Arboriculture & Urban Forestry 40(6): November 2014 especially during major storm events, the num- ber of incidents seems to be inaccurate, while later reports, sometimes years aſterwards, provide a more balanced view; the fatalities and injuries were caused by factors other than trees falling on people. Similarly, the level of detail reported is variable. In some cases, reports are sparse and no other headlines were detected to offer additional details. In other cases, the images or news video show type of failure and tree genus along with the names of the people affected, which allows for much better cross checking. With such variability, the data can only ever be used to derive order of magnitude estimates. And because the data were intended to show the relative serious- ness of the problem, this variability is acceptable. Six years of data are now in place and some trends are apparent. Most (but not all) incidents involve a wide range of adverse weather conditions, varying from gusty winds, heavy rain, and wind, through to hurricanes. While tree failures in extreme weather are highly likely, failures in less severe weather are also frequent. Using the data from the United States, the United Kingdom, Canada, and Australia, the most common type of incident is trees or tree parts falling on passing vehicles, such as cars, bicy- cles, motorbikes, ATVs, and snowmobiles (~46%), closely followed by pedestrians, horseback riders, or people outside in their yards (~40%). People inside buildings, (houses, mobile homes, barns) account for about 14% of incidents. Tree failures onto houses occur in most major storms but fatali- ties or injuries arising from the failure are less com- mon. Electrocutions due to trees on power lines do occur, but not oſten. Incidents involving dead trees are common. Incidents involving trees with no obvious defects prior to failure are reported, but whether the tree would actually have passed a competent risk assessment before failure is less obvious. There have been several high-profile inci- dents in the United Kingdom dealing with this issue. Overall, it is clear that the numbers are low. The six-year average (2008 to 2013) of deaths and inju- Table 1. Six-year averages. Tree Australia USA Canada UK fatality 3 66 2 5 Limb fatality 2 12 2 1 Total fatalities 5 78 4 6 Tree injury 6 75 3 13 353 ries is shown in Table 1 (rounded to the nearest whole number). Note that the incidents recorded only involve people who were killed or injured. The headlines reviewed include many reports of trees falling down causing property damage, but within that data set, few tree failures cause death or injury. Major storms regularly bring down doz- ens if not hundreds of trees in any one event. But the exact number of trees that failed is almost never recorded. The data shown here only con- cerns fatalities and injuries. Note that the numbers of fatalities and injuries do not necessarily equal the number of incidents since there may have been more than one fatality and or injury per incident. The results collected to date suggest that the magnitude of the problem is low as a percentage of national populations. The large-scale numbers are shown in Table 2 as likelihoods of being killed or injured within the six-year period documented. In examining the articles and images in the docu- mented reports, it is obvious that many incidents involved dead or seriously compromised trees within striking distance of well-used target zones. These trees could easily have been recognized and should have been removed or treated to reduce the risk lev- els. But not all incidents involved dead trees. Trees with clearly defined problems ought to be simple to recognize. Assessment by people who are trained in tree biomechanics and tree risk assessment protocols should help. But not all trees with defects will fail right away, and not all are automatic candidates for removal. And then there are tree failures where even the best risk assessor would not have had any reason to predict failure any time soon. Even the strongest trees fail when the conditions overwhelm them, and it would be unreasonable to expect otherwise. While each event is undoubtedly of concern, does the general problem warrant large expendi- tures of time and money? The answer is not simple. A reasonable approach demands a response propor- tional to the problem. One the one hand, the costs of inaction or proven negligence are high, oſten sums Limb injury 2 34 0 2 Total injuries 8 109 3 15 Avg. no. of incidents 11 134 5 18 Total no. of incidents in the six-year period 64 806 32 111 ©2014 International Society of Arboriculture
November 2014
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