Dear Rod,
In my experience, most Gums cant be killed even if you're
trying! Personally, I would decide on a 'safe height', take a metre off of
that, and get out the chainsaw! No need to be elegant.
Gums have a reserve of nutrients stored in their roots that they use to
start again after fire or other major catastrophe, such as being split in
two by lightening, or being torn in two by storm.
We cut a beautiful gum down quite far at home, (it was just a stump) and
shoots came out of that. You end up with a short bush, rather than a tall
gum. I am not sure what happens after that, as we moved houses.
Don't take my word as gospel, but I do believe in the regenerative qualities
of gum trees.
:0)
Jade.

http://www.botany.hawaii.edu/bot350/1996/Kini/k7.htm
LIGNOTUBERS

An unusual feature of a major group of eucalypts known as the mallees is
the presence of lignotubers, a mass of subterranean vegetative buds
associated with substantial food reserves. These tubers develop during the
first year of growth as two globular swellings in the axils of the
cotyledons. If the top of a seedling which has developed a lignotuber is
destroyed, growth can resume almost immediately. Lignotubers can produce new
growth during the life of a tree at intervals for over a century. As a
result of these lignotubers, the eucalypts are difficult to eradicate as
they are somewhat resistant to herbicides.

About 50 eucalypts species, known as marlocks, branch into multiple trunks
don't have lignotubers. (Gardiner, 1966). Species that lack lignotubers
produce seeds prolifically as an alternate survival strategy (Pyor,1976).

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