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

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:

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:

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. 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