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[Reply #15 on:]

It remains a dubious claim,  in my opinion.

There are many different ways in which wood "loses mechanical properties".   Abrasion,  rot,  fatigue, splitting, variations in moisture.
To assume this is related to "loss of mass",  or indeed "density",  seems naive.  Wood also shrinks and swells under various conditions,
which, by definition, affects the density.

You refer to wood rotting.  This usually involves the wood becoming wet.  Wood which has become wet,  and is rotting,  will
appear to have a higher density than it had when it was dry.

Then there are hundreds of species of wood to consider.

[Reply #16 on:]

You refer to ultrasound testing of wood.   That's a good approach.   Sound wood will tend
to transmit sound,  unsound wood will tend to absorb it.   You can measure the rate of attenuation
of the ultrasound and draw conclusions from that.

Be that as it may,  there are many ways wood can become unsound,  without "losing mass", as you claim.