Dear deadwooders

We are trying to get a better understanding of the decay dynamics of Eucalyptus
obliqua CWD in our Tasmanian cool wet sclerophyll forests, so we can get our
(very) long-term management of these forests in order. These forests have some
of the highest volumes of CWD in the world (typically 600-1200 m3/ha). We know
logs take a long time to decay here, but we don't know how long and the
historical record in these forests doesn't go back far enough to guide us. A
further complication is that the disturbance dynamics of these forests are
complex, consisting of intermittent wildfires, occasionally stand-replacing, and
probably a steady trickle of treefalls in intervening years/decades.

We have been trying the chronosequence approach, and a doctoral student working
with us recently sampled CWD from clearfelled coupes because we could be sure of
the year in which the log hit the ground. However, clearfelling only started in
the 1960's, and when we plot density loss against time it's as easy to fit a
horizontal straight line (i.e. no density loss over the past forty years) as it
is to fit an exponential decay curve. If we do fit a decay curve, we get a
t0.95 of 319-676 years (depending on which logs we exclude). This doesn't fit
with our expectations - we anticipated something closer to 200 years - but it's
the only 'hard' scientific data we now have, so unless we find a better way this
is what we'll have to use for our modelling. It's possible it's because
clearfelled, non-harvested logs are exposed to a hot burn after felling, which
may sterilise the log and slow down initial decay. It may also be because
clearfelled logs are derived from living trees, whereas natural logs may mostly
come from the eventual collapse of standing dead trees, which would already be
partly decayed when the hit the ground. But then again, logs in natural forest
exposed to wildfires might be expected to be periodically burnt too, though
probably not to the same extent as in a clearfell, while a proportion of natural
logs should also emanate from living trees rather than standing dead trees.

I'm wondering if anyone can recommend any other techniques that might help us?
We have two main problems: (1) not being able to date the year of death of logs
older than the 1960's'; and (2) not being sure whether the density
loss/chronosequence method will ever work, even if we overcome problem 1.
Regarding the first, are there any techniques to date the year of death of logs,
for instance using chemical ratios or even isotopes? We have considered looking
at the structure and composition of surrounding vegetation instead to give an
indication of time since last wildfire, but this is very approximate and doesn't
allow for the log having been there since before the last wildfire. Regarding
the second problem, are there any workable alternatives to the density
loss/chronosequence approach? I have read a bit about using respiration as a
measure of decay rates (e.g. using slaked lime), but have been put off by the
fact that rates measured over short periods (hours or days) are often considered
unrealistically rapid. There is also the issue of how to bag up some of these
logs, which can be huge.

Anyway, if anyone has any suggestions, I'd be keen to hear them.

Thanks for your time

Simon Grove





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Dr Simon J Grove, Conservation Biologist,
Biology and Conservation Branch,
Division of Forest Research and Development
Forestry Tasmania, GPO Box 207, Hobart, Tasmania 7001, Australia.
Tel. 61 3 6233 8141. Fax 61 3 6233 8292.
Email: simon.grove@....
Web addresses: http://www.forestrytas.com.au/
http://www.warra.com


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