30 data, Edberg and Berry (1999) found that decay of roots or the lower trunk was a contributing factor in 83% of coast live oak (Quercus agrifolia) failures. In contrast, Edberg et al. (1994) found the majority of recorded Monterey pine (Pinus radiata) failures were limb failures. It is difficult to accurately predict tree failure, but controlled and observational studies have been conducted to help provide guidance on the tree and environmental factors that lead to overturning or stem breakage (Edberg et al. 1994; Wessolly 1995; Edberg and Berry 1999; Kane 2008; Kane and Clouston 2008). The available body of related scientific research can be used to compliment the past observations and experiences of arborists and urban foresters in identi- fying the factors that elevate a tree to a higher poten- tial for failure (Kane et al. 2001). Johnson (1999) found 84% of trees that failed during summer storms in Minnesota, U.S., had pre-existing defects. Most of the defects contributing to tree or branch failure could likely be identified and mitigated with appropriate tree planting and mainte- nance practices (Johnson 1999). Likewise, following both Hurricane Andrew in 1992 and the 2004–2005 hurricane seasons in Florida, U.S., trees that had been properly pruned (i.e., open and evenly dispersed crowns that had not been topped) withstood the hurri- canes better than trees that had been improperly pruned or not pruned at all (Duryea et al. 1996; Duryea and Kampf 2007). In contrast, trees with poor structural forms and/or defects, such as codominant stems, were more susceptible to hurricane forces. In a more deliberate study, Hickman et al. (1995) evaluated 695 native oak (Quercus spp.) trees in a California, U.S., recreational area that had previously been assessed for failure potential. They found that, of the original 695 trees, 60 (8.6%) had failed within seven years of the original study. Of the tree defects and site parameters assessed (decline, soil, wind, butt, trunk, root, limb, irrigation frequency, lean), decline, trunk condition, and lean were identified as being the key factors in predicting whole-tree failure. More recently, Koeser and Smiley (unpublished data) reevaluated trees risk as assessed using the ISA BMP method prior to Hurricane Matthew. Of the 20 trees assessed as having an imminent likelihood of failure, 19 (95%) failed during the storm. In contrast, 38% of the trees assessed as having a probable likelihood of failure, failed. Trees rated as having possible or ©2019 International Society of Arboriculture Klein et al: Risk Assessment and Risk Perception of Trees improbable likelihoods of failure had similar failure rates at 15% and <1%, respectively (Koeser and Smi- ley, unpublished data). Consequences of Failure Consequences of failure can be minimal, such as damages that results in minor repairs (e.g., fixing a small fence or disruption to landscape lighting) or they can be much more severe with regards to public safety (e.g., injuries that lead to hospitalization/death; Smiley et al. 2011). Schmidlin (2009) compiled wind-related tree failures in the United States that resulted in the loss of human life. There were 407 deaths from 1995 to 2007 at an average of 31 deaths per year (1 in 9.7 million). Ellison (2007) cited a study (Anonymous 2006) that estimated the likeli- hood of being killed by a tree in the United Kingdom was 1 in 5 million. Similarly, Fay (2007) pointed out that the Health and Safety Executive Sector Informa- tion Minute equates the likelihood of being killed by a tree in a public space to be 1 in 20 million. This was slightly lower than the 1 in 18.7 million likelihood of being struck by lightning (Health and Safety Execu- tive 2007). Regardless of the estimate, Ball and Watt (2013b) noted that in the UK, deaths and serious inju- ries resulting from tree failures are extremely rare, and that it is unlikely that the number of these inci- dents could be reduced without instituting strict mea- sures that might pose adverse effects to both the labor force and the environment. Visual Tree Risk Assessment While advanced risk assessment methodologies and technologies exist, the majority of risk assessments are conducted visually. Intuitively, visual assessments may not catch defects that cannot be seen externally on the tree, such as internal or incipient decay (Dol- win et al. 1999; Guglielmo et al. 2007; Smiley et al. 2011). However, research contrasting visual assess- ments to resistance microdrilling has shown that external signs and indicators of decay can be quite reliable for some species, especially when internal decay is significant (Koeser et al. 2015). Similarly, Terho’s (2009) post mortem assessment of decay in urban trees found that fruiting bodies were a common external sign of the presence of internal decay among Tilia spp., as well as a good indication that the tree had reached its threshold for strength loss. In addition to fruiting bodies, Kennard et al. (1996) noted that the presence of wounds, cankers, and cracks could be
January 2019
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
