Intense fires, erosion and streams
Increased soil erosion can be an important damaging impact of
intense fire late in the dry season. The storms of the following
wet season, initially striking soil bared by the fire, are likely
to wash substantial quantities of topsoil and ash into streams.
Results include a loss of nutrients from the land and reduced
stream water quality.
Research as part of the Kapalga fire experiment compared water
quality in ephemeral streams draining areas burnt early or late in
the dry season or left unburnt. Early storms, before the start of
permanent wet season flow, produced run-off only from the
late-burnt area, and this contained high concentrations of
sediments, nitrogen, phosphorus, iron and manganese. Potential
effects of such early flows include rapid growth of algae and fish
kills in downstream billabongs and rivers.
During storms throughout the wet season, the sediment
concentrations in flow from the late-burnt area were two to three
times higher than those in run-off from the other sites. Sediment
loads at these levels could cause silting in some watercourses,
accompanied by increased growth of aquatic plants. Another risk is
that the higher iron and manganese levels in run-off from savanna
burnt late in the dry season could cause unacceptable increases in
the concentrations of these elements in town water supplies.
Fire and the nutrient cycle
Despite the impression conveyed by lush wet season growth, the
soils of Australia's tropical savannas are generally very low in
nutrients. This makes it essential that the nutrients stored in
decomposing plant material are released relatively quickly to
support new growth.
Termites and various micro-organisms play a big part in ensuring
that happens. Fire is the other main contributor as new growth can
draw on the nutrients left behind in ash. Smoke carries a
proportion of the nutrients into the air, but these generally
return to ground close to where they originated. Some nutrients are
lost in gaseous form.
Research on the effects of fire on biodiversity by CSIRO at
Kapalga in Kakadu National Park has indicated that any net losses
of most nutrients should be too small to have an impact. Nitrogen
is the important exception; the study showed losses during annual
fires that completely burn the grass cover might exceed inputs from
biological nitrogen fixation, leading to a gradual depletion of
soil reserves. Hence, in savannas like those in Kakadu, the best
option may be annual early dry-season fires that leave substantial
patches unburnt, or more complete burns every two or three years.
The findings make it clear that nitrogen cycling is one of the
factors to be considered when determining fire regimes for tropical
savanna regions.
