200year old cedar in Dorset..

[Paul Barton]

A statement from the National Trust about this tree: National Trust's South West Blog – Duke of Wellington?s tree at Kingston Lacy

[treeseer]

While not everybody was in agreement, it was felt that the major surgery needed would have spoilt the appearance of the tree and therefore the garden in which it was a feature."

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How 'major' a surgery was proposed? Were those specifications included in the consultants' reports? Specs are often left out of reports seen in the US, diminishing their usefulness by leaving managers to fear the worst.

 

Repeating Barrell's previous work done to his specs from the 1980's may well have sufficed. Why was this approach rejected? Not sophisticated enough for 2014??

 

Crunching numbers--based on blatantly bogus formulas, of data derived from imperfect machines, imperfect software, and especially imperfect operators (myself included)--is just one part of an objective assessment.

 

Objectivity comes from direct observation and analysis of the living tree. The use of machines and mathematics, as in tomography and pull tests, does not guarantee an objective assessment. After all, the decisions on what to test, where, how, why, when, and how to analyze the results--these are subjectively made.

 

Overreliance on machines and math while downplaying the tree's response and adaptations is VOODOO!

 

Fears of 'spoilt appearance' were not founded on specs for proper pruning. BS3998: If, owing to decay or structural weakness, there is a need to prevent failure in a veteran tree, lapsed pollard or lapsed coppice stool, some kind of crown reduction (see 7.7) should normally be adopted as the main solution.

7.7 Crown reduction and reshaping

COMMENTARY ON 7.7

Crown reduction alleviates biomechanical stress by reducing both the

leverage and the sail area of the tree, and can allow retention of a tree in

a confined space. It can also be used to create a desired appearance or to

make the tree more suited to its surroundings. Unlike topping (see 3.28

and Annex C), it retains the main framework of the crown and therefore

a high proportion of the foliage-bearing structure, which is important

for the maintenance of vitality."

 

Attached on retrenchment, fwiw.

RETRENCHING HOLLOW TREES FOR LIFE 131206.pdf

[Paul Melarange]

Fascinating thread and some really good posts.

On the matter of the safety factors Paul M has quoted:

‘Due to the level of uncertainty involved in any numeric approach, a factor of safety of 1.5 is required in the static load test’

A 1.5 factor in structural engineering is tiny given the probable accuracy of data on the material properties. 1.5 is approaching aviation engineering standards where material properties are pretty rigorously measured. FOS of 4-5 would be typical of those used in static structures (e.g. buildings). It does depend on how you arrive at your FOS and there are alternative methods but Id like to know exactly how the 1.5 FOS is arrived at

Paul M’s recommendation for static load testing is very sensible and surely this was a case where empirical evidence should have been applied. But hindsight is a wonderful thing

That said, 'load testing' had just been undertaken by the recent storm. Maybe this had revealed evidence of progressive failure? I don’t know.

 

Pete,

Thanks so much for posting. You are absolutely right and you make some really interesting points. Apologies for the delay in responding. I have discussed your post with my colleague Paul Muir of Treework Environmental Practice (the Uk's leading expert in tree statics) and he has written the following response:

 

Pete Bannister is correct. A safety factor of 1.5 is what engineers would be likely to use when designing an aircraft, and higher safety factors are incorporated during the design of buildings, for example. While there is more information and a higher degree of certainty regarding the material properties applicable to aircraft construction, I would suggest that accepting a safety factor of 1.5 is more to do with cost considerations whilst also requiring the aircraft to be light enough to actually leave the ground. The engineers would prefer higher safety factors, but this would require more expensive materials, a heavier plane and greater running costs.

 

The aviation comparison is actually appropriate. Tree statics developed from a university study in Germany (Lothar Wessolly) that looked at lightweight structures in nature. Remember trees have to balance competitive height growth with available resources (costs), so there is an evolutionary advantage to not over-investing in safety margins. Trees only start to achieve high safety factors during the mature phase (because they can’t stop producing annual increment). Even then lower order branching will have lower safety factors – twigs break on an annual basis.

 

In terms of the static load test a safety factor of 1.5 is a desired minimum. I am comfortable that this is (generally) acceptable. There are a lot of margins for error within each element of the model. Most importantly, I suspect that the design wind load that the tree is required to carry is almost always an over-estimate in our calculations.

 

There is a further important consideration. Trees are structures that change. When considering a measured safety factor an appropriate interpretation will involve considering how much the safety factor is different from what would be expected. A safety factor of close to 1.5 for an early mature tree might be the best one can expect, even if defect free. For a late mature tree with a large stem diameter a safety factor of up to 10 or more, might be expected, so a result of 1.5 would perhaps be cause for concern. All balanced against the vigour of the tree. What potential is there for the situation to improve rather than deteriorate.

 

My experience is that trees with large stem diameters, like the cedar at Kingston Lacey, have very high (theoretical) safety factors (i.e. the safety factor one would expect if they were defect free), and that even though these might be extensively reduced through decay (say from a safety factor of 10, to one of 3 or 4), the residual safety factor is still higher than one might expect for an early mature defect free tree. In other words these old trees are less likely to fail. The implication is that strength loss formulae are fundamentally flawed in terms of being appropriate failure criteria. Which is stronger - a rope with a 10 ton breaking strength with half the fibres cut or a rope with a 1 ton breaking strength?

[Jcarbor]

I got an email off Jeramy Barrell tother day, he had posted a video about the tree being felled. He put forward his case for not felling such a great tree. you can view it on u tube. personally i think it should have been guarded and retained.

[Tom D]

Thats interesting David, perhaps the aesthetic argument would hold water with a less historically significant tree, I am not for preserving all veteran trees at all costs. I would hope that in the future the NT will assess the cultural, historical, and arboricultural significance of the tree whilst deciding on an appropriate course of action.

[PhilBeech]

Their response is very clever and diverts attention cleanly away from the arboricultural consultancy aspect of the decision.

[David Humphries]

I would hope that in the future the NT will assess the cultural, historical, and arboricultural significance of the tree whilst deciding on an appropriate course of action.

 

I hope this is the case too Tom, but would also add 'ecological' aspect to the above list of significances.

 

I've considered my NT membership over this episode, but think it would serve no point in not renewing as the vast majority of visiting members will not be overly affected by the physical loss of this one tree and in the short term my children would suffer through the lack of access to these sites.

 

I doubt penning my individual concerns to the trust would also serve any real purpose (unless of course a nation wide petition arose)

 

Perhaps better to casually engage & talk with members & employees at the various sites around the country as and when opportunity arises.

 

Would be such a shame to lose any more of these mature heritage specimens (like the ones below from Ickworth) earlier than they need to or should be removed.

 

 

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[treeseer]

The images of the Wellington tree showed that it lost its top(s) a while ago, with flat ~20 cm cuts. Wondering how these happened?

 

If that's what the NT imagined would happen again, that makes their decision to fell somewhat more understandable.