Conserving Biodiversity in Brigalow Regrowth - School of ...

Managing dense Brigalow regrowth:
To thin or not to thin?
Brigalow regrowth is often very dense (sometimes > 15,000 stems per hectare) because of the plentiful root suckers
produced following clearing. Very high densities are particularly common after multiple pulling attempts on deep
clay soils with gilgais.
Stems compete intensely with one
another and grow slowly, so slowly
that dense stands are often described
as “stunted” or “locked up”. Given
enough time, dense brigalow regrowth
will almost certainly grow into
woodlands akin to virgin stands, but is
it possible to speed up the process by
thinning manually? The answer is yes.
An experimental thinning trial in dense
27 year old regrowth near Millmerran
showed that thinning to 4000 – 6000
stems per hectare produces the best
results for woodland recovery and
wildlife habitat. More severe thinning
will open up the canopy and allow
grasses to establish. While this might
seem a positive result for grazing
purposes, it is not so good for wildlife.
High grass cover, particularly buffel
grass, increases the risk of fire (see
pgs 10 & 19), which will slow down the
restoration process by killing trees or
promoting even thicker regrowth.
If the goal is to manage brigalow
regrowth for wildlife habitat, then
canopy cover should be maintained at
more than 50% (i.e. on average more
than half of the ground is shaded by
tree canopy) in southern Queensland
and northern New South Wales. In
central and northern brigalow regions
we recommend maintaining tree cover
at 70% because buffel grass grows
more vigorously in these regions.
In younger regrowth (e.g. 10–15 years
old) it will be necessary to retain more
stems in order to maintain adequate
tree cover. In these cases it may be
best to thin in stages, for example thin
down to 10,000 stems per ha initially,
then a few years later thin down to
6,000 stems per ha.
If providing habitat for native fauna
is not the primary goal of managing
regrowth, then the manager might
want to encourage the growth of
introduced pasture grasses and
manage fire risk by grazing. It is
16
important to know that sites that are
managed in this way will not support
the same richness of fauna that they
would if managed in the way we
describe, as wildlife habitat. However,
from a wildlife perspective, some trees
are better than no trees at all, and so
the more common fauna will even use
regrowth areas which are managed for
grazing and stock shelter.
Thinning methods
Obviously ringbarking takes a lot of
time and effort, and there may be more
efficient methods available. Regardless
of the method used, secondary
suckering will occur less if thinning is
done after good rain. When thinning, it
is important to retain a range of stem
sizes in a “natural” arrangement.
For example, a completely random
selection of stems yields very natural
arrangements.
Thinning for carbon
Trees sequester carbon as they grow,
but some species sequester more
carbon than others. Brigalow is a
high biomass species, meaning that
it stores a lot of carbon compared to
other species from a similar climate.
Brigalow regrowth therefore has
considerable potential to be used
as carbon sinks. Although thinning
releases some carbon (through the
decay of treated stems), thinned
stands will recoup this carbon quickly
and reach their carbon capacity sooner
than unthinned stands. Although
slower, unthinned stands are still likely
to sequester carbon reasonably
quickly.