Tree Risk - notes

 Notes from Tree Risk Assessment Qualification (TRAQ) class taught by Kevin Eckert for the Arizona Community Tree Council.  October 2022 

About 25 people are killed each year by falling trees in U.S.

Removal should only be an option when mitigations are not sustainable.  We have an ethical obligation to do no harm.  The benefits of trees are important.  

A risk is meaningless without a time frame.

Imminent - if a tear in the tree is weathered, the failure is not Imminent

Probable - failure would likely occur during normal (expected/seasonal) extreme weather.

Possible - failure would likely occur during abnormal (hurricane, tornado, derecho, etc) extreme weather.

Wounds

Thicker sap trees tend to compartmentalize better

If sap is oozing, touch and smell it:

Sticky = ok, its sap

Sour = bacteria

Slimy or foamy = fungus

Wet = just water

Drought

Tree dying from bottom up is over-irrigation

Tree dying from top down is drought (under-irrigation)

Failure

Soil failure vs root failure:  roots in air or roots break and stay in ground

Codominant is not a fault.  

Included bark is a fault because weak area doesn't hold stems.  

Aspect ratio near 1:1 for codom stems is a fault because don't have enough wood to wrap around union.  This is common cause of failure in pines, especially CA fire investigations.

Decay

Worse on compression side and at the base.  Hollow poles are strong. Most stress in tree is around the shell, so sapwood carries most weight, not heartwood (which is more brittle).

Heartwood - least concern

Sapwood - will start seeing tip dieback. 2nd worst.

Root - will also see tip dieback.  Not too bad if not structural; crown dieback will balance root dieback.

Basal - worst.  

Types of Response growth:

Reaction wood from tension/compression

Flexure wood from movement

Wound wood - e.g. lip around wound. More growth shows more stress

Likelihood of failure increases/decreases based on load (e.g. wind exposure) / response growth

Stress

Reproduction is sign of stress in trees (plants):  orchard trees cut to fruit more, but decreases lifespan.

Vigor/health does not always reduce tree failure risk

Adding N can increase leaf area and stem elongation, but that can increase load on stem and if stem doesn't thicken then can increase tree failure risk, even while increasing tree "vigor".  Sometimes reducing growth (ie use growth regulator) can improve mitigations.

Sheer plane: where tension and compression meet

Cracks: transverse (fiber failure) or longitudinal (with the grain)

Age

Old trees: retrenchment = natural process of aging, shortening and widening = more stable

Trees naturally move with wind.  Mass damping shows importance of inner branches and leaves.  Helps tree stems "undulate like snake in the wind".  Sharp bends or lack of inner leaf area can inhibit natural movement.

Construction

New concrete can leach lime/calcium oxate which raises soil pH and can harm some plants if too much

Adventitious branches = "water sprouts" (from topping = non ANSI cuts) = epicormic.  Poor attachments can rip out.

Shigo said:  "don't err on the side of safety….just do it right!"  :: Do the best you can based on your education, training, and experience.  

"Improper" Tree Hazard Assessments?

The Summer 2020 issue of FSEEE Forest News argues that the Forest Service improperly uses tree hazard assessments to justify closing public lands.

Based on the article, it appears that FS spokespeople have not always done a good job communicating the reason for post-fire closure areas.  There are many reasons to close areas post-fire, but tree risk assessments by themselves would probably not justify backcountry trail closures. 

The FS Field Guide for Hazard Tree Identification and Mitigation cited in the article applies a numerical probability assessment to hazards by scoring Failure Indicators and adding these with Damage Potential to obtain a numerical Hazard Rating.   This methodology would be familiar to any arborist and follows standard arboriculture practices.  If there are dead or damaged trees around developed FS facilities or campgrounds than clearly there would be a high Hazard Rating and it would make sense to close those areas until the hazard can be mitigated.  But applying this same methodology to backcountry trails would result in scores that are relatively low because of the sparse or nonexistent targets for the hazard trees. So on this point the Forest News article is correct: hazard trees alone do not justify backcountry forest closures.

However, there are other hazards in recently burned areas besides dead trees, including active restoration/revegetation projects, burned-out root systems, unstable soils, potential debris flows, and obscured or impassable trails.  It may make sense to close these areas to the public until trails can be repaired or ongoing restoration work is completed.