Qualitative Tree Risk Assessment: Risk Ratings for Targets and Trees

Summary: The qualitative tree risk assessment matrix pegs risk levels to failure and impact likelihood or unlikelihood and to severity of associated consequences.

A blocked road with a tangle of downed utility poles and trees represents a significant consequence in tree risk assessment categorizations of tree failure and impact likelihoods; Flintstone, Walker County, northwestern Georgia; Thursday, April 28, 2011, 13:10:26: Duane Tate, CC BY 2.0 Generic, via Flickr

The article Qualitative Tree Risk Assessment in the February 2012 Arborist News arrives at risk levels by ascertaining event likelihood and consequence ratings and at risk level evaluations by applying qualitative criteria.

The likelihoods of branch or tree failure and of specific target zone impacts become the "combined likelihood of a failure impacting a target" within given timelines. The terms unlikely, somewhat likely, likely, very likely communicate combinations of failure and impact for single and for "multiple targets with different values and occupancy rates." The derivative qualitative risk rating matrix designates the estimated consequences "based on the value of the target and the harm that may be done to it."

Consequences express "factors that may protect the risk target from harm," fall characteristics and distance, monetary and non-monetary target values from client perspectives and part size.

Low-value, personal injury-free, repairable, replaceable damages from small-, medium-, large-sized branches and power disruptions impacting beds, fences, landscape lighting and structures fetch categorizations as negligible consequences. Low to moderate damage from branches impacting decks, roofs or structures, slight disruptions to neighborhood traffic and residential power or very minor injuries garner minor categorizations. Disruption of distribution primary or secondary voltage power lines or secondary-street traffic, moderate- to high-value damage to structures or vehicles and personal injury have significant consequences. Death or hospitalizable injuries, disrupted arterial traffic, high-voltage distribution and transmission power lines or motorways and high-value damage to occupied houses or vehicles invoke severe consequences.

Qualitative tree risk assessment judges whole-tree risk aggregations of failure modes and risk targets as independent events since individual risk ratings cannot be added or multiplied.

Tree risk assessors know overall, whole-tree risk ratings for individual trees with multiple failure modes and risk targets as the "failure mode having the greatest risk." Mitigation of high-risk failure modes leaves overall risk ratings higher, lower or unchanged because of "residual risk associated with that tree, including the remaining risk factors."

The tree risk assessment matrix mentions low risk levels for minor consequences from somewhat likely failure and impact likelihoods and for negligible consequences from unlikely likelihoods. It notes moderate levels for minor consequences from likely or very likely failure and impact likelihoods and for severe or significant consequences from somewhat likely likelihoods.

Qualitative tree risk assessment matrices observe high levels for significant consequences from likely or very likely failures and impacts and for severe consequences from likely likelihoods.

Severe consequences from imminent failure and very likely impact likelihoods prompt categories of extreme risk and recommendations that "mitigation measures be taken as soon as possible."

Subjective perceptions of mitigation aesthetics, costs and inconveniences, risk and safety quell or quicken intolerance or tolerance of high-risk trees and risk tolerance and action thresholds. Because of subjective intolerances and variable tolerances, authorities such as councils, municipalities, property managers and utilities typically reveal acceptable risk thresholds in their risk management plans. Aesthetic, budget, geo-historical and safety-related concerns by tree risk managers and tree-related benefit and loss concerns by tree risk assessors sculpt risk tolerance and mitigation thresholds.

The qualitative tree risk assessment matrix translates target and tree zone vulnerabilities into mitigation thresholds, according to co-authors Sharon Lilly, Nelda Matheny and E. Thomas Smiley.

A large tree in extreme closeness to a picnic area could pose the severe consequences of hospitalizations and/or fatalities according to a qualitative risk assessment of tree failure and impact likelihoods: Joseph OBrien/USDA Forest Service/Bugwood.org, CC BY 3.0 United States, via Forestry Images

Acknowledgment

My special thanks to:
talented artists and photographers/concerned organizations who make their fine images available on the internet;
University of Illinois at Urbana-Champaign for superior on-campus and on-line resources.

Image credits:

A blocked road with a tangle of downed utility poles and trees represents a significant consequence in tree risk assessment categorizations of tree failure and impact likelihoods; Flintstone, Walker County, northwestern Georgia; Thursday, April 28, 2011, 13:10:26: Duane Tate, CC BY 2.0 Generic, via Flickr @ https://www.flickr.com/photos/fdtate/5729300293/

A large tree in extreme closeness to a picnic area could pose the severe consequences of hospitalizations and/or fatalities according to a qualitative risk assessment of tree failure and impact likelihoods: Joseph OBrien/USDA Forest Service/Bugwood.org, CC BY 3.0 United States, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=5054010

For further information:

Gilman, Ed. 2011. An Illustrated Guide to Pruning. Third Edition. Boston MA: Cengage.

Hayes, Ed. 2001. Evaluating Tree Defects. Revised, Special Edition. Rochester MN: Safe Trees.

Marriner, Derdriu. 18 February 2012. “Qualitative Tree Risk Assessment: Falling Trees Impacting Targets.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2012/02/qualitative-tree-risk-assessment.html

Marriner, Derdriu. 10 December 2011. “Tree Risk Assessment: Tree Failures From Defects and From Wind Loads.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2011/12/tree-risk-assessment-tree-failures-from.html

Marriner, Derdriu. 15 October 2011. “Five Tree Felling Plan Steps for Successful Removals and Worker Safety.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2011/10/five-tree-felling-plan-steps-for.html

Marriner, Derdriu. 13 August 2011. “Natives and Non-Natives as Successfully Urbanized Plant Species.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2011/08/natives-and-non-natives-as-successfully.html

Marriner, Derdriu. 11 June 2011. “Tree Ring Patterns for Ecosystem Ages, Dates, Health and Stress.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2011/06/tree-ring-patterns-for-ecosystem-ages.html

Marriner, Derdriu. 9 April 2011. “Benignly Ugly Tree Disorders: Oak Galls, Powdery Mildew, Sooty Mold, Tar Spot.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2011/04/benignly-ugly-tree-disorders-oak-galls.html

Marriner, Derdriu. 12 February 2011. “Tree Load Can Turn Tree Health Into Tree Failure or Tree Fatigue.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2011/02/tree-load-can-turn-tree-health-into.html

Marriner, Derdriu. 11 December 2010. “Tree Electrical Safety Knowledge, Precautions, Risks and Standards.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2010/12/tree-electrical-safety-knowledge.html

Smiley, E. Thomas; Matheny, Nelda; and Lilly, Sharon. February 2012. "Qualitative Tree Risk Assessment." Arborist News 21(1): 12-18.
Available @ http://html5.epaperflip.com/Viewer.aspx?docid=8b0e5722-c46f-47d0-b61c-a2bc00f5ce1c#page=14