of the park. The tree stands alone, independent from the surrounding canopies. Upon arrival it was clear that this tree had been recently pruned (this may have been done after the tree failure event). Deadwood has been removed from the canopy and there was no debris on the ground in proximity to the tree. One pruning cut stood out from the typical dead-wooding cuts: the pruning wound appeared to be live wood and there was visible evidence of bark tearing on the underside of the branch collar. The pruning wound is approximately 20cm in diameter. Tearing on the underside of the branch collar can be associated with a limb failing in a manner where the tension side of the limb breaks (the top side), causing the limb to tip down and then to tear back on the compression side (the underside of the limb) back towards the limb attachment point. The limb would have fallen freely from this point with no obstructions below (no other tree branches etc) and a target on the ground would be vulnerable. We cannot say with certainty that this is the location of the failed branch that caused the fatality, however it is likely that a branch failed from this point.
The question becomes why did this branch fail? Trees fail for a variety of reasons involving defects and tree related stress. A term that has been used by some arborists in media reports to describe this particular failure is Summer Branch Drop (SBD). Richard W. Harris made the following comments in the Journal of Arboriculture 9(4) April 1983: “Apparently sound limbs occasionally break out of mature trees during calm summer weather. High xylem pressure and/or weakening of the cell wall bonding in the xylem accompanied by increased limb weight may be responsible.” This explanation points to a series of issues including tissue shrinkage, moisture change and gas release inside of branches all contributing to a failure. These types of unexpected failure events have been documented during periods of drought in combination with warm temperatures. While this SBD theory exists, it should also be noted that these failures (be it during a storm or a calm sunny day) are almost always associated with some form of tree defect (affecting the structure) and/or poor tree vigor (affecting the tree health). A connection can be made that periods of drought place stress on a tree, increasing susceptibility to failure, aggravating existing defects and increasing the likelihood of tree failure.
What can be done to prevent this from happening? Proactive and regular maintenance plays a critical role in tree preservation and hazard mitigation. It has been stated in other SBD events that failure occurred to limbs that are horizontal in direction, often extending beyond of the main canopy drip-line. These heavily weighted limbs act as a leaver and are often referred to as tension beams that will eventually crack right through due to the load. Typically these tension beams will begin cracking in small increments until they finally release in full.
It can be noted that this particular tree does show a very open and spreading canopy, with several long horizontal limbs. This form is not unusual for Siberian Elm trees and does make it more prone to failure due to the added weight, leverage and poor taper. Urban trees in general are more susceptible to developing this spreading canopy structure. In a forest setting, trees in close proximity to one another encourage upright growth, whereas urban trees frequently do not have these restrictions and may spread more than usual. Additionally, Siberian Elm trees were badly affected by the Toronto ice storm in December, 2013. Many sections of these trees failed. This created openings in the canopy, exposing the tree structure to new environmental forces, such as wind load on parts of the tree that have never been exposed to wind before. This can initiate the formation of defects and cause failure. Storm damage also had the negative effect of stimulating sprouting growth in response to the lost foliage. Sprouting growth is weakly attached and often creates a “lions-tail” effect of a bushy end on a long poorly tapered limb. This kind of structure is also more prone to failure. One may speculate in the case of the Siberian Elm in Trinity Bellwoods Park that species played a significant role in the failure of the tree branch.
While performing regular tree maintenance pruning, close inspection should be carried out by the arborists directly in the tree. Certain defects may be revealed at this time that would not be apparent from a ground inspection. In addition to removing dead branches, poorly attached branches or branches with weak unions, arborists should carefully evaluate long, overextended horizontal limbs. Reduction pruning cuts can be made on these limbs to reduce the weight, tension and wind resistance on the branch.
To provide protection from limb breakage or branch union failure, the arborist may install additional tree support. Cabling and bracing are effective methods of supporting a tree.
Directly addressing tree health must also be a part of tree maintenance. A tree under stress will be more prone to failure. It is important to note that although a tree may have full foliage, it does not mean it is structurally sound, nor can it immediately be assumed that the tree is healthy and vigorous. Periods of drought are never good for plants and efforts must be made to keep trees watered. Improvements can also be made to the soil that the tree is rooted in. This will encourage water penetration into the soil and improve the available nutrients to the tree.
Trees are an extremely important part of the urban environment. They provide oxygen and shade, and contribute to feelings of well-being and improved mental health. The tragic failure of the tree in Trinity Bellwoods Park reminds us that trees are living woody plants that are not structurally engineered by humans. Trees within our cities must be maintained to prevent these kinds of tragedies from occurring. Within the urban environment, trees require special attention for two crucial reasons. Firstly, the urban environment creates unusual conditions (unlike forests) and places additional stress on the trees. Secondly, trees within the city situate humans as potential targets for any tree failure. Not every tree failure can be predicted: however, regular and thorough maintenance by tree professionals greatly reduces potential risk and increases the longevity of the tree.
Cohen & Master Tree and Shrub Services Ltd.
ISA Certified Arborist 0973A