Forest soils - linking fire and ecology

Australian forest managers have long been challenged to justify the environmental credentials of their practices. This includes the key practice of fuel reduction burning whose merits have been fiercely debated since Victoria’s 2009 ‘Black Saturday’ bushfires.

Fuel reduction burning, particularly if done too frequently, can have environmental impacts. However, periodically using controlled low intensity fire is far less damaging to the environment than the alternative of allowing unburnt forests to develop heavy fuel accumulations that can be burnt in high intensity summer conflagrations.

This view is supported by some academic study and a larger cache of anecdotal evidence based on observational case studies, as well as in-house research by forest management agencies. However, due to the tremendous variability of Australian forest types, there is still much to be learnt about the effects of fire on forest ecology.

Recent research in the Western Australian jarrah forests goes some way towards redressing this knowledge gap. Conducted by Jaymie Norris, as part of his Bushfire CRC PhD research at the University of Western Australia, this research has looked to the condition of forest soils under various burn intensities for clues about the impact of fire on ecological processes. Its findings have ramifications for forest management policies and practices, including for developing strategies to address climate change.

Within our forests, microbes are integral to all ecosystem processes. While we can’t see them, they are continually recycling nutrients and maintaining the productivity of our forests. Without microbes, our forests may not even exist.  Mr Norris’s research has focused specifically on understanding changes in the patterns of microbial communities (diversity, function and activity) in relation to soil carbon and nutrients as the most sensitive indicators of shifts in ecosystem processes in a post-fire landscape.

His studies were undertaken within 27,000 hectares of jarrah forest in the Perth Hills which had been burnt by a bushfire in January 2005. This had created a mosaic of forest burnt at different intensities. Within this landscape, the researchers took soil samples from:o   unburnt forest,

o   forest burnt at low intensity, where the understorey was largely consumed but overstorey crowns were not fully scorched, and

o   forest burnt at high intensity, where there was significant scorching and leaf lossof the crowns of overstorey trees (Fig. 1).

These samples were then analysed for a range of carbon (C) and nitrogen (N) fractions, including microbial C and N.

The analysis showed that fire affects soil carbon and nitrogen pools differently according to its intensity. High intensity fire tends to homogenise the distribution of microbial biomass C and N, while soil burnt at low intensity showed a more patchy distribution compared to unburnt forest. Overall spatial variability of soil resources tends to be greatest with low intensity fires.

During low intensity fire, soil may remain below or near the critical temperature for nitrogen volatilisation (220°C), thereby resulting in greater loss of carbon relative to nitrogen.

High intensity fire has the opposite effect. Soil temperatures may greatly exceed 220°C, meaning that the loss of carbon relative to the loss of nitrogen is small. These differences in the sensitivity of soil C and N fractions to temperature have major implications for assessments of the stability and resilience of carbon in burnt forests, as well as for the quantification of carbon sequestration in fire-prone forests.

At high fire intensity, although much nitrogen is modified or lost, carbon may remain in the soil at high concentrations. In severely burnt sites, this may mean that the remaining C is protected in the form of charcoal rather than unburnt particulate organic carbon. The ecological role of charcoal remains unknown with debate in the scientific literature largely inconclusive. This is an area that requires further research.

Prescribed burning in the WA jarrah forests is generally at low intensities and is likely to maximise the variability of soil resources. Conversely, the research suggests that both the absence of fire or wildfires of higher intensity, create less complex landscapes.

This suggests that there is greater microbial biomass ‘insurance’ for the recovery of key ecological processes associated with low intensity fire. This finding supports the contention that patch mosaic burning is, to an extent, self-managed and represents ecologically-appropriate management.

The implications for forest management

By quantifying the relationship between fire intensity and variation in soil resources, this research contributes to a better understanding of the long-term impacts of fire regimes on the key ecological functioning of jarrah forest. This includes greater certainty about how different fire intensities will affect the pattern of post-fire regeneration.

It has also fostered a betterunderstanding of the carbon cycle of fire-proneeucalypt forests. This is particularly relevant dueto the large areas involved and the potential for fire to have a strong impact on carbon pools.

Incorporating this knowledge into existing models such as the National Carbon Accounting Toolbox (NCAT) and the FullCAM model will further improve understanding of how regional carbon cycles are linked to fire and forest management. This is very important given the need to refine land management policies in the context of addressing climate change.

Further, the assessment of impacts of fire and soil will be of increasing importance with the adoption of Article 3.4 of the Kyoto Protocol, which introduces requirements towards full accounting and the need to differentiate between human-caused and other GHG emissions.

A more detailed discussion of this research can be found in Bushfire CRC Fire Note No. 57 as part of the research adoption program of the Bushfire CRC – www.bushfirecrc.com; or by contacting Jaymie Norris, now with Victoria’s Department of Sustainability & Environment – jaymie.norris@dse.vic.gov.au