SC insertions...
----- Original Message -----
From: Mark Hinsley
To: UK Tree Care
Sent: Tuesday, January 22, 2008 8:23 AM
Subject: RE: BS:5837 20% offset question
Hi Ian
The notion that you cannot offset the RPA would suggest you are treating the
RPA as the extent of the root system, rather than a rule of thumb for the
amount of root loss that a tree will tolerate without detriment. Once you
take on board the fact that the roots extend well beyond the RPA then off
setting be 20% simply alters the location of the retained roots - it does
not impact upon the actual amount of root system retained.
SC Mark seems to be spot on. Again, I have not looked closely at BS 5837
in a while. But the US protocols, principally Matheny & Clark (based loosely
on the previous BS 5837) applied properly should wind up with a Critical Root
Zone (CRZ), Tree Protection Zone (TPZ) or other analog (that might be roughly
analogous to RPA) that is quite conservative, that gives an opitimum chnace
of tree survival w/o major impact. But even with that conservatism, the
typical root spread of a tree extends well beyond a CRZ or TPZ, well beyond
typical crown drip lines (some sources suggest 1-2-3 x the drip line). So A)
as Mark suggests there is lots of room to shift the zone or area around and
still keep sufficient roots for optimum survival, and B) lattitude to recue
the area understanding performance may be less than optimal.
SC Following on from the mention of "drip line," some protocols still use
that (or some fraction or multiple) as the standard for root protection. The
concept of "open grown" is found in the US literature in that context. An
open grown tree of X trunk diameter is likely to have a drip line or crown
spread of Y. But a tree with the same Xd but grwon in a crowded group is
likely to hace a crown spread of <Y, maybe 0.5Y or 0.25Y. But much the same
root spraed, so for a tree that is not "open grown," drip line is a poor
indicator of CRZ-TPZ-RPA.
SC
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