Sequestering Carbon in Native Forests Part IV

Here’s the next instalment of the Sequestering Carbon in Native Forests series. It’s fairly acedemic (I have to be serious sometimes), so get your acedemia on! If you haven’t read any of the previous posts in the series maybe check them out first; they are best read in order.

Pre-existing forests can be managed to sequester carbon through measures such as having a mix of species, managing fire regimes and keeping the forest at an ideal mix of age-classes. A mixture of species in a forest makes the system more resilient and overall better at storing carbon (Böttcher and Linder, 2010).

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This plantation looks really cool but in terms of sequestering carbon it’s not great as it doesn’t have a mix of species or age classes. PHOTO: wood report

 

As outlined above, different species react differently to factors such as water availability so in this way a resilient tree community would be made up of a variety of species so that whatever the rainfall pattern at least one species was still able to actively sequester carbon.

In the Australian bush the constant possibility of fire means that landholders with a large number of trees on their property need to have a fire management plan with efforts such as controlled burning to reduce fuel loads. (Böttcher and Linder, 2010).

Although this releases carbon dioxide, it can greatly reduce the risk of large, uncontrolled fires later and can actually help with the sequestration process if the ash goes back into the soil. For the purposes of sequestration the ideal age-class structure of a forest is fairly mixed (Böttcher, 2007).

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Controlled burns are important in Australian forest management. PHOTO: abc.net.au

If a system cannot self-regulate to achieve this, the forest should be managed; trees that are all relatively young and still growing sequester large amounts of carbon but there will be a lesser extent of the natural process that use dead roots, branches and twigs to lock up carbon in the soil as these processes take decades to fully establish.

However, if the forest comprises only of mature trees that have stopped growing the sequestering potential for that forest will largely have been reached.

Thus, once a forest has been established it should be managed (if it can’t self-regulate) so there is a mixed age-class with a mind to disturbing the ecosystem as little as possible. While it takes around 100 years for this point to be reached (possibly longer in low rainfall areas) the potential of forests to mitigate against climate change, even in those low rainfall areas, is good.

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Sequestering Carbon in Native Forests Part II

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).

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Fig. 1. Overview of main effects of forest management options for sequestration. The chart shows pools and services (grey boxes), fluxes of carbon (arrows) (Böttcher, 2007).

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).

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Fig 2. Net forest change in Australia (using forest regrowth and deforestation data) 1973–2004 (State of the Environment Report, 2006).

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).

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Fig. 3. Cumulative carbon sequestered by a permanent stand of southern pine and by a periodically harvested stand (Vled and Plantinga, 2005).

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.