SC: insertions
----- Original Message -----
From: John Hearne
To: UK Tree Care
Sent: Thursday, December 21, 2006 8:41 AM
Subject: RE: Non invasive cable brace[Scanned]
Ian wrote:
> If the focus of the applicant's justification for losing the tree is
>down to the cost of maintaining the cable then surely the argument
>doesn't stack up against the vastly more significant cost of felling
the
>tree, even if this financial decision was based on a projection of
>cabling maintenance visits throughout the remaining life expectancy of
>the tree.
Not necessarily. And anyway, if the owner's preference is to fell, he
could appeal and could make a very strong case. How confident would you
feel in defending a cable brace against robust questioning as to its
effect on adaptive growth and load transferences?
SC: As noted following where are the data either way? How robust is the
questioning if it is based on conjecture or occassional occurences.
Experience fairly srongly shows - my experience anyway - that cables are
pretty effective at managing risks wothout demonstrably increasing other
risks.
I believe the worst storm damage at Kew Gardens has been to cabled and
braced trees.
SC: might be. But what where are the data? And why. Sorry to be cynical
but this too is like the elastic crowd claiming all sort of "karate cop" or
"paradontos effect" failuers above steel cables. OK. Where are the data? I
have just not seen it in my environment with properly installed cables.
There is good, sound literature that suggests that crotch failures or even
twig failuers are a "safety valve" that prevent whole tree failure. Could
be. There are high profile cases where lots of cables were installed in the
crown and the tree failed at the base. We can argue that by keeping the
crown together, the intercepted load and the base bending monet are
increased, leading to failure. If there are base wealnesses that needs to be
considerd in the first instance. But again that is not a really compelling
argument to avoid all cabling and either remove tres or tolerate risks.
SC: We also have to ask what loading led to the increased damage in cabled
trees. And compar that to the risk that was being mitigated. Did the cabled
leader fall on the structure below or not? Did the cable leave the tree in
place to deliver benefits for x years before the storm rather than removing
the tree in the first instance?
You may provide the necessary strength to a perceived
weakness, but understanding the changes to the distribution of stresses
that you may introduce is a far more complex matter.
SC: The growing science on this (Ken James and others) is a good thing.
But we can overthink it. Gotta go back to experience. On the whole are
cable braces doing their intended job without casuing other failuers?
I suspect that, in many cases, braced trees are more vulnerable to
failure in stormy conditions, and not necessarily at the point you
expected and had attempted to guard against. If there are unanswerable
uncertainties - you simply cannot enforce.
SC: doesn't that cut both ways? Is it an inanswerable uncertainty whether
the tree will live past tomorrow? Next week? Next year? We make certain
assumptions and make management decisons in the face of uncertainty. It is
what we do. Is it really likely the cable brace will preserve the tree from
failure of that weal fork? Is it sort or marginally likely that the cable
brace might increase the failuer risk of other tree parts? Os ot a 100%
certainty that if we cut the tree down it's gone?
On the other hand, TPO powers should allow enforcement of other
essential works such as painting wounds, concreting holes, draining
water pockets, excising the slightest whiff of diseased tissue - and
sticking dead cats in cavities.
SC: The dead cat thing might be effective but if perodic cable inspection
and maintenance are a cost issue at say every two year frequency, it pales
compared to the cat replacement. What with maggot activity, bacterial action
and scavengers the carcass is not likley to last more than a month... maybe
more in Winter.
John Hearne
Tree Officer
New Forest National Park Authority
South Efford House
Milford Road
Everton
Hampshire
SO41 0JD
Telephone 01590 646677
Fax 01590 646601
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