Using trees to sequester carbon in low-medium rainfall areas has high potential because those areas are not widely used for agriculture and often are in need of regeneration.
However, which species can be planted will depend on the amount of water available and how specific species react to factors such as long drought periods, sudden floods and random cycles of wet and dry periods.
The main species used for sequestration are eucalypts, most of which have various adaptations to Australia’s unpredictable weather patterns but a tree that is stressed due to lack of water will stop growing, thus stop sequestering carbon, and may even die.
A study done by Walsh et al (2008) found that E. globulus and E. botryoides are both highly suited to sequestering carbon, but while both species can sequester during short periods of little or no rainfall neither can handle extended drought, which is all too common in low rainfall areas during el Niňo years. This makes those species suitable but not ideal.
The same study found that while E. camaldulensis, E. sideroxylon and C. maculate are not as efficient in sequestering carbon they can handle long periods of drought far better than the above two and so are better suited to low-medium rainfall conditions. This shows that while it is not ideal to plant in such conditions compromises can be made.
Above all, it is important that these planted forests are not competing for land with agriculture or pre-existing forests.
Of course, unlike plantations, these forests need not be in one big clump; they can be spread along fence lines or creeks, they can be a patchwork across paddocks or public land, anything so long as they are large enough (≥0.2 ha) to qualify as a forest. Spacing plantings like this can also minimise water needed in any one place (Battaglia, 2011).
If you want to learn more about eucalyptus plantings, the NSW Department of Primary Industries has a lot to say on the issue.
Part II of this series discusses what carbon sequestration is, as well as an overview of how to do it with trees.
“The term sequestration means achieving and maintaining a net increase in the amount of organic carbon present in a soil,” Battaglia, 2011.
Sequestering carbon using trees means managing the trees so that the net amount of carbon captured through photosynthesis is greater than the net amount of carbon released by respiration, burning, logging and so on (see Fig. 1).
Using natural biophysical processes to lock the carbon in the soils makes the captured carbon relatively stable and able to stay in that state for a long time. There are many factors inherent to the amount of carbon that can be sequestered, from rainfall to soil type to vegetation to land use.
The disturbance to Australia’s forests and soil wrought by commercial agricultural operations and land clearing cannot be overstated; between 2000 and 2004 nationally there was a net loss of 287 000 hectares of forest (State of the Environment Committee, 2006), an area larger than the Australian Capital Territory and since 1973 there has been a net loss of tree cover every year (see Fig. 2, below) (one could safely assume that since white settlement there has been an overall downward trend in tree cover in Australia but there are no concise records).
To effectively use forests to sequester carbon the landscape must be managed using whole system thinking so the needs of the environment and agriculture are balanced (Battaglia, 2011). Forests that are overly ‘managed’ such as plantations are sometimes less effective at sequestering carbon because the mortality of the trees cannot be captured as dead branches, roots and so on being returned to the soil by natural process (Binkley et al, 1997).
For this reason, the minimum length of time for a stand of trees to sequester its full potential of carbon (in best conditions) is 100 years (see fig. 3, below).
Although the diagram is specific to southern pines this trend is common to all forests. Using trees in this way is effective as it requires no great input of infrastructure or costs once the trees have been planted and it takes carbon out of the atmosphere and places it in a stable and long-term storage system.