From Savanna Links, Issue 34, 2007. Savanna Links is written and produced by the Tropical Savannas CRC.

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. 

Australia’s tropical savannas account for around a third of Australia’s land-based carbon stores. However, savannas are regularly swept by bush fires 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.

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, 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 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.

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, 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 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:

1. 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

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 CO₂, 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.

2. Carbon sequestration

Some savanna landscapes should be able to be managed to enhance storage of carbon in trees, shrubs and soils, for 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.
 

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