Tree Managers : Public Discussion

Mike Ellison Wrote:
-------------------------------------------------------
> Mark
>
> My reply is interwoven with your posting.
>
> Regards
>
> Mike Ellison
>
> --------------------------------------------------
> ----
> > Whilst the system produces statistical data, I
> > have yet to see research data to support its
> > accuracy. Based on Australian figures, the risk
> of
> > death or serious injury is extreemely low (less
> > than 1 in a 1,000,000 of population). Given
> that
> > there are more than 10 trees for every person
> the
> > odds of a tree causing problems is less than 1
> in
> > 10,000,000. How then do we account for the
> > problems we do have? Clearly these are the
> result
> > of trees that have a lot higher risk of failure
> > that are high trafic areas!
>
> ME: The data are published as refered to below.
> Without going into the figures that you have
> quoted, what you are discussing is risk of death
> or serious harm (RoH) from tree failure in the
> population as a whole, which is likely to be very
> low (in the region of 1/5,000,000 in the UK based
> on historical data). When we look at individual
> land ownership, one land owner may have a RoH far
> lower than 1/5,000,000 and anther land owner will
> have a RoH that is much higher. Whilst
> 1/5,000,000 may be the mean, there are huge peaks
> and troughs, which accounts for the fact that
> some tree owners have problems with high RoH
>
> > Given that it is these trees that are the
> subject
> > of risk it would seem that the data on these
> would
> > need to be extreemely accurate. The concern I
> have
> > is that the assesment is subjective and not
> > quantataive at this point. Yes the tree does
> have
> > girdling roots but can I say to what extent
> that
> > this has impacted on the stability of the tree
> and
> > the thigmomorphogenic responses. As a result we
> > generally err on the side of caution and assume
> > the risk is higher than it is. (Hence the
> > Wessolley Mattheck debate)
>
> ME: The starting point is the question "are there
> any significant targets?" because if there are
> not, there cannot be a significant risk of
> significant harm. Targets can be accurately
> quantified by frequency of occupation or by repair
> or replacement value. I believe that you are
> correct in suggesting that as aresult of the
> difficulty in interpreting the mechanical effects
> of decay and defects, we err on the side of
> caution. If your assessment of probability of
> failure (PoF) is cautious, this will be reflected
> in the qtra RoH outcome. However, qtra has
> substantially improved my assessment of PoF and
> therefore improved the outcome of the qtra
> assessments. And yes we can quantify and improve
> the assumptions that we are already basing our
> decisions upon.
> >
>
> > Clearly any program that considers the size of
> the
> > falling parts, the probable frequency and the
> > target must pick out the more hazardous trees
> (eg
> > the ISA hazard assesment program) but how is
> this
> > subjectivity converted into statistical
> accuracy
> > without reseach data?
>
> We use our best estimates and as we use the qtra
> procedures and observe trees that have been the
> subject of our assesments, our assessments become
> more accurate. Research data could improve our
> understanding of tree failures as a proportion of
> the opulation and this is something that I am
> hoping to assist with by encouraging cooperation
> with the ITFD and through other avenues. Have you
> had a look at the articles under the 'Resources
> for Tree Managers' section of the qtra website?
>
>
> > If any arborist can tell me the liklihood of a
> > 100mm dead branch falling I would like to know
> how
> > they do so.
>
> Based on species, size and condition, it is not
> unreasonable to expect s skilled arborist to be
> able to estimate how likely it is that the dead
> branch will fail. Currently, this skill requires
> improvement and use of qtra can only help advance
> our observational techniques.
>
>
> Instead we assume that it can fall at
> > any time and it will most certainly fail so we
> > might as well be smart and remove or at least
> > reduce it if we want to keep the habitat.
>
> This approach would fail to take due consideration
> of targets so I assume that this would be in the
> context of the target. In terms of safety, the
> majority of dead branches are irrelvant because
> either the target value is so low or the usage of
> the site is substantially reduced during the
> weather conditionsthat are most likely to result
> in failure of the branch. Judging by the
> substantially degraded state of most fallen dead
> branches, it is fairly safe to assume that they
> degrade over a period of years and that the
> failure of any particular branch in the coming
> year is by no means certain.
>
>
> On the
> > other hand what are the actual statistical
> figures
> > on death or serious injury due to the failure of
> a
> > dead branch of this size. Again here in
> Austarlia
> > the odds are very very small.
>
> To the best of my knowledge, there are none.
>
>
> > If 1 in 10,000 is an acceptable risk is there
> data
> > for 10,000 trees at that risk level to prove or
> > falcify the statistical assertion. Throwing the
> > average tree with a risk factor of less than 1
> in
> > 10,000,000 into the data soop is surely poor
> > ststistics
>
> You may have misunderstood our use of the 1/10,000
> risk of significant harm as a threshhold of
> acceptable risk. Other sources suggest that
> people generally would not spend their own
> resources to reduce an annual risk of death that
> was already as low as 1/10,000 and that a 1/10,000
> risk of death is broadly acceptable where the risk
> is imposed on the public in the wider interest. I
> am uncear as to how data for 10.000 trees would
> prove or falsify this advised limit (which is not
> a not statistical assertion). Where does your
> second point relate to qtra?
> >
> > In short what is the error factor and does the
> > data consider the mean, mode, maximum, minimum
> or
> > average liklihood of any event. What is the
> > statistical difference between the minimum and
> > maximum (the error factor) and how have these
> > figures been proved? I certainly would hate to
> > find myself in a court case because I acted too
> > quickly or because I did not act quickly
> enough.
>
> The issue of error is currently being considered,
> but we do not operate to an absolute prescriptive
> threshold and any outcome that contradicts
> intuition can be reassessed and refined if
> necessary. The system calculates the likelihood
> that a significant loss will occur as a result of
> tree failure. The data used are set out in the
> articles mentioned above and there have been
> various modifications to the process since their
> publication.