Saturday, February 12, 2011

Tree Load Can Turn Tree Health Into Tree Failure or Tree Fatigue


Summary: Jerry Bond, Ph.D. and urban forester, adds tree load to soil quality, stress raisers and tree defects to predict tree failure, tree fatigue or tree health.


snow load and wind load damage to river birch (Betula nigra) during 2002 ice storm: Randy Cyr/Greentree, CC BY 3.0, via Forestry Images

Arborists are the target audience of Tree Load: Concept even though the article's tree health concerns in the February 2011 issue of Arborist News attract master gardeners, master naturalists and tree stewards.
Jerry Bond, consultant with Urban Forestry, LLC, and Ph.D., begins the five-page article with load magnitude and movement as tree failure and tree fatigue probability indicators. He considers load, in terms of tree risk assessment, "the internal force created by the interaction of energy with a structure or one of its parts." He differentiates between complex forces on "organic structures in natural settings" and external, "simple forces exerted on homogeneous and rigid beams as used in mechanical theory."
Load ensues from an energy source's duration, quality, speed and temperature interacting with crown architecture, leaf characteristics and wood density and elasticity and endangers tree health.
Tree load falls into less or more variable categories, with dead load representing "relatively constant" weight of "above-ground wood that increases slowly throughout a tree's life."
More variable categories get designated dynamic regarding earthquakes, tree take-down top removals, vehicular impacts and wind gusts; or environmental respecting horizontal or twisting winds and ice. Live load has a narrow scope for such non-environmental and vertical forces as climbers and their rigging operations and a wide scope for all temporary forces. Designations as static load identify external, long-term, non-varying forces while those of resonance indicate "smaller forces" leading to "large oscillations" and tree fatigue or tree failure.
Categories concerning directional effects around or at right angles or parallel to length and involving less or more temporarily weighted forces join to compromise tree health.
Compression and tension, forces that dominate the axial dimension focus of risk assessment analysis, keep the effects of load paralleling length and running with the grain. Torsion, a circumferential dimension, leads load's effects around tree length while the radial dimension lets effects run across the grain and at right angles to length. Compression makes wood weaker than tension does whereas torsion maximizes such defects as cracked, long leaders and such variables as species-specific shear strength of green wood.
Tree load navigates the apoplastic continuum of columns and rows of interconnected cell walls from the crown's main stem to the roots and into the soil.
The formula of stress equaling force divided by area offers tree fatigue and tree failure if increased force over decreased areas overwhelm tree health and strength.
Absence or presence of severe defects and stress raisers, constancy and direction of load and interactions between strength and stress predict tree failure or tree fatigue.
"[A]n internal condition that leads to high localized stress" qualifies as a stress raiser, be it a bark occlusion, canker, crack, decay, dogleg, hole or notch. Illustrated images reveal the inability of decayed butts and saturated soils to keep trees anchored during violent storms and the vulnerability of cracked, decayed, fungus-riddled limbs. The final paragraphs summarize risk assessment's imagining tree load flow from treetop to root tips, identifying defects and stress raisers and rating tree health and strength.
The article targets arborists even though the author's commitment to making tree load-related lectures and research accessible to the public welcomes non-specialists to "Tree Load: Concept."

common, or European, beech (Fagus sylvatica) with cankers as stress raisers: Frank Vincentz, BB Y SA 3.0, via Wikimedia Commons

Acknowledgment
My special thanks to:
Talented artists and photographers/concerned organizations who make their fine images available on the Internet;
Virginia Polytechnic Institute and State University for superior on-campus and on-line resources.
Image credits:
snow load and wind load damage to river birch (Betula nigra) from 2002 ice storm: Randy Cyr/Greentree, CC BY 3.0, via Forestry Images @ http://www.forestryimages.org/browse/detail.cfm?imgnum=1238165
European beech with cankers as stress raiser: Frank Vincentz, CC BY SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Fagus_sylvatica_-_canker_03_ies.jpg

For further information:
Bond, Jerry. February 2011. "Tree Load: Concept." Arborist News, vol. 20, issue 1 (February 2011): 12-17.
Available @ http://www.isa-arbor.com/myAccount/myEducation/resources/CEU-Feb11.pdf

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