The relationship between the nutrient status and flammability of forest fuels
Date
1982
Authors
Mak, Edwin Hon Tak
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Abstract
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nitrogen, phosphorus, and potassium are withdrawn and calcium is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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