Can we rely on an expert witness telling us what the courts are expecting when it comes to tree risk?
In short, the answer is no because they’re an expert to the court, and not an expert for the court.
A competent arboricultural expert witness knows their limitations. Namely, their role is limited to being an expert to the court. They’re stepping way outside their field of expertise if they claim a Judge’s wisdom about how the law will evaluate tree risk-related evidence in the next case.
Claims by Arborists that they’re experts for the court should ring alarm bells. In a similar way to a Judge who, with no arboricultural training or qualifications, claims they could carry out an advanced tree risk assessment with a Static Load Test on a tree that has extensive root decay because they’ve seen it done.
In the UK, we’ve had several tree risk-related Judgments where the Judge has spotted an expert straying outside of their lane, and dipping into their legal dressing up box. Most recently in Colar v Highways England, the Judge spends a quite remarkable amount of the Judgment tearing strips off the defendant's expert. This commentary, by Gordan Exall, in 'Civil Litigation Brief' covers the issues in detail.
Perhaps, what’s of much greater concern is when a Judge is not aware that the evidence an expert gives them is critically short on expertise. Highly questionable expert evidence appears to have been pivotal in two landmark Judgments in the UK, Poll v Bartholomew (2006) and Cavanagh v Witley Parish Council (2017).
This article explores the gulf between reasonable, proportionate, and reasonably practicable tree risk assessment and management, and expert evidence in these cases.
Recently, we had a couple of enquiries asking for a copy of this article. It reviews qualitative and quantitative approaches to tree risk assessment and looks at how we could do better.
It's over two years old now and was written at the time VALID was entering the home stretch. Though VALID has evolved further, much of the article is still relevant today.
This makes for an interesting tree risk assessment case study.
An ISA TRAQ, QTRA, and VALID tree risk assessment were carried out on the same Pine trees in Western Springs, Auckland | NZ.
It involves around 200 Pinus radiata. From a risk of branch or tree failure perspective, the trees of particular interest are those that could fall onto a footpath or property.
The reports are linked.
ISA TRAQ | August 2019
QTRA | December 2019
Random tree part or tree onto footpath
1/400,000 (Size Range 4)
1/500,000 (Size Range 3)
1/1,000,000 (Size Range 2)
<1/1,000,000 (Size Range1)
1 Not Acceptable
50 Not Tolerable
We’ve got an all new Basic Tree Risk Assessment workshop!
Contact us to arrange training now.
This one day course will train your staff in how to recognise obvious tree risk features that need a closer look by an Arborist. If you’re a tree owner or manager with staff who spend a lot of time outside, this is the most cost-effective way for you to manage your tree risk.
We know duty holders prefer this training to Lantra’s ‘Basic Tree Survey and Inspection’ or ‘Highway Tree Inspection’ courses. You'll appreciate the value because the training is all about helping you fulfil your duty of care as a key part of your Tree Risk-Benefit Management Strategy.
*Basic Validators aren’t expected to survey or inspect trees. They’re able to carry out Passive Assessment (keeping an eye out for obvious tree risk features you can’t help but notice). Or, carry out an Active Assessment at a Basic level, where they’re looking for obvious tree risk features.
If you have a tree in your garden, all you need do is download this short Strategy and use a pdf editor to change the personal details. Then keep an eye out for obvious tree risk features (page 3) that you can’t help but notice in high-use areas. If you see any of these obvious tree risk features, get in touch with an Arborist who's been trained in tree risk assessment.
When you know what to keep an eye out for, trees with the highest risk are the easiest to spot. Most tree risk assessment isn’t so difficult.
Here, and in the other Strategies, Passive Assessment encourages citizen science risk assessment from those who are on the front line, enjoying the benefits that trees give us, day in day out.
This, and the other Strategies for Government and Landowner, can be downloaded from our Risk Management page.
Don't call 'high-use' zones, 'high-risk' zones.
This is from the Arboricultural Association's ARB Magazine (Autumn 2020) and can be downloaded here.
The high tree risk twilight zone
As we’re going through a hot spell in the UK at the moment, it’s time to release v8.0 of the Summer Branch Drop (SBD) Guide.
Those of you who are concerned about the risk from SBD, this should help reassure you. The overall risk from SBD is mind-bogglingly low. In this update, we have an easy to grasp explanation of what mind-bogglingly low means. The overall risk from SBD for a whole year is less than the short times it takes to cover about 5km/3mi on a drive.
250 miles (400km) drive = 1 micromort (a one in a million chance of death).
The overall risk from SBD is over one hundred times lower than this.
If you’re a duty holder, unless you’ve got a tree that’s a repeat offender, there’s no need to fanny around with confusing and ineffectual warning signs. Just download the SBD Guide from the Government section on the Risk Management page of the website.
Photo credit - Paul Barton
We recently came across a new concept called Friction. Friction is making things difficult when they could be done much easier. It’s the obstacles between what you want to do and doing it. Generally, friction is something you want to remove, and a great example of successful friction removal is Amazon. One of the reasons Amazon has been so successful is it’s taken a lot of the friction out of buying stuff. They’ve made shopping pretty much as effortless as it can be.
It made us realise that one of the things we’ve done with VALID is to remove friction from tree risk.
At the risk assessment level, the occupancy and consequence decisions in the App have little friction. The friction that comes with bafflegab (vague words) and numberwang (difficult maths) has been designed out. With a bit of training, occupancy and consequence decisions are pretty much effortless.
On the other hand, when you want assessors to be thoughtful about their decisions you add friction. In the App, you have to go through VALID letter-by-letter before you can make a likelihood of failure decision. This is adding friction where it’s important.
What do the Boffins have to say?
Whilst putting VALID together with the Risk Professor, and Bath University, we learned many lessons about risk modelling. One lesson learned was that I’m an Arborist. I mainly know about trees. There’s much I didn’t know about risk modelling. Okay, I did know risk matrices are fundamentally flawed and can’t sensibly rank risks. And you can’t apply mathematical rules to ordinal rankings. Though I was yet to find out how much I didn’t know, I was at least smart enough to realise some people knew much more than me.
How uncertain do we need to be?
Another important lesson learned was there’s too much uncertainty in tree risk to claim single figure value accuracy. It’s not credible to measure tree risk to such accuracy as 1/4, 1/300, 1/20 000, or 1/5 000 000. Nor can you realistically measure the difference between a 1/10 000 and a 1/50 000 risk. And you really shouldn’t be compounding the error by adjusting these very accurate risks by double or single figure multipliers like 0.25 or 2, 3, or 4.
With tree risk, we’re looking to measure something with high uncertainty, and our risk ratings should reflect that. With VALID there are only four tree risk ratings. None of them is a single figure value.
Here’s our Drive-by Assessment specification that we've been working on the Tasmanian Government's Department (DSG) of State Growth. This is now embedded into the Government Agency Tree Risk-Benefit Management Strategy template at Appendix A2.
DSG are working on their Traffic Management Plan and I'll encourage them to share that as well. This is really important because the risk of assessing trees on a high-use road is much higher than the risk of branches or trees falling on a high-use road.
It can be adapted to meet your needs. By way of explanation for how this is set up. It’s part of the Government Tree Risk-Benefit Management Strategy. So, you’ll need to be familiar with that to understand the difference between Passive Assessment & Active Assessment. And the three levels of Active Assessment – Basic, Detailed, and Advanced.
Zones of high confluence = High use + large trees
In VALID that’s 1400 or more vehicles a day. The speed limit doesn’t matter. For people, we’re looking at around 1200 per day. This translates to someone passing every minute or so between 7am – 7pm, Mon – Fri. Large trees, for purposes of zoning rather than assessment are trees with a DBH of about 50cm/20in or more.
Drive-bys can be carried out by a Basic Validator or a Validator. This is released under a creative commons licence, and you're welcome to replace these trained assessor titles with whoever would be carrying out a Drive-by Assessment and what their credentials are.
You can keep up with all latest news about tree risk by staying in touch.
We'll send you an occasional short dispatch from the front line every few months or so.
You'll also be first to know when new events are coming up.
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