Why plant trees for carbon farming?
Reforestation, afforestation and revegetation in Western Australia (WA) can:
- improve resource condition and provide ecosystem services (e.g. by ameliorating secondary salinity, wind erosion and biodiversity decline, carbon sequestration)
- provide a financial return (e.g. plantation forestry with softwoods and hardwoods)
- improve amenity and landscape value
- achieve combinations of these points (e.g. carbon sequestration forestry under the Emissions Reduction Fund).
For information on forestry and revegetation establishment methods, species selection check, taxation details and carbon farming methodologies, see:
- Forest Products Commission
- regional natural resource management organisations
- local nurseries
- Australian Taxation Office conservation covenant concessions
- Australian Taxation Office carbon sink forests
- approved and proposed carbon farming methodologies
There are several approved methodologies and around 30 projects have been registered in WA.
Large areas of agricultural land in WA are suitable for reforestation and revegetation activities, although there are uncertainties about their ability to generate income. These uncertainties include the opportunity cost of changing land use, the long-term price of carbon, the rate at which carbon is sequestered (tree growth) and the costs associated with establishing and managing the vegetation.
How good are trees at sequestering carbon?
Carbon sequestration rates determine the economics of carbon forestry in WA.
Growth rates of the major plantation sawlog and woodchip species are relatively well understood for the traditional forestry areas of WA, although climate change is likely to influence growth rates. There is less information about non-forestry tree species, particularly older stands in the drier areas of the grainbelt. As a result, economic analyses of carbon forestry in WA have largely used modelled tree growth rates.
To improve estimates of tree growth, models such as FullCAM, which is used in the national carbon accounts, are being updated as more data becomes available.
Research in southern Australia has shown that soil organic carbon generally does not increase for at least 30 years after replacing pasture with plantation tree species.
It is unclear whether soil organic carbon is greater under mixed species plantings compared to plantation forestry in the longer term. In plantation forests, carbon sequestered in leaf litter and deadwood is more significant than changes in soil organic carbon, but litter dynamics are less well understood than tree growth rates.
Proponents of carbon farming projects can use one of two estimates of carbon sequestration:
- modelled growth rates (possibly conservative sequestration rate but with a low verification cost)
- an inventory methodology that measures tree growth directly (accurate estimate of sequestration rate but greater measurement and verification costs).
The long-term effect of climate change on tree growth should also be factored into the calculations. Climate change effects can be beneficial or deleterious, depending on the species and site.
Harvested versus no-harvest sequestration forestry
Carbon offsets can be generated from forest or revegetation that is subject to regular harvest.
This could entail:
- handing back offsets for the carbon in the biomass that is harvested
- only applying for offsets for the average amount of carbon sequestered over the harvest rotation
- only applying for offsets for the carbon sequestered in the unharvested portion of the biomass.
There are advantages to harvested projects compared to non-harvested projects:
- the land continues to generate primary produce and income for the life of the project
- fewer carbon offsets are generated, reducing the cost of changing land use should that be desired
- integrated biomass systems provide some flexibility to respond to future changes in climate, technology and product demand
- income from carbon offsets can offset establishment costs and provide early income in longer rotation harvest systems
- potential offsets generated for carbon are stored in harvested wood products.
Mallee agroforestry is an example of an integrated farm forestry system that could potentially generate income from carbon offsets, Renewable Energy Certificates and harvested biomass.
Integrated versus block plantings
Reforestation and revegetation of agricultural land can be done in many ways, ranging from large block plantings to highly integrated alley systems, with each layout having various pros and cons. For example, block plantings can be used to target particular soil types or areas in the landscape and may be cheaper and easier to establish and manage than integrated plantings.
Integrated plantings and particularly narrow linear belts spread the environmental benefits of revegetation over a larger area and provide trees with greater access to resources (particularly water) than blocks.
Mallees growing in 2-row belts can produce 30–80% more biomass (and sequestered carbon) than an equivalent area of mallees growing in blocks. This can be particularly important in low rainfall environments or where trees are being grown for harvest.
Integrated plantings can increase costs by competing for soil water and reducing adjacent crop and pasture growth. For mallee belts, the average width of foregone agricultural income is 14 m or 8–9 m on either side of unharvested or harvested belts, respectively.
These trade-offs need to be carefully evaluated before undertaking any agroforestry project.
Opportunity cost and land
The attractiveness of carbon farming activities for Western Australian landowners can be increased by locating reforestation and revegetation projects on agricultural land with a low opportunity cost (i.e. land that is currently of limited agricultural value). A recent study found that 75% of farmers surveyed in the North-East Agricultural Region are willing to permanently revegetate unproductive soils, with many already withdrawing these areas from cropping programs, or planting them to oil mallees.
Such land might be marginally saline, inherently acidic, in areas with low rainfall or non-arable for other reasons. The dispersed nature of these areas and their relatively small size makes alternative land uses unpractical.
Carbon farming revegetation of unproductive agricultural land with no opportunity cost associated with the land-use change is clearly an opportunity, but it is currently constrained by the lack of knowledge about the carbon sequestration potential of the species that grow in these areas.