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Published: 30 January 2012

Native forest management and greenhouse gas emissions – how much do we really know?

Kathryn Page, Ram Dalal and John Raison

Harvesting of native forests needs to be included in the carbon accounting process in Australia. But, do we know enough about the greenhouse gas and carbon cycling implications of this kind of forestry?

Native forest regrowth in East Gippsland, Victoria. The effect of different post-harvest management practices on greenhouse gas emissions and forest growth rates is highly spatially variable in Australian native forests and is poorly understood.
Credit: CSIRO Ecosystem Sciences

Large areas of diverse native forest are either harvested or subjected to disturbances such as fire each year throughout Australia. To understand and incorporate native forestry activities into Australia’s greenhouse gas (GHG) accounting process, we need to understand how these disturbances affect carbon cycling and GHG emissions.

The modelling system currently used to calculate emissions and sequestrations from Australian forestry operations – FullCAM – is underpinned by a great deal of research. However, certain aspects of forestry operations, including harvesting within native forests, are yet to be fully incorporated into this system. Recently, researchers from Queensland Department of Environment and Resource Management and CSIRO reviewed current knowledge of changes to GHG emissions and carbon cycling following harvesting in Australian native forests, and identified areas requiring further research. ‘Off site’ GHG emissions from harvested products (e.g. when wood waste is sent to landfill) were not considered in the review.

Harvesting or other disturbances in native forests changes plant growth rates, respiration by soil microorganisms, carbon storage in soil and biomass, and ‘fluxes’ of the GHGs nitrous oxide (N2O) and methane (CH4). Gas flux describes the flow of gas from one place in the atmosphere to another and is usually measured as a rate of flow across a given area where the gas is produced. Following harvest, the net carbon dioxide (CO2) flux from forest systems increases, due to decreasing photosynthesis and a rise in decomposition of plant material left behind after logging. This generally turns the forest from a carbon sink into a carbon source, until plant biomass regrows and balances carbon emissions.

Soil moisture often rises following harvest, as the actively growing vegetation is no longer present to use available moisture, and temperatures can also increase when the soil is exposed to the air. In moist soil, less oxygen diffuses into the soil, and anaerobic (low-oxygen) soil conditions can develop. These conditions favour the growth of microorganisms that generate by-products such as N2O and CH4.

Increases in N2O flux are commonly observed during the first few years following harvest if supplies of inorganic nitrogen and soil moisture conditions coincide. More CH4 may be produced, or less may be used up, if the rise in soil moisture following harvest limits the amount of CH4 that diffuses from the atmosphere into the soil. This reduces CH4 uptake by methane-consuming bacteria, and can also promote anaerobic conditions, which favour soil microorganisms that produce CH4.

Over the long term, the amount of carbon stored in plant biomass and soil may also decrease in harvested forests, because it may take up to several hundred years to fully replenish the carbon removed in the harvested product. If forests are harvested on a rotation more frequent than the time it takes to recover this lost carbon, the forest will become a regrowth forest of lower carbon density. In addition, if harvest practices reduce site productivity – e.g. due to nutrient removal during burning activities or via soil erosion – they may reduce overall site quality, net primary productivity and overall carbon storage potential.

Intensively harvested and burned native forest in southern Tasmania. Harvest practices that reduce the productivity of a site may decrease site quality, net primary productivity and overall carbon storage potential.
Credit: CSIRO Ecosystem Sciences

While the general processes affecting GHG emissions and carbon storage in harvested forests are reasonably well understood, the magnitude and time course of these changes are poorly quantified worldwide. From the changes observed in forest biomass production and in N2O and CH4 fluxes, it is understood that GHG fluxes from forests typically increase following harvest and then gradually slow as primary production increases and N2O and CH4 fluxes return to background levels. International studies that have quantified these changes also show that the largest changes tend to occur to CO2 flux and that this gas will dominate GHG budgets.

In an Australian context, however, the accurate quantification of changes to GHG budgets is limited by the small amount of local research. Knowledge of primary production rates following harvest for the diverse range of species and management practices in Australian native forest environments is poor. Respiration by soil microorganisms – a key process in the changing CO2 fluxes of post-harvest forest soil – has also been little studied within the Australian environment. Therefore, the impact of harvesting on total respiration over full harvest cycles is largely unquantified. In addition, CH4 and N2O fluxes have been poorly quantified following harvesting in Australian forests.

Further research effort is needed if Australia is to accurately incorporate the harvesting of native forests into its GHG accounting processes. In particular, variations in GHG flux and forest growth over the range of forest types harvested in Australia are poorly understood and need to be more intensively examined. Harvesting and management of Australian native forests is increasingly undertaken by the private sector, where silvicultural systems are likely to differ and forest management may be carried out in conjunction with grazing and other farming activities. This also needs to be considered, and its effect on GHG production quantified.

Dr Kathryn Page is a soil scientist with the Queensland Department of Environment and Resources Management (DERM). Dr Ram Dalal is Senior Principal Research Scientist with DERM. Dr John Raison is Chief Research Scientist with CSIRO's Ecosystem Sciences.

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Published: 11 January 2012

Managing the future of the north’s aquatic biodiversity

Bradley J. Pusey

Northern Australia features prominently in visions for Australia’s future. Plans for capturing, storing and transporting the region’s water to make it available for increased agricultural development in the region and elsewhere are frequently proposed. But what is our current understanding of the biodiversity, and the ecological functions and services and of the northern region, and how might these assets be affected by such development?

A comb-crested Jacana, one of the 30 per cent of the nation’s waterbird species that are recorded in the northern region.
A comb-crested Jacana, one of the 30 per cent of the nation’s waterbird species that are recorded in the northern region.
Credit: With permission of MV Jackson

The recently published book, Aquatic Biodiversity in Northern Australia: patterns, threats and future, by the Tropical Rivers and Coastal Knowledge (TRaCK) research consortium, examines the biodiversity of northern Australia’s freshwater ecosystems, its dependency on the natural water regime and how increased water use might be managed to avoid future degradation of the region’s sensitive ecology.

TRaCK is comprised of leading tropical river researchers from five Australian Universities (Charles Darwin University, University of Western Australia, Griffith University, Australian National University and James Cook University), federal research organisations (Geoscience Australia, eriss, AIMS, CSIRO), State governments of Queensland, Western Australia and the Northern Territory, and the Northern Australian Indigenous Land and Sea Management Alliance (NAILSMA). More than 30 researchers from TRaCK contributed to the book.

Northern Australian freshwater ecosystems sustain a very rich and distinctive biodiversity. Although it comprises only 17 per cent of the continent’s land area, the region contains a large proportion of the nation’s aquatic plants and animals.

For example, waterbird diversity accounts for about 30 per cent of the nation’s waterbird species, whereas freshwater fish account for almost 60 per cent. A third of the nation’s frog species, and half the turtle species, are also found in northern Australia, as are many water dependent lizards, snakes and of course, crocodiles.

A graphic flutterer (<i>Rhyothemis graphiptera</i>).
A graphic flutterer (Rhyothemis graphiptera).
Credit: With permission of J Clark

Such high biodiversity is perhaps not surprising as the region’s freshwater ecosystems sit within a savanna landscape of globally significant ecological integrity1.

Moreover, northern Australia contains the highest concentration of free-flowing rivers in the world2. Consequently, the biological systems of northern Australia’s swamp, wetlands and rivers remain relatively pristine and capable of supporting such biodiversity riches. Importantly, the connectivity of different parts of the riverine landscape (for example, headwaters, main channels, floodplains and estuaries) and between the riverine and near-shore marine environment, remains largely intact across the region, allowing the free passage of organisms, nutrients, carbon and energy necessary to sustain high biodiversity.

A view across the majestic Hann River in the Fitzroy River catchment (Western Australia).
A view across the majestic Hann River in the Fitzroy River catchment (Western Australia).
Credit: TRaCK

“Hot-spots” of biodiversity vary between taxanomic groups, making effective conservation and protection measures difficult. The national reserve system currently does not adequately cover the distribution of all aquatic biota. For example, up to 80 per cent of all species examined had less than 5 per cent of their total distribution contained within existing protected areas.

Also of significance is the high rainfall of the region – more than 1 million gigalitres per year! This means that the area’s rivers transport huge volumes of water. The Mitchell River in Queensland, for example, conveys an average of 24 000 megalitres a day, more than any other river in Australia.

Such high flows and abundant rainfall also mean a great diversity of aquatic ecosystem types, across a huge area. For example, floodplain wetlands are so vast that they may cover about 25 per cent of the catchment area of some basins. They are hotspots of biological production which is later transported widely throughout river systems and is, therefore, of enormous ecological, cultural and economic significance.

Measuring fish during fish surveys on the Daly River, Northern Territory.
Measuring fish during fish surveys on the Daly River, Northern Territory.
Credit: TRaCK

However, despite the huge amount of rainfall, it is highly seasonal and often unpredictable in incidence and volume. Most of northern Australia incurs an annual water deficit (rainfall minus evapotranspiration) of about 1000mm. This places extreme pressure on the region’s biota and greatly limits production. Most rivers cease to flow, often for periods of up to nine months, and contract back to series of isolated refugial waterholes. Such large contractions in available aquatic habitat constrain the development and maintenance of high biodiversity.

In intermittent rivers (the dominant river type across the region), aquatic species are either forced to retreat to these refuges, which become increasingly less hospitable as the dry season progresses, or seek shelter underground. Refugial waterholes often persist only when in contact with shallow groundwater.

A prawn shell on cracking clay, Fitzroy River, Northern Territory. The northern region is characterised, and tested, by extremes of wet and dry.
A prawn shell on cracking clay, Fitzroy River, Northern Territory. The northern region is characterised, and tested, by extremes of wet and dry.
Credit: TRaCK

Groundwater is also critical in sustaining high levels of biodiversity in larger rivers. For example, the highest levels of fish biodiversity are found only in those rivers that have perennial flow sustained by groundwater contributions during the dry season (for example the. Daly, Mitchell, Wenlock and Jardine rivers).

Groundwater resources have been identified as suitable for development and exploitation in order to support agricultural expansion. The 2009 Northern Australia Land and Water Science Review3 suggested that further development of agriculture in northern Australia would most likely be of a mosaic style, where agriculture would be confined to areas defined by the coincidence of suitable soils and exploitable groundwater.

A helicopter being used by TraCK researchers to rapidly collect water samples from a remote area of the Fitzroy River, Western Australia
A helicopter being used by TraCK researchers to rapidly collect water samples from a remote area of the Fitzroy River, Western Australia
Credit: TRaCK

Such a development option is less likely to have negative impacts on freshwater biodiversity than broad-scale agriculture and large-scale water capture and diversion schemes, which are sometimes proposed.

A greater focus on sustainable use of resources and a greater appreciation of the values of aquatic biodiversity from economic, cultural and spiritual perspectives suggest that alternative visions for the future of northern Australia are gaining traction. These visions recognise that the economic value from commercial fishing, recreational fishing and tourism more generally, is significant.

Similarly, the substantial dependence of Indigenous household economies on riverine production (for example fish, crustaceans, and birds etc. are used for food), represents another, significant form of economic value.

Combined with the central cultural and spiritual importance of the region for Indigenous peoples (about 60 per cent of the population), this multiplicity of values require that the maintenance of healthy aquatic ecosystems and the rich and productive biodiversity they sustain is key to a sustainable future for the north.

Horticultural development on the Daly River, Northern Territory.
Horticultural development on the Daly River, Northern Territory.
Credit: TRaCK

A set of guiding principles for improved management of aquatic biodiversity was proposed by the book’s authors. These are:

  1. Prioritise and protect high value aquatic ecosystems;

  2. Address current threats;

  3. Explore development options and their consequences carefully;

  4. Improve planning processes to secure environmental water allocations;

  5. Improve the available information base; and

  6. Improve public awareness and engagement.

Although visions of the future are useful, a clear focus on the present is critical. Aquatic systems across the north are currently threatened by feral animals (including pigs, buffalo, and alien fishes), weeds, altered fire regimes and a range of other diffuse pressures including resources extraction. We need to urgently and efficiently develop counter measures to these immediate threats to limit further and potentially widespread degradation of the region’s unique riches, and to ensure its broader values and future are assured.

Bradley J. Pusey is a researcher at the Tropical Rivers and Coastal Knowledge (TRaCK) research consortium, and editor of Aquatic Biodiversity of Northern Australia: patterns, threats and future. 2011. Charles Darwin University Press, Darwin.

1 Woinarski J, Mackey B, Nix H, and Traill B. 2007. The Nature of Northern Australia: Natural Values, Ecological Processes and Future Prospects, ANU E Press, Canberra.
2 Vörösmarty C J, McIntyre P, Gessner M, Dudgeon D, Prusevich A, Green P, Glidden S, Bunn S E, Sullivan C, Reidy C, and Davies P M (2010). Global threats to human water security and river biodiversity. Nature 467:555–561.
3 Stone P. (2009). Northern Australia Land and Water Science Review. Final report to the Northern Australia Land and Water Taskforce, CSIRO Publishing, Canberra.

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