At the beginning of 2006, two of the most severe
cyclones in decades affected Australia’s north: Larry and
Monica. Devastating as the storms were, they are providing an
opportunity for us to learn more about how cyclones affect the
dynamics of the savannas.
While Larry struck near Innisfail wreaking havoc along the wet
tropics, Monica tore a path through Arnhem into the
savannas—and was possibly one of the most severe cyclones to
affect the Australian coastline in recorded history. The eye
crossed the Arnhem Land coast about 35 km west of Maningrida on 24
April 2006. The Maningrida airport recorded a maximum wind gust of
150 km/hr, but near the eye, gusts of up to 350 km/hr were
estimated.
Researchers from NT Department of Natural Resources, Environment
and the Arts (NRETA), Bushfires NT and CSIRO are now taking the
opportunity to study the after-effects of Cyclone Monica on savanna
vegetation and wildlife. Researchers flew over Cyclone
Monica’s Top End path to map the destruction left in her
wake. NRETA spatial scientist Craig Hempel estimated the area of
greatest destruction to be about 7000 square kilometres in Arnhem
Land, encompassing a large part of the Goomadeer River catchment
between Maningrida and Jabiru.
“The destruction ranges from coastal mangrove habitats,
Melaleuca swamps, sensitive sandstone country and through Eucalypt
woodlands,” Craig said. “In the worst affected areas,
90 per cent of the vegetation was destroyed. In many areas only
partial tree trunks remain, in others there is complete
defoliation.
“It will take decades for the environment to recover from
a cyclone of this intensity and management of the area is crucial
to ensure recovery happens.”
There is also concern about the long-term impacts of fire on
cyclone affected areas. NRETA Bushfires NT scientist Andrew Edwards
said the damage the debris left in the wake of Cyclone Monica had
increased fuel loads for fires.
“With the fire season upon us, an increase in fuel loads will
cause fires of increased intensity and this could have implications
for the recovery of the area,” he said.
“The example of the effects of Cyclone Ingrid on Cobourg
Peninsula in early 2005 showed that vegetation only has one chance
to recover after such a destructive event. Fires of greater
intensity occur and prevent trees from re-shooting.
“If the area does not recover the native vegetation has
the potential to be replaced by weeds, such as gamba and mission
grasses that could dominate the area and further suppress the
re-introduction of native species.”
The maps produced from the survey will allow scientists to
quantify the disturbance to each habitat in relation to wind
direction and intensity.
“This will help build a picture of what role these events
have in shaping the coastal landscape and focus management on
sensitive areas,” said Craig.
Andrew Edwards said the information gathered from the survey has
influenced fire management this year.
“When we saw the serious extent of destruction we gave advice
to park managers, Aboriginal rangers in Arnhem Land and Bushfires
NT operational staff who have created strategic burning regimes to
reduce fuel from debris in the area,” Mr Edwards said.
“We have tried to strategically burn parts of the area in
an effort to prevent wildfire,” he said. “The aerial
survey is the first step in assessing the type and extent of damage
and will provide a basis for further studies into the
cyclone’s impacts.”
The damage done
More recent on-ground work by the Djelk Rangers and Staff of
Bushfires NT, NRETA and CSIRO has estimated tree damage from the
cyclone. Near Maningrida airport, about 20% of savanna trees were
snapped off or toppled by the cyclone. This proportion grew quickly
towards Junction Bay where the eye crossed the coastline. Here
about 80% of trees were knocked down. Many of those still standing
had been rattled to death, with the bark stripped off and the fine
roots broken. Long-term plots have been established to monitor
recovery of the vegetation.
Cyclones possibly have had a much greater role in the dynamics
of savanna vegetation within 100 to 150 km of the coast than has
been previously recognised. This research will enable the
relationship between wind speeds and tree damage, and the role
of severe winds in tree dynamics, to be assessed.
The cyclone may have an upside for some of the region’s
wildlife. Because of the massive destruction, there are now greater
numbers of hollows and logs, used by many species of wildlife as
habitat.
The mapping information will be used by organisations including
the Bureau of Meteorology, CSIRO, Charles Darwin University, Parks
North Australia and traditional owners, who will work in
collaboration with the NT Government to further investigate the
type and extent of the damage to ecosystems within the area to aid
future management.
