Earth and Space News: International Society of Arboriculture ISA Arborist News February 2012

Showing posts with label International Society of Arboriculture ISA Arborist News February 2012. Show all posts

Sunday, February 19, 2012

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:

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

Saturday, February 18, 2012

Qualitative Tree Risk Assessment: Falling Trees Impacting Targets

Summary: The qualitative tree risk assessment matrix examines likelihood or unlikelihood of failed branches or trees impacting specific target zone targets.

A tree risk assessment eventually could determine that a 600-year-old white oak tree (Quercus alba) has sufficient decay and rot to pose tree failure likelihood that would damage Revolutionary War headstones in the surrounding cemetery; Basking Ridge white oak on the grounds of Basking Ridge Presbyterian Church in Bernards Township, Somerset County, north central New Jersey; Monday, Jan. 1, 2007, 00:02: Jared Kofsky, CC BY SA 3.0 Unported, via Wikimedia Commons

Context parameters, as communication flow, evaluation method, legal requirements or policies, limitations and objectives, antecede tree risk assessment, according to the article Qualitative Tree Risk Assessment in Arborist News for February 2012.

Probability and consequence formulas beg comparing other risks and trees in quantitative approaches to tree risk assessment's "systematic process to identify, analyze, and evaluate tree risk." Quantitative approaches calculate estimates from accurate, precise data and methods even though "little systematically collected data on which to base probabilities" compromise quantitative tree risk assessments.

Inherent ambiguity and subjectivity daunt qualitative approaches whose determinations and evaluations of risk levels depend upon clear terminology and defined ratings of likelihood, consequences and risks. Qualitative numerical tree risk assessment systems entertain incorrect mathematics by estimating relative risk ratings from risk factors expressed as categorizations numbered ordinally for addition or multiplication.

Risk assessment method choice fancies data and information availability, level of detail, needs of decision makers, requisite expertise and resource availability and reasonability for potential consequences.

Risk managers and tree owners gather uncertainty sources from limited predictability of decay progression, response-grown wood, traffic and occupancy rates, tree failure consequences and weather events. Their qualitative tree risk assessment report has a comparative matrix of tree risk rating by likelihood and by consequences for clients, controlling authorities or societal standards. Their risk assessor identifies tree risk categorizations by the impact consequences of tree failure likelihood from anticipated loads and tree defects, response growth and structural conditions.

Compounded effects and problem-mitigating growth variably jeopardize stability in qualitative tree risk assessment since "Not all conditions and defects have a significant impact on tree structure."

Risk assessors know of tree failures from critically combined conditions, defects and triggers, such as rain, snow and wind loading events beyond the site's seasonal norms. They list as the timespan for imminent, improbable, possible or probable tree failure likelihood either as a one-year interval or as an inspection interval until re-inspection.

Inspection and time intervals mention improbable failure likelihoods for branches or trees unlikely to fail during normal weather conditions and possibly in many severe weather conditions. They note as possible likelihoods unlikely failures of branches and trees during normal weather conditions and as probable likelihoods woody plant failures under normal weather conditions.

Qualitative tree risk assessment offers branches and trees already or "most likely" failing around the corner, even without increased load or significant winds, as imminent likelihoods.

Target occupancy rates for risk assessors and, with aggravations or mitigations of falling trees for arborists, provide estimated tree failure impact likelihoods for target zone targets.

A remote chance of impacting a specified target in the target zone qualifies the failed branch or tree for categorization as very low likelihood of impact. The failed branch or tree with an unlikely chance of impacting a specified target in the target zone receives the categorization of low likelihood of impact. Almost equal likelihood and unlikelihood and great likelihood of impacting specified target zone targets respectively summon failed branches and trees medium and high likelihoods of impact.

The qualitative tree risk assessment matrix ties target impact and tree failure likelihoods and unlikelihoods, according to co-authors Sharon Lilly, Nelda Matheny and E. Thomas Smiley.

Qualitative tree risk assessment could offer tree failure likelihoods for paper birch (Betula papyrifera Marsh.) with multiple defects overlooking playground: Joseph OBrien/USDA Forest Service/Bugwood.org, CC BY 3.0 United States, via Forestry Images

Acknowledgment

A tree risk assessment eventually could determine that a 600-year-old white oak tree (Quercus alba) has sufficient decay and rot to pose tree failure likelihood that would damage Revolutionary War headstones in the surrounding cemetery; the Basking Ridge white oak is on the grounds of Basking Ridge Presbyterian Church in Bernards Township, Somerset County, north central New Jersey; Monday, Jan. 1, 2007, 00:02: Jared Kofsky, CC BY SA 3.0 Unported, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Old_Tree_in_Basking_Ridge.JPG

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.