Here are some pictures and a tree story shared with me by Mark Hartley, the arborist who has helped Newcastle to try to get the message out that the Laman Street trees are stable. (more…)
Posts Tagged ‘Mark Hartley’
It occurred to me recently that the chance of many people making the time to read an arborist’s report is remote. So I thought I’d take a tried and true approach and give the best bits to you in instalments. This instalment is about casebooks. (more…)
Mark Hartley, the arborist commissioned by the community to do a peer review of the reports on risk and tree health in Laman Street, won their highest award, the Award of Merit. (more…)
I think it’s an annual event, but living nearby I can tell you it feels a lot more frequent than that. What did I read just today? The days drag but the years fly by. Anyway, it was a great spot for a few of us to man a possie/pozzie (how does one spell that?) just outside the pub to ask people to sign our petition. We were up to 6500 in total a week ago so it should be almost 7000 by now. I wonder what the record is for a Newcastle issue? And still be unsuccessful?
The wind was a bit of a nuisance and it rained every now and again but that’s probably not a bad thing – it must have sent some punters home early instead of encouraging then to keep drinking. Which reminds me that one supporter made the comment that binge drinking leading to violence is a much bigger risk in Newcastle than Our Fig Trees. It’s been a fantastic thing for everyone (except a number of hotel owners) that hotels lock people in and out after 1am now. Violence has dropped impressively. What a shame this hasn’t been an impressive enough trial to convince our state Premier to encourage this elsewhere.
Talk of risk reminded me of a conversation between a local arborist and Mark Hartley (the experienced arborist who did a peer review for us of the council’s arborist reports). An expert with local knowledge gave Mr Hartley some information regarding the frequency of fig tree failures over a decade or so. These figures make a difference to the ‘probability of failure’ used in quantified tree risk assessment (QTRA).
‘… on page 9 [of your peer review], you conservatively estimated the risk of fig tree failure at 1:500 and I think that is a little out. With the benefit of local knowledge I can state that I have personally witnessed at least 4 entire tree failures in Newcastle over the last 10 years (only Hill’s figs) including the Pasha Bulker storm event, and 2 catastrophic failures of the majority of the canopy that would have been included in the same class because of the size of the part, although the nature of the failure cause was different. There are 1200 Figs left in Newcastle of a mature size.
So 6 large or complete failures over a 10 year period out of approximately 1200 trees, what would that make the probability of failure? …Would that increase or decrease the outcome of the probability?’
…Based on your data in 12,000 tree years (10 x 1,200) there have been 6 failures which means a rate of failure has been 1 in 2000. I have estimated the Probability of Failure to be 4 times higher than those records indicate. (…there is some reason to be [conservative]
This means that the trees outside the gallery would need to be 20 times more likely to fail than the average fig tree in council’s care. It also means that the least conservative estimate for the probability of failure (PoF) provided by the council’s experts is 20,000% out….
In addition there is some evidence to suggest that root severance contributed to at least two of the failures. If these are eliminated then the failure rate due to defects alone is 1 in 3,000. Furthermore, the trees outside the gallery have all been assessed as being free from the significant included stem structure that resulted in the two structural failures … whilst limb failure is possible, loss of large portions of the tree is very unlikely.
If these two structural failures are eliminated then the rate of failure has been 1 in 6,000 This is close to the Probability of Failure of 1 in 5,000 that I said I believe can be achieved with light reduction and thinning.
This means that the Laman street trees would need to be 600 times more likely to fail than the average tree without cut roots and significant structural defects in the trunks in order to achieve the Probability of Failure adopted by [one of council’s experts] and 800 times more likely when considering [another].