Tree Risk News

In the UK, Durham County Council (DCC) is the latest major duty holder to adopt a VALID approach to managing and assessing tree risk.

Here's DCC's Tree Management Policy.

DCC have integrated VALID's Government Tree Risk-Benefit Management Strategy model template into their policy.

All DCC's risk-benefit assessments are made under the protective umbrella of their Policy. Here's how they've zoned their occupancy, and go about their assessments in their Tree Inspection Procedure.

DCC's greatest tree risk asset is their policy and procedure.

After policy and procedure, DCC's most valuable risk asset isn't their 4 Validators (trained Arborists). It's the nearly 60 Basic Validators we've trained over the last week.

Basic Validators are trained to pick up on Obvious Tree Risk Features they can't help but notice as they go about their day to day work.

Basic Validators can also carry out Active Assessment at a Basic level, whenever they perform general 'inspection' procedures as part of their work.

If there's a red risk tree in County Durham, it's a Basic Validator who's most likely to spot it long before a Validator does.

*We're not keen on the word 'inspection' when it comes to tree risk. Inspection implies a level of detail that's seldom necessary if you're reasonable, proportionate, and reasonably practicable about managing the risk.

In DCC's case, the word 'inspection' is embedded in the description of many procedures carried out by Highways Officers, Rangers, and the like, so they've kept it.

Bark inclusions - A Feature or Fault?

Some nice evidence based arboriculture helping us move on from seeing bark inclusions as being 'Defects'.

Trees Adjust the Shape of Branch Unions to Increase Their Load-Bearing Capacity

Putting the benefit into risk

We're playing around with a new definition of risk.

It's inspired by CAPUR's, 'Improving Society's Management of Risks: A Statement of Principles.'

CAPUR's definition is…

"Risk an idea about what might happen in the future (good or bad)"

We very much like the clarity that 'risk' can be good or bad in this definition.

Mind you, I'm not sure what work 'in the future' is doing, that's not already done by 'what might happen'.

It'd be a better definition as.

"Risk an idea about what might happen (good or bad)"

I think it's much clearer definition of the threat v opportunity core of risk that ISO 31000's tries to encapsulate with its cryptic definition.

"Risk is the effect of uncertainty on objectives"

Both are leaps and bounds better than the most commonly used definition of risk…

"The likelihood of something bad happening"

There are no benefits to risk in this definition.

In VALID's risk model. An Acceptable risk is one where each year there are more than a million good outcomes for every bad outcome.

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.

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.

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)

...etc

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.

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.

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

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.

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).

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

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.

Don'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.

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.

these 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'.

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.

Risk
=
Likelihood of Occupancy
AND
Likelihood of Failure
AND
Consequences

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.

Or

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.

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 & Benefits