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chronological age. Harvest will be timed according to the yield per length of row so as to manage the
cost of harvesting.
As mallees grow the proportions of their biomass in the form of wood, leaf, twig and bark varies
(Peck et al, 2011). The biomass composition varies according to many factors other than mallee size,
perhaps the most significant being the availability of soil moisture; with better access to moisture,
mallees can sustain higher leaf areas. Mallees growing in dense blocks are typically observed to have
small crowns at the top of the plant, whereas mallees in narrow belts with greater access to usable
soil moisture will carry heavier crowns, sometimes extending almost to the ground.
However across all species observed in the work of Peck et al (2011), the trends are that with
increasing age or size, the proportion of wood increases, the proportions of leaf and twig both
decline, and bark varies relatively little. There are differences between species and within species
there are also differences between saplings, or previously unharvested mallees, and regenerating
coppice.
In saplings:
• Wood varies between 10% and 30 % of biomass for 10 kg mallees to about 45% for mallees
weighing 100 kg. The range observed in the smaller mallees is partly according to species.
• Twig and leaf vary together and are very similar proportions of total biomass. In small mallees
both fractions represent 30% to 40% of biomass, falling to 20% to 25% for large mallees.
• Bark typically comprises between 5% to 10% of biomass for all mallee sizes, though in E. kochii
ssp plenissima, the proportion of bark rises to 15% in large mallees at the expense of leaf and
twig proportions.
In regenerating coppice:
• Wood varies between about 15% in all species for small coppice to 25% to 40% in 100kg
coppice, with different species having different proportions in these larger mallees.
• Leaf and twig again vary together, being about 40% each in small coppice, down to about 30% in
large coppice.
• Bark is again a small proportion, being about 5% of biomass and a little higher in large E. kochii
ssp plenissima.
The market requirements for biomass composition, the efficiency of harvesting, and the economics of
biomass production all influence the composition of the biomass. At this stage, before market
development has occurred, we can only define trends and directions of influence.
Wood is likely to have the highest value as a fuel because it has low ash content (Peck et al 2011; Wu
et al, 2011). It is also the biomass component most likely to have other market potential, such as
charcoal production. It will be easier to separate wood chip from the rest of the biomass than it will
be to subdivide the rest of the biomass into leaf, twig and bark.
Leaf is likely to have next highest value as it contains the eucalyptus oil (2-3% of total fresh weight
of leaf) and after distillation, the bulk of the material may represent a useful relatively low grade fuel
source. This residue may contain undesirable levels of alkali metals and alkaline earth metals, and
other problematic elements such as chlorine. Recent work has recorded reduced ash content after
hydrodistillation (Wu et al, 2011) but steam distillation may produce different results.
Twig will have value as a relatively low grade fuel source but often with less ash than the leaf and
bark (Peck et al, 2011).
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