Concerns about the potential environmental impacts of gamba
grass (Andropogon gayanus) were first raised in the early
1990s. However, the push for effective and coordinated control
strategies were hampered by the lack of published scientific
information about its biology and environmental impacts1. In
response to this knowledge gap, Drs Samantha Setterfield and
Michael Douglas from Charles Darwin University have led several
research projects over the last decade—investigating both
gamba’s biology and evaluating its impact on ecosystem
invasion.
Gamba can establish in intact ecosystems
Gamba grass has rapidly spread along roadsides and other
disturbance corridors since its introduction. However, its ability
to establish and spread in relatively undisturbed sites with intact
canopies was, until recently, vigorously denied.1
To address this knowledge gap, Samantha and Michael ran controlled
trials in late 2000 specifically investigating the effect of soil
and canopy disturbance on the establishment of gamba grass. Results
showed that gamba can establish and survive regardless of canopy
cover or soil disturbance, although modified or disturbed land does
increase the grass’s chance of successfully
establishing.2
After these trials it was now clear that gamba grass threatened a
much wider part of the savannas than had previously been
thought.
Gamba grass changes fire regimes
Further studies examined the effect of gamba grass invasion on fuel
loads and fire intensity.
Compared with native grasses, gamba grass forms taller, denser
stands, that cure later in the dry season, resulting in substantial
changes to savanna fire regimes.3 Native grass fuel loads are
typically 2–4 tonnes/ha4, whereas gamba fuel loads are
typically 11–15 tonnes/ha3 and may be as high as 30
tonnes/ha. The higher fuel loads of gamba grass support early dry
season fires that are about eight times more intense that those
fuelled by native grasses.3 Later in the dry season, when the gamba
grass was more fully cured, fire intensities were almost 25 times
as high as these recorded in adjacent native grass savannas.5
Gamba grass reduces tree cover
It was suggested that this increase in fire intensity in areas
where gamba had invaded, could lead to a decline in trees and
shrubs, resulting in a process called the ‘grass–fire
cycle’.3 To test if high-intensity gamba grass fires were
leading to increased numbers of tree deaths, the CDU team combined
historic and current aerial photography of areas in the Darwin
rural area, together with field surveys. They found that over 12
years there was a 50% reduction in tree canopy cover.6 This
dramatic change in the structure of savanna vegetation demonstrates
the serious risk that gamba grass poses to the savannas across
northern Australia.
Gamba grass alters hydrology
Further collaborative research, led by Dr Lindsay Hutley (CDU),
examined patterns of soil moisture dynamics in native grass and
gamba grass plots. Of particular interest was soil-drying patterns
in the dry season and how this varied with soil depth. Evergreen
savanna trees exploit deeper and deeper soil horizons for moisture
uptake as the dry season progresses. Sensors were installed in
adjacent gamba and native grass plots to a depth of 1 m to track
these patterns of soil moisture through three wet–dry
seasonal cycles.
Lindsay found that the gamba plants used larger amounts of water
than native grasses.7 Water use also occurred for a longer period
into the dry season, with the deeper rooted gamba extracting soils
moisture from depth. This meant that a larger deficit in soil
moisture developed under gamba grass plots at these deeper horizons
(50 and 100 cm), suggesting increased competition between evergreen
trees and dense gamba grass stands for soil moisture. This enhanced
competition represents another stress that gamba grass invasion
imposes on the savanna ecosystem, particularly the woody
components.
Gamba grass alters nitrogen cycling
Natalie Rossiter has completed her PhD on the effects of gamba
grass on nitrogen cycling. Natalie found that gamba grass invasion
changes the amount of nitrogen stored in various components (pools)
of the savanna ecosystem (e.g, vegetation, litter, and soil) and
the rate of transfer of nitrogen among these components (fluxes).
Compared to plots dominated by native grasses, grass-nitrogen pools
in gamba grass plots were seven times higher, soil nitrate
availability was three times lower and soil ammonium availability
three times higher.8
The large changes in soil nitrogen availability may be due to gamba
grass inhibiting the process of nitrification in the soil, as it
does in its native range in Africa. Ammonium is its preferred
nitrogen source8, so by preventing nitrification and accumulating
ammonium, gamba can increase its own competitive superiority over
native grasses. This phenomenon may help explain the apparent
paradox of a highly productive grass thriving in a low-nitrogen
ecosystem. Natalie also found that the larger above-ground nitrogen
pool, and the higher fire intensity in gamba grass plots, doubled
the fire-mediated nitrogen losses (via the process of
volatilisation) 5. In the long term, large, frequent, fire-mediated
nitrogen losses in savannas invaded by gamba grass are likely to
reduce levels of soil nitrogen.
Ongoing research
Despite the widespread community concern over gamba and its
continued spread in northern Australia1, the grass is still not a
declared weed in either the Northern Territory or Queensland, and
there are no restrictions on its sale or use. However, researchers
are now providing the scientific data to demonstrate the risks
associated with this invasive grass, and consequently the urgent
need for a strong and coordinated management response.
The TS–CRC project has led to an ongoing research program
aimed at improving management of tropical invasive grasses. The CDU
team (Samantha Setterfield, Michael Douglas, Adam Drucker, Natalie
Rossiter and Kristine Brooks) are working in collaboration with
NRETA’s Dr Keith Ferdinands and Piers Barrow, and staff from
the WA Department of Food and Agriculture, Queensland EPA and
Biosecurity QLD to develop a set of best practice guidelines for
invasive grass management including control techniques and mapping
and monitoring guidelines.
The team is also working with Dr Lisa Wainger (University of
Maryland) to develop economic-based tools to prioritise management
actions. The suite of projects is funded by the Natural Heritage
Trust Joint Steering Committee and the Department of Environment
and Heritage through the CERF and Defeating the Weeds Menace
programs.
