Tree Risk Assessment: Tree Failures From Defects and From Wind Loads

Summary: Tree risk assessment helps tree specialists Sharon Lilly, Nelda Matheny and E. Thomas Smiley to associate tree failures with defects or with wind loads.

Open-grown trees in windy areas need to be monitored for severity of wind load effects; windswept Monterey cypresses (Cupressus macrocarpa) in San Mateo, Silicon Valley, Northern California; Thursday, March 29, 2007, 22:00:00: Philippe Teuwen (doegox), CC BY SA 2.0 Generic, via Flickr

Tree risk assessment, evaluation and management allow arborists, assessors and managers to anticipate and treat tree failure, according to Tree Risk Assessment: A Foundation in the December 2011 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 balance benefits and safety. Beauty and security concern tree risk managers whose responsibilities in property management and ownership count upon budgets and resources for contracting tree risk assessors and arborists. Considerations of aesthetics and of well-being dominate tree risk assessment since prescient inspections and preventative interventions depend upon identification and interpretation of hidden and visible defects.

Arborists emphasize cosmetic and practical aspects of providing lightning protection, pruning, removal, replacement, support and transplant services that enhance quality of life and value of property.

Trees function as beneficial beauties by anchoring soil against erosion, breathing in carbon dioxide, breathing out oxygen, offering fruits and nuts, providing shade and sheltering wildlife. The compaction, extreme temperatures, mechanical injuries, nutrient deficiencies, overfeeding, overwatering, pathogens, pests, pH imbalances, poisons and under-watering that do not kill trees given them shorter lifespans.

Tree risk assessment has a "limited ability to predict natural processes" such as decay progression rates and response growth and non-visible defects such as bark occlusions. Tree risk assessors inspect branch patterns and sizes since branches that are low-lying, thick-clustered and under 1 inch (2.5 centimeters) in diameter generally fail less forcefully.

Treetop-high branches over 4 inches (10 centimeters) in diameter join dead, decayed, decaying, dying trunks in enlarging impact zones to more than 1.5 times tree heights.

Gravity keeps fallen branches over 4 inches (10 centimeters) in diameter within drip circles the radii of the longest branches and root circles thrice tree heights. It leads to identifications of targets that may suffer "serious damage, injury, or death" since "Tree failures during normal weather conditions are sometimes predictable and preventable."

Tree risk assessment mentions targets by "the amount of time that they are within the target zone" to determine constant, frequent, occasional or rare occupancy rates. Targets such as buildings, electric lines, parking and play areas and roadways need to receive constant, static classifications since they are "fixed or not readily moved."

Mobile targets such as animals, pedestrians and vehicles and such movable targets as picnic tables, sculptures and swing sets occupy constant, frequent, occasional or rare classifications.

Constant, static targets prompt predictable if not preventative tree risk assessment damage estimates that prove to be less or more prescriptive and proscriptive through site factors. Construction, extreme weather events, groundwater levels, microbursts, precipitation patterns, prevailing winds and wind exposure as well as soil compaction, erosion and saturation qualify as site factors.

Trees reveal lower wind load tolerances where freezing rain, heavy snow and severe storms are rare by tolerating "wind speeds that commonly occur in an area." Research shows that "Tree failures under normal wind speeds are usually associated with serious, uncorrected, or unmitigated structural defects or other conditions, alone or in combination."

Tree risk assessment tells arborists, master gardeners, master naturalists and tree stewards to watch for hidden and visible defects and for prevailing and severe wind loads.

field demonstration of pre-removal inspection of wind load-caused damage to English oak (Quercus robur): Victoria Whaley/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:

Wind load effects may be severe for open-grown trees in windy areas; windswept Monterey cypresses (Cupressus macrocarpa) in San Mateo, Silicon Valley, Northern California; Thursday, March 29, 2007, 22:00:00: Philippe Teuwen (doegox), CC BY SA 2.0 Generic, via Flickr @ https://www.flickr.com/photos/doegox/477454523/

field demonstration of pre-removal inspection of wind load-caused damage to English oak (Quercus robur): Victoria Whaley/Bugwood.org, CC BY 3.0 United States, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=5432254

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. 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; Nelda Matheny; and Sharon Lilly. December 2011. "Tree Risk Assessment: A Foundation." Arborist News 20(6): 12-20.

Smiley, E. Thomas; Nelda Matheny; Sharon Lilly. 2011. Best Management Practices - Tree Risk Assessment. Champaign IL: International Society of Arboriculture.