TS-CRC Student project - The impacts of invasive grasses on ecosystem processes in Australia's savannas

Gamba grass in the wet season
Above, gamba grass during the wet season

High intensity gamba grass fire
Gamba produces much higher intensity fires than native grasses

Charles Darwin University

Natalie Rossiter

Background | Aims and scope | Impacts on nitrogen dynamics | Impacts on fire regimes | Supervisors | Bibliography | More information |


In northern Australia, concern has been raised about the impacts of introduced grass species, in particular Andropogon gayanus (gamba grass). Gamba grass is a perennial species of African grass that was introduced in the Northern Territory as a replacement for native pastures, however gamba grass is now invading savanna ecosystems throughout the Top End (Howard 2002). This invasion could have significant consequences for native communities, as well as ecosystem function and stability, comparable to the dramatic effects of invasion documented in other ecosystems (D'Antonio & Vitousek 1992, Vitousek et al. 1996). This is of particular concern as gamba grass invasion has the potential to alter all three determinants of savanna functioning: nutrient and water availability and fire regimes. However, at present these consequences are largely unknown, as there is little information on the ecosystem effects following the replacement of native grasses with gamba.

Aims and scope

The objective of this study is to quantify the changes in ecosystem processes that occur following the establishment of gamba grass. This project will use a comparative approach to determine how the invading grass may alter:

  • Nitrogen pools and fluxes and
  • Fuel characteristics and fire regimes.

Impacts on nitrogen dynamics

One of the most significant changes in resource dynamics caused by invading grasses is the alteration of the nitrogen cycle (Evans & Belnap 1999). Exotic grass invasion can lead to significant decreases in soil nitrogen (N) availability (Ibarra-Flores et al. 1999, Evans et al. 2001). In turn, alteration in the availability of nitrogen may drive changes in species composition (Tilman 1982), creating the potential for a feedback cycle to emerge, where an invader may alter species composition and modify the ecosystem controlling community dynamics.

Nitrogen availability is a major constraint for plant growth in savannas (Medina & Silva 1990, Solbriget al. 1996). Plants generally respond by producing biomass with lower nutrient concentration. However, exotic grasses tend to preferentially allocate nitrogen to the production of leaves (Baruch 1996). It has been suggested that exotic grasses in Australia’s savannas require more nutrients than native grasses in order to achieve their higher growth rates and that they seem to use nitrogen more efficiently (Baruch et al. 1985, Bilbao & Medina 1990). The higher biomass in communities invaded by exotic grasses indicates that the amounts and rates involved in nutrient cycling should be higher than in native savannas. It is possible that the high nitrogen requirements of the exotic grasses may deplete the soil nitrogen reserves in the savannas. Exotic grasses may also affect savanna nitrogen cycling by influencing the chemistry and quantity of litter entering the soil organic matter (SOM) pool, and the microclimate conditions in which decomposition occurs (Mack et al. 2001). The alteration in N availability may be further compounded by the effect of exotic grass on fire regimes and the potential for significant nitrogen loss via volatilisation, due to frequent, intense fires.

Impacts on fire regimes

Exotic grass also has the potential to drastically alter fire regimes (D'Antonio & Vitousek 1992). Exotic grass invasion can initiate a positive feedback cycle with fire, causing a decrease in tree cover and the conversion of forests and woodlands to grasslands. D'Antonio and Vitousek (1992) termed this process the grass–fire cycle. It has been suggested that exotic grass invasion could be creating a grass–fire cycle in the savannas of northern Australia (Bowman 1999, Russell-Smithet al. in review). Particular concern has been expressed that invasion by gamba grass could be altering the fire regime characteristics of the savannas, promoting intense, late dry season fires (Williams et al. 1997, Bowman 1999, Russell-Smith et al. in review). Gamba grass is a highly productive grass, which significantly increases the fuel load in savannas, producing up to 10 times the biomass of native grasses (Barrow 1995, Bowman 1999, Howard 2001, 2002). It grows in excess of 4.5 m high, and unlike native grasses, it remains upright throughout the fire season, creating a tall standing fuel load.

Furthermore, anecdotal evidence suggests that gamba grass dries later in the dry season than native grasses. In areas invaded by gamba, this leads to a high fuel load in the mid to late dry season, which has the potential to produce intense, late-dry season fires. Others have speculated that the high fuel loads of gamba grass can build up in a single growing season, supporting fires every year, and sometimes twice a year (Cook 1991).

Such changes in fire regimes have the potential to initiate a grass–fire cycle on a broad ecological scale in the savannas of northern Australia. Fire intensity in the early dry season in savannas invaded by gamba grass is up to 12 times higher than that in native grass savannas (Rossiter et al. 2003). However, this study was based on only limited observations, and there is an urgent need for a more comprehensive study of the effect of the effect of exotic grass invasion on the prevailing fire regimes in the savannas of northern Australia.

An understanding of savanna ecosystem processes such as fire regimes, and nutrient, cycling is fundamental to understanding savanna function. Such information is vital to ensure the sustainable management of Australia’s tropical savannas. Native grasses have been identified as important elements of the savanna ecosystems, as they are significant sources of carbon (Eamus et al. 2001) and water (Hutley et al. 2000) flux in savanna ecosystems and they contribute largely to the existing fire regimes in the savannas. Furthermore, nutrient and water availability and fire regimes have been identified as the three key determinants of savanna function (Solbrig et al. 1996) and any alteration to these ecosystem processes due to invasion is likely to have significant long-term ecological consequences.


Dr Samantha Setterfield (NTU)
Dr Michael Douglas (NTU)
Dr Lindsay Hutley (NTU)
Dr Garry Cook (CSIRO-Sustainable Ecosystems)


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Williams, R. J., Gill, A. M., and Moore, P. H. R. (1998). Seasonal changes in fire behavior in a tropical savanna in northern Australia. International Journal of Wildland Fire 8 :227-239.


Ms Natalie Rossiter
PhD Student
Charles Darwin University
Tel: 08 8946 6469

Fax: 08 8946 6847

Faculty of Science, Bld 42