The relationship between the nutrient status and flammability of forest fuels
An investigation is reported of the relationships between nutrient status of foliar and litter materials from Australian forests and the three components of flammability, namely, ignitability, sustainability, and combustibility. The Limiting Oxygen Index (LOI) method was chosen as an index of ignitability and sustainability, and combustibility has been measured by Thermogravimetric Analysis . (TGA) and Derivative Thermogravimetric Analysis (DTG). Results derived from LOI and TGA are seen to be more reproducible and reliable than other methods commonly used in m~asuring forest fuel flammability. The first study examines the general relationship between a number of chemical attributes and the flammability of the leaves of 47 species. A significant correlation between nutrient status and flammability was found, foliar calcium in particular, appearing to have most effect on flammability. Because of this, the major study involves a comprehensive evaluation of relationships between flammability and nutrient concentrations in green foliage, freshly fallen leaves, and litter decomposing on the forest floor. Thirteen species have been used, drawn from the vegetational gradient, dry sclerophyll forest to rainforest. Ignitahility and sustainability of forest fuels are greater in materials with lower nutrient concentrations, including the eucalypts, although there are exceptions to this generalisation. Alternatively, combustibility, as defined by the TGA technique, is lower in foliar and litter materials with low nutrient status, again including the eucalypts. The three flammability indices do not appear to respond directly to seasonal variations in nutrient concentrations in green foliage or changes in nutrient concentrations during the decomposition process. However, flammability generally increases at leaf senescence and litter fall, during which substantial amounts of nitrogen, phosphorus, and potassium are withdrawn and calcium is accumulated. A number of characteristics of the eucalypt forest contribute to its considerable flammability. 'These include the slow decomposition rate of eucalypt litter, the physical structure of the litter layer, the ignitability and sustainability of eucalypt litter, and a number of combustion characteristics. The relevance of the standard TGA index of combustibility is reviewed and an alternate index suggested, which may be more appropriate to the flaming combustion of the forest floor. It is concluded that low nutrient concentrations and flammability in eucalypt are related, but possibly in an indirect evolutionary way. The evolutionary response of eucalypts and other species to low nutrient soils may have involved changes in the structure and components of leaf tissues contributing to their particular flammability characteristics.
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