annual report 2011 - Forestry Tasmania

Old trees: the elderly at the heart of the forest community
Dr Simon Grove
Simon.Grove@forestrytas.com.au
Ten years ago, Forestry Tasmania initiated a
comprehensive research program on the ecological
importance and management of rotting logs, or
coarse woody debris (CWD). Research on the internal
composition of rotting logs has now put in place one of
the final pieces of this program, and demonstrates that
it’s not only in human societies that the elderly are the
heart of the community.
The various log decay classes are used to interpret
forest stand dynamics and wildfire history, and to
understand forest biodiversity values and carbon
cycling capacity. Surveys or inventories of CWD
attempt to categorise logs into discrete decay-classes
based on readily observable features, but this assumes
that the external appearance of a log is a good
predictor of its internal state of decomposition. This
study tests that assumption.
A total of 64 Eucalyptus obliqua logs were selected from
locations in the Warra Long-Term Ecological Research
site, representing the full range of decay classes,
from Class 1 (freshly fallen) to Class 5 (becoming
incorporated into the soil humus layer). We assumed
that a log’s diameter was a reasonable surrogate for
its age, that is, that logs less than 60 cm diameter
derived from regrowth-age trees and logs greater
than 60 cm diameter derived from mature trees, and
extracted three thin slices (‘biscuits’) from each log.
Twenty different rotten-wood types were identified
from the appearance of the wood (texture, colour,
feel and smell). Decay by brown-rotting fungi (which
preferentially attack cellulose, leaving lignin) was
distinguished from decay by white-rotting fungi
(which attack both cellulose and lignin), and the
proportion of each biscuit occupied by each rottenwood
type or airspace and the extent of biomass loss
were measured to determine how well the decay-class
system reflects a log’s internal state of decomposition.
There was a strong relationship between a log’s
externally determined decay-class and its loss of mass.
This accords with earlier studies that showed that a
log’s decay-class gives a good indication of how long
it has been decomposing. Since a log’s relative mass
is related to its carbon content, this gives information
on the contribution that rotting logs make to carbon
cycling.
However, a log’s external appearance can mask
considerable internal variation in the proportions of
individual rotten-wood types. Some of this variation
is related to a log’s position on the continuum of
decomposition from freshly dead wood to wood
humus. But in addition we found that logs derived
from regrowth-age trees decomposed differently
from those derived from mature-age trees. Smaller
logs presumed to be from regrowth-age trees were
more likely to have decayed from the sapwood
inwards towards the heartwood, and to have
decayed predominantly by white-rotting fungi.
Larger logs derived from mature-age trees were
more likely to have decayed from the heartwood
outwards towards the sapwood, and to have
decayed predominantly by brown-rotting fungi.
These apparently simple differences between
logs of different sizes, and this of potentially
different origins, have important ecological and
management implications. They also help account
for the findings of other studies at Warra that
highlighted the biodiversity differences between
regrowth-age and mature-age trees and the logs
arising from them. Mature-age trees harbour more
species, and more unique species, than regrowthage
trees, and the same applies to the logs derived
from these trees. Mature-aged trees are in many
ways the heart of the forest community, and
forests lacking them will lack a large amount of
their potential biodiversity.
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