Three Tree Risk Assessment Levels: Limited Visual, Basic and Advanced

Summary: Sharon Lilly, Nelda Matheny and E. Thomas Smiley hinge three tree risk assessment levels -- limited visual, basic, advanced -- on Scope of Work statements.

Scope of Work statements prior to visual, basic and advanced tree risk assessments describe the trees under assessment and the level of risk of assessment to be conducted; scenario of rotten tree fallen on house: Paul A. Mistretta/USDA Forest Service, CC BY 3.0 United States, via Forestry Images

Limited visual, basic and advanced tree risk assessment levels are preceded by Scope of Work statements, according to Tree Risk Assessment: Levels of Assessment in the April 2012 issue of Arborist News.

Sharon Lilly of the International Society of Arboriculture, Nelda Matheny of HortScience, Inc., and E. Thomas Smiley of Bartlett Tree Research Laboratory begin with work statements. Scope of Work statements confirm the client's budget, the local authority's inspection, permit and report requirements, the property's unrestricted access points and the written report's recipients. They describe the areas or trees to be assessed, the level to be conducted and target specifications for branch diameter, lean, live crown ratio and taper.

Work scope statements elucidate methods of reporting, needs for documentation and timetables for inspections and reports as well as ratings of risks and recommendations for mitigation.

Limited visual assessments through drive-by, fly-by or walk-by fit in as the first and "fastest but least thorough" level in the three tree risk assessment levels. They give their inspectors opportunities to assess "large populations of trees" after storms and for "obvious defects," specified conditions and "imminent and/or probable likelihood of failure." They hold status as inventories of conditions and defects for specific tree locations, sizes and species unless individualized, tree-by-tree analyses and evaluations meet risk assessment criteria.

Limited visual assessments include such "obvious defects" as "dead trees, large cavity openings, large dead or broken branches, fungal fruiting structures, large cracks, and severe leans."

"[C]ertain conditions of concern, such as lethal pests or symptoms associated with root decay" join specified concerns to be eyeballed behind windshields, from above or in-person.

Level 1 assessments keep "large populations of trees" inspected by agencies, landowners and municipalities on limited budgets and by utilities wary of electric transmission system threats.

The basic assessment process, as the second of three tree risk assessment levels, lets the inspector do a careful, complete walkabout of one tree or more. Level 2 inspectors make "detailed visual" inspections of the individual tree's branches, buttress roots and trunk and of the surrounding site, for hidden and obvious defects. They need no tools unless the Scope of Work permits binoculars, cavity probes, clinometers, diameter tapes, hand-held magnifying glasses, root-excavating trowels, tape measures or trunk-sounding mallets.

Basic assessors observe such above-ground, external, lower-crown factors as cavities, cracks, dead bark, decay, fungal fruiting bodies, hollows, nesting holes, pests, response growth and weak unions.

Advanced assessments, as the most expensive and intensive of three tree risk assessment levels, "provide detailed information about specific tree parts, defects, targets, or site conditions."

Aerial inspections by climbs, ladders or lifts, decay testing, soil profiling, storm/wind load analyses, tree ring analyses and trunk lean measurements qualify as advanced assessment techniques. The most sophisticated, "[s]pecialized equipment, data collection and analysis, and/or expertise" result in "a qualified estimation," not "an accurate measure," of below-ground, external and upper-crown factors. Cavities, cracks, decay and embedded bark sound differently than functional wood during full-fluted and resistance-recording drilling, in wood stress wave analyses, and on time-of-travel-generated multi-dimensional tomograms.

Assessors throw caution to the wind when their tools breach a tree's outer tracheid, inner latewood ring and radial xylem parenchyma and innermost new xylem walls.

Sonic tomography instruments are useful in tree risk assessment by helping arborists to detect decay: Joseph O'Brien/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:

Scope of Work statements prior to visual, basic and advanced tree risk assessments describe the trees under assessment and the level of risk of assessment to be conducted; scenario of rotten tree fallen on house: Paul A. Mistretta/USDA Forest Service, CC BY 3.0 United States, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=1505095

Sonic tomography instruments are useful in tree risk assessment by helping arborists to detect decay: Joseph O'Brien/USDA Forest Service/Bugwood.org, CC BY 3.0 United States, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=5034033

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. 19 February 2012. “Qualitative Tree Risk Assessment: Risk Ratings for Targets and Trees.” Earth and Space News. Sunday.
Available @ https://earth-and-space-news.blogspot.com/2012/02/qualitative-tree-risk-assessment-risk.html

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. April 2012. "Tree Risk Assessment: Levels of Assessment." Arborist News 21(2): 12-20.
Available @ http://viewer.epaperflip.com/Viewer.aspx?docid=632bea95-6e56-46d2-a603-a2bc00f42840#?page=12