In an earlier contribution to this discussion,
Jules wrote:

Are there tables anywhere for the dry weights of timber of different
species by volume? I realise it's all likely to be somewhat
hand-wavy...

Google is your friend! Typing "specific gravity of wood" into the search box
gives lots of hits, including this one:
http://www.simetric.co.uk/si_wood.htm

Since most wood floats in water, if you assume it has an SG of 1 (1 Tonne
per M^3) you will build in a reasonable safety margin.
--
Cheers,
Roger
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