"P. van Rijckevorsel" wrote in message
...
The answer is very simple when a tree just starts out, after germination,
all of its cells (100%) are alive. As it grows bigger and bigger, every
year the percentage of living cells drops. It only reaches zero when
the tree is truly and completely dead.
PvR

Thanks, but I wonder if it's really that simple.

If we plot a curve of living cells to total cells I bet we
wouldn't see anything approaching a straight line going
from 100% to 0% over the life of the tree. I'd think we'd
see a couple of years at the beginning when all cells are
alive, then a gradual decline as the tree grows up and out,
then perhaps a levelling off that lasts for years as the
mature tree lives out its life, then a steady and perhaps
rapid decline.

I'm particularly interested in that middle, mature period
when the tree has achieved its growth but is still very
viable.

Alan