Earth and Space News: tree risk assessment

Showing posts with label tree risk assessment. Show all posts

Saturday, August 11, 2012

Tree Risk Assessment Mitigation Reports: Tree Removal, Tree Retention?

Summary: Sharon Lilly, Nelda Matheny and E. Thomas Smiley discuss opting for and recording tree removal or tree retention in tree risk assessment mitigation reports.

Tree risk assessment mitigation reports assess tree removal or, preferentially, tree retention; risk tree (right) in park near Tawas City, Iosco County, Lower Peninsula of Michigan: Joseph O'Brien/USDA Forest Service/Bugwood.org, CC BY 3.0 Unported, via Forestry Images

Tree risk assessment mitigation reports are records of the inspections, remedies and reviews of risky trees, according to Tree Risk Assessment: Mitigation and Reporting in the August 2012 issue of Arborist News.

Sharon Lilly of the International Society of Arboriculture, Nelda Matheny of HortScience, Inc., and E. Thomas Smiley of the Bartlett Tree Research Laboratory begin with risk-taking. Master gardeners, master naturalists, tree risk assessors and tree stewards consider tree-based risks regarding the likelihood of tree failure and impact and target-based risks respecting consequences. Target-based thinking drives clients and tree risk assessors to decide immediately upon "the ultimate security of risk elimination based on tree removal" over tree retention options.

Residual risk endures in target-based options since "With tree removal, that residual risk is brought to near zero; however, even stumps can pose some residual risk."

Tree-based thinking finds "possibilities for retaining trees when practicable" since "Trees offer many benefits," such as anchoring soils against erosion, releasing oxygen and removing carbon dioxide.

The "preventive and remedial" mitigation process gets the "likelihood of failure or the likelihood of impact" and their consequences to a level "acceptable to the client." Tree retention mitigation heads toward drainage systems, lightning protection and structural support brace rods, cables, guys and props to reduce wet soil-, storm- and wind-induced failures. Crown-raising, pollarding, reduction pruning, thinning and tree growth regulators increase wind dissipation and transfer while relocating, rerouting or restricting mobile and movable targets inhibits risky access.

Tree risk assessment mitigation reports juggle information, such as residual risks and risk ratings of extreme-risk, high-risk, moderate-risk or low-risk, for tree removal and retention options.

The risk ratings keyed from inspections into tree risk assessment mitigation reports keep work priorities on schedule since extreme-risk trees are mitigated "as soon as possible." They lead to mitigating high-risk trees "as soon as it is practical" and moderate- and low-risk trees whenever pruning cycles and work budgets and schedules converge.

Verbal, videographic, work order or written-up report manages one-time or repeated progression from inspection to intervention, re-inspection and re-intervention for tree removal or tree retention. It notes assessment date and assessor name, inspection level as limited visual or basic or advanced, scope of work, site factors and tree identification and location. It operates as detailed documentation of response growth, storm patterns, structural defects, target mobility and occupancy rates and tree failure and site impact likelihood and consequences.

Tree risk assessment mitigation reports protect "both tree risk assessor and client from misunderstandings" by providing information on residual risks and limitations on inspections and inspectors.

"[U]ncertainties related to trees and the loads to which they are subjected" qualify as limitations on assessments since "scientific study of tree failure is relatively young." Inspectors investigating tree removal and tree retention options reveal as limitations on performance anecdotal information, hidden defects, inaccessible sites, nascent methodology, seasonal pests and unpredictable winds. Limitations suggest inspection intervals of eight to 16 months, one to two years and three to five years for pest-prone, high-risk and low- to moderate-risk trees.

Assessment, mitigation, re-inspection and re-intervention take assessors and clients close to de-mystifying how trees avoid failure and what wind directions and speeds cause trees to fail.

Measuring a tree's vitality with an electrical conductivity meter is helpful in tree risk assessment retention or removal: USDA Forest Service-Northeastern Area/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:

Tree risk assessment mitigation reports assess tree removal or, preferentially, tree retention; risk tree in park near Tawas City, Iosco County, Lower Peninsula of Michigan: 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=5030027

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. 16 June 2012. “Internally Stressed, Response Growing, Wind Loaded Tree Strength.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2012/06/internally-stressed-response-growing.html

Marriner, Derdriu. 14 April 2012. “Three Tree Risk Assessment Levels: Limited Visual, Basic and Advanced.” Earth and Space News. Saturday.
Available @ https://earth-and-space-news.blogspot.com/2012/04/three-tree-risk-assessment-levels.html

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. August 2012. "Tree Risk Assessment: Mitigation and Reporting." Arborist News 21(4): 14-18.
Available @ http://viewer.epaperflip.com/Viewer.aspx?docid=de6602f7-b34e-413f-b53b-a2bd009f8b0d#?page=14

Saturday, June 16, 2012

Internally Stressed, Response Growing, Wind Loaded Tree Strength

Summary: Sharon Lilly, Nelda Matheny and E. Thomas Smiley find that internally stressed, response growing, wind loaded tree strength defies tree failure.

Tree strength response of woundwood, which is produced as a response to cambial (tissue layer between inner bark and wood) damage, compartmentalizes tree decay by reinforcing large cavities; a protective screen preserves hard outer rim and protects the cavity: USDA Forest Service-Northeastern Area/USDA Forest Service/Bugwood.org, CC BY 3.0 United States, via Forestry Images

Internally stressed, response growing, wind loaded tree strength accounts for some trees surviving failure-worthy events, according to Tree Risk Assessment: Loads and Growth Response in the June 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 two formulas. Formula 1 calculates bending moment M from force F times lever arm distance D for the magnitude of lateral forces levering against the tree crown's center. Arborists describe load as resulting from "various forces acting on a structure" and designate gravity pulling downward and climbers, epiphytes, rigging operations and wind operating dynamically.

Downward and dynamic forces exert "stresses and strains in the tree structure" against which drag, as wind velocity squared, is "the most common, wind-related lateral force."

Winds flow more forcefully against taller trees and at higher altitudes even though drag functions under the influences of air density, wind resistance and wind-intercepting surfaces.

Greater streamlining-induced reconfigurations of branches, leaves and twigs on the wind-intercepting "frontal area and volume of crown facing the wind" go with more forceful wind speeds. Internal stresses from the loads of climbers, epiphytes, gravity, rigging operations and winds have a mathematical definition in Formula 2, whereby stress equals force over area. Stress involves separate, simultaneous or successive forces of compression squeezing tree fibers, shear sliding them in opposite side-by-side directions, tension stretching them and torsion twisting them.

Cross-sectional areas decreased by stress-raising cankers, notch cuts and sharp-angled bends join to overload, and overwhelm into failure, internally stressed, response growing, wind loaded tree strength.

Mass damping by different-sized branches, leaves and twigs moving helter-skelter and proximity to other trees keep loads less stressful by dissipating or transferring wind energy elsewhere.

Internally stressed, response growing, wind loaded tree strength leads to trees withstanding "stress without failure" while breaking stress lets others suffer "stress sufficient to cause failure." It makes possible such response growth potentials or presences as bulges, buttress roots, corrected leans, intact bark, pest-free crowns, ribs, taper and well-developed, wide root flares. Strong trees need to generate new wood, known as response growth, that responds to "damage or loads, which compensates for higher strain (deformation) in marginal fibers."

Species-specific amounts observe inputs from energy reserves and light, nutrient and water resources while response growth distribution overlaps with mechanical stress, repeated movement and structural weakness.

Internally stressed, response growing, wind loaded tree strength produces reaction wood "in response to a stimulus or load," such as growth from cuts and toward light.

Compression wood qualifies as reaction wood whose response growth patterns in conifers "is composed of cells that have expanded" to compensate for lower- or leeward-leaning stems. The type of reaction wood called tension wood reveals response growth patterns of cells contracted to compensate for a stem's "upper or windward side" in hardwoods. Woundwood strengthens cavities or wounds whose cambial damage stimulates chemically distinct, decay-resistant, dense wood and whose enlarged, inward-curling "ram's horns" support openings that cannot close over.

Response growth tells master arborists, master gardeners, master naturalists and tree stewards that tree strength may constitute "sufficient structural compensation" to avoid failure and survive overloads.

Young red pine (Pinus resinosa) displays snow pull damage on lower branches; strong, upper branches resisted the snow force: Steven Katovich, CC BY 3.0 United States, via Forestry Images

Acknowledgment

Woundwood, which is produced as a response to cambial (tissue layer between inner bark and wood) damage, compartmentalizes tree decay by reinforcing large cavities; a protective screen preserves hard outer rim and protects the cavity: USDA Forest Service-Northeastern Area/USDA Forest Service/Bugwood.org, CC BY 3.0 United States, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=1408012

Young red pine (Pinus resinosa) displays snow pull damage on lower branches; strong, upper branches resisted the snow force: Steven Katovich, CC BY 3.0 United States, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=1593060

For further information:

Smiley, E. Thomas; Matheny, Nelda; and Lilly, Sharon. June 2012. "Tree Risk Assessment: Loads and Growth Response." Arborist News 21(3): 12-16.
Available @ http://viewer.epaperflip.com/Viewer.aspx?docid=6e0f74c1-8c91-4709-94be-a2bd00ae9609#?page=12