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Urban Tree Risk Assessment- Perceptions, Reality, and Reliability. (Part 2)
Dr. Andrew Koeser
University of Florida
(To read Part 1, see our February 2024 issue.)
Lesson Two: We carry our biases and perceptions with us into our assessments
In another study, conducted again in collaboration with Tom Smiley and Bartlett Tree Experts, we analyzed the risk assessments and mitigation recommendations provided by nearly 300 arborists for a selection of trees. We found that the individual arborist evaluating the tree had the greatest influence on the assigned risk rating, even exceeding the impact of the tree itself. Furthermore, arborists with more credentials, particularly those related to tree risk assessment, and greater experience were less likely to recommend tree removal as a mitigation strategy. Interestingly, when controlling for other factors, arborists with children were more likely to suggest tree removal. These findings highlight the inherently subjective nature of tree risk assessment, which remains dependent on individual human judgment.
Lesson Three: The way we categorize tree defects currently is problematic
As I amassed storm data sets containing tree risk information, a peculiar pattern emerged. Several commonly monitored tree defects consistently failed to predict failures during storms. This observation prompted my team and me to conduct an international literature review, searching English, French, Chinese, Japanese, Portuguese, and Spanish databases, to identify additional studies examining pre-storm defect data. Unfortunately, we found few additional studies where defect data was collected and analyzed before a significant weather event.
Furthermore, studies comparing failure rates of non-defective and defective trees often revealed similar or even higher failure rates in the non-defective group. This unexpected finding prompted me to revisit my own data sets to replicate these results. However, the extensive list of defects currently evaluated by the industry renders encountering a "defect-free" tree highly unlikely.
This poses a significant challenge. Numerous studies have established that both defective and non-defective trees can survive storms, including low-intensity hurricanes. While pioneering research in the 1980s examined only a few defect categories, our current industry practice considers dozens, many of which remain uninvestigated and might solely reflect issues impacting timber suitability, not necessarily storm resilience. Instead of continuously expanding this growing list of potential risk factors for arborists to navigate, focused studies analyzing specific tree defects under actual storm conditions are crucial to identify which can be safely removed from the list.
See Part 3 in next month's TREE Press.
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