Tree Risk AssessmentNelda’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

• Forceso wind

o gravity

o ice/snow

o rain

• The tree cannot be stronger than wood

4. When the wind blows hard, trees fail

6

Tension Compression

WIND

Failure in wind

If root/soil strength is greater than trunk strength, trunk breaks.

7

WIND

Failure in windIf trunk stronger than soil/roots, tree uproots.

Soil shear Root tensile shear

So many variables!• Wind

o speed, timing, gusts, duration

• Crowno area, density, flexibilityo height

• Stem o diameter, strength

• Rootso depth, spread, strength

• Soilo 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

10

Wind gust

Trees in wind

11SWAY

12

Trees in wind

Ken James, 2005

13

Trees in wind

Ken James, 2005

“..a complex, looping pattern that responds to the changing wind gusts.”

14

6. Branches help dissipate wind energy

15

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

7. Strength loss formulas aren’t much help

• Buckling failure in cylinder

t/R

16

• Bending stress of hollow cylinder

(d4/D4) x 100

% S

tren

gth

lo

ss (

Lo

ss i

n I

)

% Hollow

0

25

50

75

100

0 25 50 75 100

Graphic: Brian Kane

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, 2nd 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

Glenn Haas 2003

100

80

60

40

20

0

>95% = significant

<95% = not significant

Degree of Certainty

Science

<51% = not guilty

>51% = preponderance of evidence

>75% = compelling evidence

>90% = beyond reasonable doubt

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. Cleaves1994

Certainty and sound professional judgment

Glenn Haas 2003

Sample tools– research– principles– reasons– decision analysis

• expert opinion• professional experience

– appropriate processes– administrative and historical records

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 isand how he/she isaffected

Perception of risk

Cognitive + Emotion

THINKINGlogic

reasonscientific deliberation

FEELINGinstinctiveintuitive

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 howmuch 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