UKTC Archive

RE: Risk of windthrow

Subject: RE: Risk of windthrow
From: Dominic Scanlon
Date: Dec 20 2007 11:21:37
Scott wrote:

<<  SC Windthrow is a complex phenomenon with multiple variables.  Very
basically the wind applies a horizontal force which is transferred and
multiplied over the tree height (Mattheck's lever arm) and creates a
"bending," "tipping," or "overtuning moment,"  call it Mo.  The tree is
anchored into the soil by a root system.  The extent of the root system
together with the cohesive strength of the soil create a "resistance
moment," call it Mr.  If Mr>Mo the tree stands.  If Mo>Mr the tree blows
over.

  SC.  Sandy soils may have lower cohesive strength than others, but
sandy soils also drain better.  A soil with high strength when dry may
lose strength when saturated such that Mr declines enough that it
becomes <Mo and the tree overturns.>>

And you have to consider adaptive growth as well - does a tree on sandy
soil grow differently to one on a claey soil - so that it an stand up?

  <<Japanese maples are going to be much shorter than say oaks or other
"shade" trees, exepreince winds of lower velocity and thus present a
lower Mo in a given soil than taller trees.  So Japanese maples provide
a good comparion against trees of similar heights in similar soils, but
not to the wind stability of taller trees.>>

I wasn't considering the maples when thinking about wind throw - the
arboretum has oak woodland on a ridge and it suffered badly in the 87
gale but other than that there was no site specific wind throw problem.
The posting was a cheap excuse to get my anecdote in! The arboretum is
down the road from Merrist wood so I guess a few forum members have been
there.

But that said I cam across a site in Devon recently that had a very
'loose' soil and where there had been repeated wind throw over many
years, leading me to assume that this soil was a 'problem' in terms of
tree loss.  When I say loose this is an attempt to describe very soft
ground conditions and lots of evidence of partial and total wind throw -
quite bizarre.

Dom

  


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