Tree Risk Assessment Nelda s Top Ten Nelda Matheny & Jim Clark HortScience, Inc. January 2009
1. Trees are optimized structures Biological and structural functions Wood is added to areas of high mechanical stress
2. Body language gives clues The manner in which a tree grows reveals the pattern of stress
3. Trees are shedding organisms Give up parts to survive Forces o o o o wind gravity ice/snow rain The tree cannot be stronger than wood
4. When the wind blows hard, trees fail
Failure in wind WIND If root/soil strength is greater than trunk strength, trunk breaks. Tension Compression 6
Failure in wind If trunk stronger than soil/roots, tree uproots. WIND Soil shear Root tensile shear 7
Wind o So many variables! Crown o o Stem o Roots o Soil o speed, timing, gusts, duration area, density, flexibility height diameter, strength depth, spread, strength texture, sheer strength, moisture, depth Harris 1998
5. Wind is a dynamic force Wind is a restless force that varies in speed and direction during stormy weather. Ken James 2008
Trees in wind Wind gust 10
Trees in wind SWAY 11
Trees in wind 12 Ken James, 2005
Trees in wind..a complex, looping pattern that responds to the changing wind gusts. Ken James, 2005 13
6. Branches help dissipate wind energy 14
6. Branches help dissipate wind energy Trees are a complex dynamic system in which branches interact to prevent dangerous sway motion developing. Ken James, 2005 15
7. Strength loss formulas aren t much help Bending stress of hollow cylinder (d 4 /D 4 ) x 100 Buckling failure in cylinder t/r 16
100 75 50 25 0 0 25 50 75 100 % Hollow Graphic: Brian Kane % Strength loss (Loss in I)
Kane s Summary Wagner s and Coder s formulas were least reliable Coder s formula had the most underestimates Mattheck s formula had the fewest underestimates, but 2 overestimates Bartlett formula worked well, but still had 3 underestimates DO NOT RELY SOLELY ON FORMULAS FOR RISK ASSESSMENTS
8. Hazard and Risk aren t the same
8. Hazard and Risk aren t the same Risk: The likelihood that a tree or tree part will fail and cause injury or damage. Hazard: A tree or tree part that is likely to fail and cause damage or injury, and the likelihood exceeds an acceptable level of risk. ISA 2006, Glossary of Arboricultural Terms, 2 nd ed.
What is risk? What is hazard?
9. Risk assessment involves judgment Judgment is a process of estimating, valuing or choosing in essence it is a thinking process. David A. Cleaves 1994
Degree of Certainty 100 80 60 40 20 >95% = significant <95% = not significant >90% = beyond reasonable doubt >75% = compelling evidence >51% = preponderance of evidence <51% = not guilty Glenn Haas 2003 0 Science Judiciary The degree of certainty used in decision-making by the judiciary is lower than that of the scientific community
Judgment The quality of judgments reflects the level of rationality and scientific rigor in the judgment process. David A. Cleaves 1994
Certainty and sound professional judgment Sample tools research principles reasons decision analysis expert opinion professional experience appropriate processes administrative and historical records Glenn Haas 2003
9. Risk assessment involves judgment Judgment based on observations investigations and testing data and information Combined with our experience training scientific knowledge education
10. Feelings about risk drive decisions Willingness to accept risk varies widely There is no defined threshold
Perception of risk Individual and social values Culture Experiences Level of education Who the person is and how he/she is affected
Perception of risk Cognitive + Emotion THINKING logic reason scientific deliberation FEELING instinctive intuitive Emotion plays an essential role in making risk judgments
10. Feelings about risk drives decisions How we perceive risk is reflected in our advice to clients. We can t tell people how much risk they should live with.
Nelda s top 10 1. Optimized structures 2. Body language 3. Shedding organisms 4. Wind is a mighty force 5. Wind is dynamic 6. Branches detune tree 7. Formulas? 8. Hazard vs. risk 9. Judgment 10. Perception of risk
U. S. Arboriculture A.N.S.I. o Standard for Tree Risk Assessment I. S. A. Best Management Practice Tree Risk Assessor Qualification Research Summit Bibliography