Tree Risk News

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  • 2024-01-12 12:30 PM | Admin (Administrator)

    Think Safety Factor, not Strength Loss

    To help stop unnecessary tree felling or damaging crown reduction tree work to your most important trees. We've updated and much improved our explanation of why t/R ratios don't work.

    Why t/R ratios don't work

    Think Safety Factor, not Strength Loss

    Explains why you can't make a credible decision about Likelihood of Failure, or the risk, based on a residual wall thickness from, sonic tomography, micro-drills, or estimation with a sounding hammer.

    Why t/R ratios don't work

    Isn't t/R = 0.3 a guide? Or a starting point?
    We've seen occasional comments like this on our social media feeds.

    If Why t/R ratios don't work hasn't persuaded you, t/R = 0.3 doesn't work as a guide, or starting point. Here's some additional explanation about why.

    t/R = 0.3 could be too much hollowness for a particular species of tree (Material properties). With particular crown dimensions and location (Load). And a particular stem diameter and geometry (Form).

    Improve the Material properties, and t/R = 0.3 would be fine.

    Or, lower the height, or have a more sheltered location (Load), and t/R = 0.3 would be fine.

    Or, increase the section modulus by widening the diameter, and t/R = 0.3 would be fine.

    t/R = 0.4 could be too much hollowness for a particular species of tree (Material properties). With particular crown dimensions and location (Load). And a particular stem diameter and geometry (Form)


    And so forth, through every t/R ratio.

    t/R = 0.3
    A tree that's 50% hollow might be a tree with a good Residual Safety Factor.

    t/R = 0.7
    A tree that's 9% hollow might be a tree with a Residual Safety Factor that's too low.

    Your critical t/R could be any ratio. It's just one part of the 'Form' in the tree Safety Factor puzzle.

    Why t/R ratios don't work

    When to use 'Why t/R ratios don't work'
    You're welcome to use Why t/R ratios don't work to question tree felling or crown reductions when sonic tomography or micro-drill readouts are used to justify them.

    Similarly, this will help you question media posts showing photos of stumps from felled trees, which are decayed or hollow, to justify the work.

    Hollow stump porn

    Sonic tomography, micro-drills, and sounding hammers can be a really useful kit. But, even when they're used well, they're only telling you a small part of the Likelihood of Failure story.

  • 2024-01-12 12:00 PM | Admin (Administrator)

    James Chambers at TMA, Paul Melarange at ThinkTrees, and Ian Barnes at Barnes Associates are concerned about poor Sonic Tomography leading to unnecessary tree felling or damaging crown reduction tree work.

    To help those of you who get Sonic Tomography carried out by Picus, Arbotom, or ArborSonic, they've published this guide of what to look out for.

    Sonic Tomography - Decay Detection Considerations

    Sonic Tomography - Decay Detection Considerations

    This is such a useful collaborative project, we'll forgive them for using the Safe word in it :-)

    Here's a nice example of where a 'red line' is a 'red flag' in a tomogram.

    t/R = 0.3 makes no sense.

    t/R = 0.3 makes even less sense when the cross section is asymmetric.

    Or when and the decay is off centre.

    Sonic Tomography t/R ratio 0.3

  • 2023-12-17 4:42 PM | Admin (Administrator)

    VALID's Risk Assessment Model

    We're at the end of another successful tree risk training and advisory tour of NZ & AU. On this trip, we've been told some Aussie Arborists are not yet sure about upgrading their approach to tree risk with VALID because of a point raised in Peter Gray's Tree Risk Assessment Review for Arboriculture Australia's 'The Bark' magazine (2020).

    Tree Risk Assessment Review

    Advantages & Disadvantages
    In the article, Peter suggests a disadvantage of VALID is:

    "The underlying mathematics running the App have not been made available for peer review. VALID claims to have used the services of a respected and independent maths professor to develop and test the App but this must be accepted without any chance to review and criticise it."

    This 'disadvantage', is in fact an 'advantage'.

    Here's why.

    Our Risk Professor
    First, the 'Maths Professor' is in fact a 'Risk Professor'.

    He's Professor Willy Aspinall. Willy is the Cabot Professor in Natural Hazards & Risk Science at Bristol University, and he said this about the 'underlying maths'.

    "We have stress-tested VALID and didn't find any gross, critical sensitivities.
    In short, the mathematical basis of your approach is sufficiently robust and dependable for any practical purpose."

    VALID is the only tree risk system that's been put together with a Risk Professor, who's an internationally distinguished expert in assessing risk in the natural environment. Surely, this is an 'advantage', and not a 'disadvantage'.

    3 points about making the 'underlying maths' available.

    1) It would give away our intellectual property, and anyone could copy it. Similarly, Google doesn't make its search algorithm available.

    2) We're planning to write a paper with the Professor once he's less busy being an expert witness on the risk to Japanese nuclear reactors from an Aso volcano super-eruption .

    Or on the fallout from the New Zealand Whakaari/White Island volcanic eruption.

    3) How the matrices are constructed in TRAQ is not available for peer review. Neither are the Monte Carlo simulations in QTRA available for peer review.

    Another consideration is, the underlying maths won't help a Duty Holder or Risk Assessor with their decision-making. Indeed, some might think they know better and try to numberwang the model to game the risk they want.

    We were also told, some Duty Holders and Arborists are concerned about how they would explain their decision making if they ended up in court.

    If you use VALID, it's extremely unlikely you'll end up in court. If you did, as long as you don't make a green decision when it should be red - which is incredibly difficult to do - we've got your back.

    The Bolitho Test

  • 2023-08-03 9:42 AM | Admin (Administrator)

    Don't do it

    VALID's first ever publication was a Summer Branch Drop (SBD) Guide, way back in 2018.

    The reason we got this guide out first was, in the UK we had a stinking hot dry spell in July 2018. It triggered many enquiries from Arborists and Duty Holders concerned the risk from SBD was something they needed to manage urgently. Otherwise they could end up in court. That unless they hastily erected signs, fenced trees off, or pruned them, they'd be liable if a death or injury happened.

    Keen to bring some common sense to managing the risk from SBD. We rolled up our sleeves, did the research, and got the guide out within a couple of weeks.

    5 years on, and we've updated our Summer Branch Drop Guide to v9. Mainly to include a section about why it's unwise to put up warning signs about Summer Branch Drop. We've had enough people ask whether we can make this point clearer and explain it. So we did.

    Summer Branch Drop SignDon't put up signs like this

    As ever with our publications, we've waived copyright on the SBD Guide and released it under a creative commons licence. Anyone is free to use it.

    The CliffsNotes
    If your trees have no history of SBD, warning signs are unnecessary because the risk is Acceptable.

    If you have a tree that has a history of SBD, and the risk is not Acceptable or Tolerable, warning signs are not an effective way to manage the risk.

  • 2023-07-26 7:12 PM | Admin (Administrator)

    Risk minimisation is an aim you often see in tree strategies. Like the recently released Sustainable Forest Initiative Urban and Community Forest Sustainability Standard.

    Lots of great stuff about sustainability in urban and community forests. BUT, one of the 5 PRINCIPLES is to 'minimize risk' from trees.

    On the face of it, minimising risk seems like a desirable and reasonable aim. Until, that is, you carefully think about what minimising risk actually means. What the costs and consequences of minimising risk actually are.

    To minimise a risk is to make it as small as you can. No matter what the costs, or the loss of benefits.

    If your goal is risk minimisation, whatever the risk, you can always make it smaller.

    This These go to 11 scene from This is Spinal Tap nails the issue.

    Tree Risk Minimisationthese go one louder

    In the same way that 11 is 'one louder' than 10. Whatever the risk, you can minimise it by making it 'one lower'.

    There's always some work you can do to a tree, or the occupancy below it, to minimise the risk by making it 'one lower'.

    If someone dies, or is injured, or property damaged. It's really easy for a Claimant to show the risk wasn't minimised, because the risk could've been 'one lower'.

    The language we use in tree risk is super important because it affects how we think about it, and the decisions we make.

    We know there are levels of risk that are so low they're Acceptable or Tolerable. When risks are this low, you don't need to minimise the risk and go 'one lower'.

  • 2023-05-31 10:24 AM | Admin (Administrator)

    As part of our ongoing efforts to take the bafflegab out of tree risk with our Mind Your Language series.

    Let's have a look at why the phrase Risk of Harm grinds our gears.

    Here are 5 reasons to stop using Risk of Harm when we're talking about the risk from trees or branches failing.

    Likelihood of Occupancy
    Likelihood of Failure

    1) Doublespeak?
    Harm (Consequences) is one of the risk inputs that's used to work out what the risk output is.

    It makes little sense to have just one of the risk inputs being carried over and repeated as a risk output.

    The Risk of Harm is the same as saying, the Risk of Consequences.

    It's even odder when you consider the other risk inputs, the Likelihood of Occupancy or Likelihood of Failure, aren’t repeated in the risk output. They're left behind.

    When we're talking about what the risk is, we don’t say.

    The Risk of Occupancy.


    The Risk of Tree Failure

    Here's the Risk of Harm issue framed in a Q&A format.

    Q) What’s the risk of that branch falling onto someone and injuring them?

    A) The risk is Acceptable.

    Or, you could say.

    A) The risk of the branch falling onto someone and injuring them is Acceptable

    It's odd to say.

    A) The Risk of Harm is Acceptable.

    It’s odd because you’ve carried over the injuring someone Consequences in the question through to the answer and labelled it Harm.

    Yet, you’ve not carried through the Likelihood of the person being hit. Or the Likelihood of the branch failing from the question over to the answer. Both Likelihood inputs have been left behind.

    The Risk is ALL three elements.

    2) Do trees deliberately inflict injury?
    Harm is defined as a physical injury to a person that's been deliberately inflicted.

    Given the amount of abuse trees get from people, it'd be difficult to blame them if they did deliberately inflict physical injury on us.

    Of course, a tree can kill or injure us, but it can't deliberately inflict injury on us by falling or shedding a branch.

    3) Isn't death a worse Consequence than harm?

    Obviously, death is a greater Consequence than a physical injury, or harm.

    If the Consequences are a death, then Risk of Harm is diluting the Consequences input.

    4) Can property be harmed?
    Damaged or destroyed, yes. But deliberately injured?

    5) Less makes more sense
    Risk, instead of Risk of Harm, removes two redundant words, that aren't helping us understand what we're measuring.

    Risk is both simpler and clearer than Risk of Harm.

  • 2023-02-19 7:55 AM | Admin (Administrator)

    Taking the Defect Out of Tree Risk - Article Update

    'Taking the Defect Out of Tree Risk' is published in the current NZArb 'Tree Matters' magazine.

    It first appeared in the UK's Arboricultural Association's (AA) Arb Magazine (Spring 2001).

    There are a couple of important improvements in the latest version.

    Like the Obvious Tree Risk Features Guide, which now includes construction damage.

    We've also improved the VALID Likelihood of Failure mnemonic to prepare for v2 our Tree Risk App. In our excitement about dumping the red DEFECT and replacing it with the neutral DECAY in the AA version, we neglected how this affected some subheadings.

    Tree Defects Hazards v Features BenefitsTree Defects & Hazards v Features & Benefits

  • 2023-02-03 8:42 AM | Admin (Administrator)

    Any assessment of risk from trees falling should begin with a Tree Risk-Benefit Management Strategy.

    Whether you're a Duty Holder or a Risk Assessor, here are 5 valid reasons why the Strategy is your greatest tree risk asset.

    1 The Context
    A Strategy establishes the context (ISO 31000) of the risk universe in which you're working. It sets out your stall by defining your 'risk objectives', and setting out the rules of the game.

    2 What you're doing
    It makes it clear what a Risk Assessor is doing when they're looking at a tree. Why they're doing it. And what the limitations are.

    3 The donkey work
    The Strategy does more than 95% of your risk assessments for you.

    4 Underwritten
    Your insurers will be happy to underwrite you.

    5 The truth of the matter
    The Strategy 'speaks truth to power'.

    Tree Risk Expert Witnessspeaking truth to power

    In the extremely unlikely event of an Acceptable or Tolerable risk happening. And a claim is being considered. It's the Strategy that puts you in the best position to disarm an Expert Witness who struggles with risk literacy.

    Or regulate a Risk Entrepreneur, who profits by talking up the risk.

    The Strategy also gives you defence in depth by speaking truth to Judges, Coroners, and Enforcement Officers.

    If you want to know what a Tree Risk-Benefit Management Strategy looks like, we've got a whole bunch of free templates on our Risk Management page.

  • 2023-01-19 9:58 AM | Admin (Administrator)

    We know from pedestrian data, the centre of built-up areas have a Likelihood of Occupancy that's so high, on average more than one person is exposed to the risk.

    Occupancy is Very High.

    Tree Risk Target People - Very High Occupancy
    More than one person exposed to the risk

    We also know from traffic data, the busiest roads have a Likelihood of Occupancy that's so high, on average more than one vehicle is exposed to the risk.

    Occupancy is Very High.

    Tree Risk Target Vehicle - Very High Occupancy
    More than one vehicle exposed to the risk

    Where we have busy roads next to busy footpaths in towns and cities, we know the combined occupancy of people AND traffic is so high, on average more than one person AND one vehicle is exposed to the risk.

    Occupancy is Very High.

    More than one person AND more than one vehicle exposed to the risk

    We know other tree risk assessment systems systematically undervalue Very High Occupancy. From the training we’ve delivered, we also know tree risk assessors have been poorly trained to recognise both Very High and High Occupancy.

    What all this means is unless you’re using VALID, and have had Likelihood of Occupancy training (it’s really easy once you're calibrated), you’ll be undervaluing the occupancy where it matters most.

    If you're undervaluing the occupancy, you'll be undervaluing the risk.

    You’ll be undervaluing the risk by at least a whopping factor of x10 or x100.

    A 1:1M risk might be as high as 1:10K.

    A Low Risk might be a High Risk.

  • 2022-10-02 10:45 AM | Admin (Administrator)

    If you've heard of Daniel Kahneman, you'll likely have read his excellent Thinking Fast & Slow, where he shares his Nobel Prize winning work into behavioural economics. All that fascinating stuff about how ingrained biases often have reasonably foreseeable impacts on our decision making.

    His most recent interest has been a collaboration with Oliver Sibony and Cass Sunstein, exploring the role of 'Noise' in decision making.

    Noise is the significant variability in decisions you get where there shouldn't be any. Such as when Doctors diagnose disease, or when Judges sentence criminals.

    Or when Arborists make tree risk assessment decisions.

    Earlier this year, More or Less' Tim Harford had a short chat with Daniel Kahneman about how to reduce Noise by practising Decision Hygiene.

    If you want to dive into the Decision Hygiene part, it starts at 5.50.

    What was eye-opening about this discussion into the benefits of practising Decision Hygiene is that it's embedded in VALID.

    Arborists' tree risk assessments are notoriously noisy. Ask 10 Arborists to risk assess a tree and you'll likely get 12 very different decisions.

    One of VALID's benefits is we've designed it to reduce this Noise. When it comes to Occupancy and Consequences, consistent and good decision making is so 'quiet' you don't need to be an Arborist to do it well.

    In tree risk assessments, we get most Noise with Likelihood of Failure decisions.

    When you make your Likelihood of Failure decision in VALID, you're guided to evaluate all the likelihood of failure attributes, independently of each other, before you make a decision.

    Tree Risk Assessment - Likelihood of Failure

    This VALID approach to Likelihood of Failure decision making is what Daniel Kahneman is calling Decision Hygiene.

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