Download Savannas and the carbon storage story

The tropical savannas cover
around 20% of the earth’s land
surface and while they have
fewer trees and stored carbon
than rainforests or temperate
forests, their extent makes them
significant in the global carbon
cycle. So how does carbon
cycling work in the tropical
savannas and where is the
carbon stored? How much is
emitted and absorbed and
are these processes changing?
Such questions were the focus
of several Tropical Savannas
CRC projects. By Peter Jacklyn.
Photo: Jason Beringer
climate
Lindsay Hutley checks on a 23-metre tower threatened by fire at the Howard Springs study site. The
five-year study measured heat, moisture and CO2 flux and was able to show that despite regular fires,
the surrounding woodlands absorbed around two tonnes of carbon per hectare per year.
Savannas and the carbon storage story
A
ustralia’s tropical savannas account for around
a third of Australia’s land-based carbon stores.
However, savannas are regularly swept by
bushfires that release many tonnes of carbon
dioxide (CO2) into the atmosphere.
To find out how much carbon is stored in Australian tropical
savanna ecosystems, researchers Lindsay Hutley (CDU), Dick
Williams (CSIRO) and PhD student Chen Xiaoyong (CDU)
dug up roots and chopped down trees in plots around Darwin
and Katherine in the Northern Territory and then weighed and
analysed the woody material to estimate the carbon stored.
They found these savanna woodlands store, on average,
around 25–35 tonnes of carbon above and around 20 tonnes
of carbon in roots beneath each hectare. Around two to three
times as much carbon per hectare is stored as organic material in the soil. Using airborne radar data they estimated the
equivalent figures for a broader woodland landscape in the
Wildman River region of the NT which were around 70–80
tonnes a hectare for the carbon in trees and roots. These figures
are less than for densely forested landscapes which can store
more than 150 tonnes of carbon a hectare but much greater
than for grasslands.
animals and bushfires. It is thought that this process is effectively ‘thickening up’ the woodlands of Howard Springs
every year, particularly through increasing the number of
shrubs. It is likely that this thickening is a response to the
changes in the carbon cycle itself—to the higher levels of
CO2 in the atmosphere and possibly increasing rainfall over
north Australia.
This thickening may be somewhat different to the thickening reported from the drier savannas in places like the Victoria
River District and the Queensland Gulf of Carpentaria where
the increase in woody shrubs may not only be responding to
increased CO2 but also to changes in grazing, fire patterns
and climate. Also, while the Howard Springs site has regular
fires, these are usually not the hot, late dry-season wildfires
that plague some parts of the savannas like central Arnhem
Land (see diagram facing page). Such fires consume a substantial amount of tree material and are likely to be major
sources of CO2.
Nevertheless, broader analyses estimate that the wetter
savannas as a whole (the Kimberley, the NT Top End, Queensland’s Cape York Peninsula and northern Gulf) are probably
absorbing around a tonne of carbon per hectare every year on
average. Although this rate of sequestration is low compared
to forest plantations, it is greater than estimates for African
savanna woodlands and other savannas around the world (e.g.
see Elmar et al, 2004).
This is probably because our eucalypts are so well-adapted
to the wet-dry tropics, rainfall is generally high in this region
(greater than 1200 mm annually) and we have more trees
and shrubs that are evergreen, absorbing CO2 all year round.
Given the vast area of these landscapes this is a significant
‘sink’ for CO2.
This sink will presumably only last as long as there are
enough nutrients in the soil and water available to sustain a
yearly increase in savanna trees and shrubs—sooner or later
a limit will be reached and savanna ecosystems will no longer
How much carbon is emitted and absorbed
each year from savannas?
A study site in un-grazed savanna woodland near Howard
Springs, Darwin was used to measure heat, moisture and
CO2 flux over a five-year period from 2001–2006. The CO2
flux was measured by an instrument atop a 23-metre tower
above the woodland.
The study was able to show that despite regular fire the
Howard Springs woodland ecosystem actually absorbs around
two tonnes of carbon per hectare each year. In other words,
every year two tonnes more carbon is incorporated into the
wood, grass and soils in each hectare of the Howard Springs
woodland than is emitted to the atmosphere by the plants,
2007
Why the carbon cycle is important
MANY elements cycle from the atmosphere to living things, the oceans or the
soil and then back to the atmosphere,
but the cycling of carbon atoms is particularly important. One reason is that
carbon dioxide (CO2) plays a key role in
trapping heat in the atmosphere—one
of the basic mechanisms be­hind the
greenhouse effect, which raises temperatures near the earth’s surface.
Another factor that makes the cycling of
carbon important is that carbon plays
a central role in combustion—burning—and in the last 200 years we have
dramatically changed the carbon cycle
through burning fossil fuels, which has
released large volumes of CO2 into the
atmosphere. Also, substantial areas of
forest have been cut down, removing
a pathway for CO 2 absorption (see
diagram above). Consequently, recent
times have seen the amount of CO2
in the air increase and the amount of
oxygen decrease.
Because there is so much
oxygen in the air, the oxygen
drop is hardly noticeable,
but as there is very little CO2
(0.038% of the atmosphere)
the extra CO 2 from burning and deforestation has
caused a dramatic rise as
shown in the graph at right.
This has contributed to an
enhanced greenhouse effect
in recent decades. Given
the potentially serious consequences for the earth’s
climate of this enhanced
greenhouse effect, great
importance is now placed on
ways of reducing CO2 emissions and on reducing the
CO2 already in the air.
Atmospheric CO2 concentrations over the last 20,000 years from various sources including ice
cores. The grey bars show the reconstructed ranges of natural variability for the past 650,000
years. Note the dramatic increase in recent times. Modified from IPCC AR4 2007.
The global carbon cycle where black numbers indicate the annual flows of carbon in pre-industrial times and red
numbers indicate recent human-caused annual flows. The flows and stores are in billions of tonnes or Gigatonnes
of carbon GtC. Source: IPCC AR4 2007
be able to absorb extra carbon every year. Climate change and the
spread of gamba grass may also result in more intense fires and
the current trend of sequestration may shift to CO2 emission.
emissions from cars and absorption resulting from the cessation of land clearing are counted.
Savanna fires are considered to be anthropogenic as they
are mostly lit by people for one reason or another, but the
consequent emission of CO2 by fire and re-absorption by
savanna plants is not counted because these processes are
considered to cancel each other out. The new findings outlined here have not yet been taken into account. The official
figures of emission from savanna fires are therefore not CO2,
but those from methane and nitrous oxide. Like CO2 these
The official figures?
How do these findings square with official greenhouse accounts for the Northern Territory which show that savannas
are major emitters of greenhouse gases through fires? These
accounts look at human-caused—anthropogenic—emission
and absorption of greenhouse gases only. So for example,
Cont. pg 3
Savanna Links <savanna.cdu.edu.au>
Savannas and the carbon storage story
Cont. from pg. 2
gases also trap heat and are greenhouse
gases—although their overall contribution to the greenhouse effect is less than
that of CO2. Unlike CO2, these gases
cannot be re-absorbed by plants and so
are included in the current greenhouse
emission accounts.
What is striking is that even counting methane and nitrous oxide, savanna
fires still accounted for 47% of NT
greenhouse emissions in 2000. This
underscores how, despite the savannas
being an overall CO2 sink, places like
Arnhem Land with frequent wildfires
emit huge amounts of greenhouse gases.
Applications
There are two broad practical applications of this research:
Abating emissions from savannas
It should be possible to abate or offset
substantial greenhouse gas emissions
by reducing the incidence of intense,
tree-consuming wildfires. This is now
happening in west Arnhem Land where
several Indigenous communities are
reducing the incidence of late dry-season wildfires, consequently reducing
greenhouse gas emissions (see Eureka
Prize for West Arnhem Land Fire Project,
page. 2).
These communities are receiving
payment for this work to the tune of $1
million a year for 17 years from Darwin
Liquid Natural Gas, a large energy consortium which can then offset the emissions abated against its own greenhouse
gas emissions. Although substantial
quantities of CO2 are being abated in
this work, as outlined above, they cannot currently be counted in the official
figures. Even so, methane and nitrous
gas emissions, which can be counted,
and are equivalent to more than 250,000
tonnes of CO 2, were abated by the
project in its first two years.
Importantly, agreements like this
provide enormous benefits apart from
greenhouse gas reduction. These include long-term employment and linked
social and economic benefits and increased protection for biodiversity and
cultural values. Such strategies may
also be possible in other fire-prone high
biodiversity areas like the Kimberley.
Carbon sequestration
Some savanna landscapes should be
able to be managed to enhance storage
of carbon in trees, shrubs and soils, for
The aftermath of late dry-season wildfire in west-central Arnhem Land in 2004. Wildfires in such a
landscape are a major CO2 source. Picture courtesy of Andrew Edwards ( Bushfires NT)
example by reducing the incidence of
fire. Unlike abatement schemes, land
managers could be paid for additional
carbon stored rather than emissions
abated. If carbon storage in woody
vegetation can be accurately assessed
or counted, areas on cattle stations could
be excised and used for ‘carbon farms’.
In the absence of fire, growth of
tropical eucalypt trees would absorb
CO2 from the atmosphere relatively
rapidly. Pastoral enterprises could thus
be managed for grazing and carbon
sequestration. A key activity would be
fire management—a major fire could
consume the tree crop and emit stored
CO2 to the atmosphere. However, losses
may not be large as tropical savanna
trees are not highly flammable, can recover from fire events and litter would
be the dominant fuel consumed.
References
IPCC Fourth Assessment Report
<ipcc-wg1.ucar.edu/wg1/wg1-report.html>
Elmar M., Veenendaal, Olaf Kolle, Jon
Lloyd 2004, ‘Seasonal variation in energy
fluxes and CO2 exchange for a broad-leaved
semi-arid savanna (Mopane woodland) in
Southern Africa’, Global Change Biology, 10
(3), 318–328.
Williams R.J, Hutley L.B., Cook G.D., Russell-Smith J., Edwards A., Chen X. 2004,
‘Assessing the carbon sequestration potential
of mesic savannas in the Northern Territory,
Australia: approaches, uncertainties and potential impacts of fire’, Functional Plant Biology,
31: 415–422
Contacts and more information
Dr Lindsay Hutley, Charles Darwin University,
<[email protected]>
Intergovernmental Panel on Climate Change
website <www.ipcc.ch/>
TS-CRC carbon dynamics project
<www.savanna.cdu.edu.au/research/projects/
carbon_dynamics.html>
Monash University Savanna Fire Experiment
<arts.monash.edu.au/ges/research/climate/
fire/index.php>
Climate change report
CSIRO has released a report on the
latest information on climate change in
Australia and its likely causes. Climate
Change in Australia updates projections of changes in temperature, rainfall and other aspects of climate that
can be expected over coming decades
as a result of continued global emissions of greenhouse gases.
Developed by CSIRO and the Bureau
of Meteorology, in partnership with the
Australian Greenhouse Office, the report also states that droughts are likely
to become more frequent, particularly
in the south-west; evaporation rates
are likely to increase, particularly in
the north and east; high fire danger
weather to increase in the south-east;
and sea levels will continue to rise.
Download the full report:
<www.csiro.au/resources/ps3j6.html>
<www.climatechangeinaustralia.gov.
au/>
2007