Interesting reading on Tannin content in Wattle
Below some excerpts from a wattle book I found online (wrt to tannin content of SA wattle bark) (I attach book in case you wish to look at some of the other info available therein):
Bark thickness can vary, being thinner on the top of
the tree and branches; it also varies with age and
vigour of the tree. Sherry (1971) quotes the earlier
work of Williams and of Taylor reporting the direct
relationship between thickness and tannin content and
his own correlation coefficients of 0.780 and 0.755
between percentage tannin content and the thickness
of undried and dried bark respectively. The
percentage tannin content of the bark decreases with
increasing height above ground (with a corresponding
increase in non-tannin content), and whereas 27.4%
tannin was found in the bark of the lower branches of
nine-year old trees only 6.7% was found in the twigs
of trees of the same age (Sherry 1971). As would be
expected the degree of taper in the stem of the tree
can influence the thickness of bark at breast height.
Beard (1957) found that ten-year-old trees in Natal
(South Africa) of the same breast height diameter as
those in the south-eastern Transvaal may yield
identical weights of undried bark although the ratio of
bark thickness to tree diameter is greater in the
Transvaal than in Natal. This is due to the greater
degree of taper of the trees in Natal and because they
are 40% taller. These trees also yield 30% more
timber. When the yields of undried bark range from
20 to 21.5 tons/ha the ratio of timber/bark yields is
about 4.1 and this increases when bark yields per
hectare increase (Sherry 1971).
The site of a plantation appears to have an effect on
tannin content. Barks from ‘moist’ sites are usually
thinner and shrink more than the barks from ‘dry’
sites, and as expected from previous comments have
lower tannin contents. The means of tannin content
of trees grown in stand densities of 1500, 1375 and
1250 trees/ha with ages 7, 9, 11 years were
respectively 37.3%, 37.0% and 35.5% for rainfalls of
933, 996 and 1209 mm (Sherry 1971). Low winter
temperatures may produce thicker barks and hence
higher tannin contents. This suggestion has been
made (Sherry 1971) to explain the higher tannin
contents - namely a mean of 45.1% - of bark from a range of sizes of 11-year trees collected in northern
Natal (South Africa) when compared with the tannin
contents - namely a mean of 42.7% - of bark collected
from comparably sized trees grown on similar soils in
the midlands of Natal.