Fire and organic substrates: soil structure, water quality and biodiversity in far southwest Western Australia

Document Type

Book Chapter


Backhuys Publishers


Faculty of Computing, Health and Science


School of Natural Sciences / Centre for Ecosystem Management




This chapter was originally published as: Horwitz, P. , Judd, S., & Sommer, B. (2003). Fire and organic substrates: soil structure, water quality and biodiversity in far southwest Western Australia. In Ian Abbott and Neil Burrows. (Eds.). Fire in ecosystems of south-west Western Australia: Impacts and management (pp. 381-393). Location: Backhuys Publishers. Original book available here.


Organic matter in soils will accumulate according to a series of interrelated factors, namely the climate (particularly temperature), topography, moisture of the soil, the rate of litter fall, the role played by soil micro flora and microfauna, the soil protective capacity (including mineralogy) and the local fire regime. In simple terms, with more organic matter, surface moisture will increase, hence the likelihood that micro climates can influence, even 'edaphically' control, fires. The reverse holds too: the drier the soil the more likely it will be to ignite when exposed to fire. Organic matter can thereby influence locally the fire regime by regulating soil moisture. In places where organic matter is likely to accumulate at the soil surface and contribute significantly to the development of the soil, such as shaded areas, depressions or other cool, moist places, wetland characteristics follow. Water quality associated with such wetland soils has a tendency towards stained, slightly acidic, stratified, dystrophic water, with bottom layers depleted in oxygen. Individual organic-rich wetlands or wetland systems have their own unique suite of invertebrate and vertebrate animals, and vascular and non-vascular plants. These biota are likely to be dependent upon local hydrology and physico-chemical characteristics regulated by the relationship between climate and the presence and condition of organic matter. Our hypothesis is that the dynamic biophysical characteristics and states of these local organic-rich systems can be altered permanently or at least disrupted by the application of an inappropriate fire regime. With frequent hot fires, or any frequent regime designed to systematically remove organic matter, fire can become a geomorphological agent by burning the soil, altering water quality by exposing anaerobic soils to air (i.e. acidification following the exposure of iron sulphides), and threatening elements of biodiversity dependent on the organic matter/moisture relationship. We support this hypothesis with evidence from several recent and on-going studies .and in doing so construct a partial bio-geography of south-west Western Australia (WA) , emphasizing the relictual nature of organic-rich, moist places, and the edaphic control of fire.