We had a meeting this week to determine how we will introduce changes to
our tree management / survey database to conform to BS5837:2012.
Has anyone else noticed that the two formula proposed for calculating
the combined stem diameter of multi-stemmed trees (sections 4.6.1a and
4.6.1b) are not mathematically equivalent? The formula for trees with
more than 5 stems generates a smaller combined diameter (where stems are
not equal in diameter). While I am not too concerned about the
implications for the actual areas generated in terms of protection of
the root zone, I wonder if this was intentional, and if so what the
logic was - other than for sensible data collection.
As an example, if one actually measured the diameters of each stem of a
hazel stool with 10 stems, then inputted these into the 2-5 stems
formula this does not generate the same combined diameter as if one
entered the average stem diameter of these measured stems into the >5
stems formula. As the 2-5 stem formula is based on the formula for
calculating the equivalent area of a "virtual" single stemmed tree with
the intention of representing the multi-stemmed tree, I wonder what the
5 stems formula is based on.
Depending on the diameters used the differences are significant and are
of course exaggerated when calculating a corresponding RPA.
We only noticed this when we explored the potential for allowing a
short-cut when surveying, involving recording one average stem diameter
for trees with 2-5 stems, where the trees were insignificant or perhaps
where the stems are of very similar (and small) magnitudes.
Not a very interesting issue, I know, but...
Paul Muir
Arboricultural Consultant / Contracts Administrator
Treework Environmental Practice, The Old Rectory, Pilgrims Way, Chew
Stoke, BS40 8TT
M: 07966 647906
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