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 |

Background

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.

Supervisors

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

Bibliography

Barrow, P. (1995). The Ecology and Management of Gamba Grass (Andropogon gayanus Kunth). Final Report to the Australian Nature Conservation Agency. Northern Territory Department of Primary Industries and Fisheries, Darwin.

Baruch, Z. (1996). Ecophysiological aspects of the invasion by African grasses and their impacts on biodiversity and function of neotropical savannas. In: Biodiversity and Savanna Ecosystem Processes (eds.O. T. Solbrig, E. Medina, and J. F. Silva).pp.79-93. Springer-Verlag, Berlin.

Baruch, Z., Ludlow, M., M., and Davis, R. (1985). Photosynthetic responses of native and introduced C 4 grasses from Venezuelan savannas. Oecologia 67 .

Bilbao, B., and Medina, E. (1990). Nitrogen use efficiency for growth in a cultivated African grass and a native South American pasture grass. Journal of Biogeography 17 :421-425.

Bowman, D. M. J. S. (1999). Introduced grasses: triumph or Trojan horse? Savanna links 10 :1-3.

Byram, G. M. (1959). Combustion of forest fuels. In: Forest Fire Control and Use (ed.K. P. Davies).pp.61-89. McGraw-Hill, New York.

Cook, G. (1991). Gamba grass: impending doom for the Top End Savannas. CSIRO TERC Newsletter 91 :2.

D'Antonio, C. M., and Vitousek, P. M. (1992). Biological invasions by exotic grasses, the grass/fire cycle, and global change. Annual Review of Ecology and Systematics 23 :63-87.

Eamus, D., Hutley, L. B., and O'Grady, A. P. (2001). Daily and seasonal patterns of carbon and water fluxes above a north Australian savanna. Tree Physiology 21 :977-988.

Ehrenfeld, J. G., and Scott, N. (2001). Invasive species and the soil: effects on organisms and ecosystem processes. Ecological Applications 11 :1259-1260.

Evans, R. D., and Belnap, J. (1999). Long-term consequences of disturbance on nitrogen dynamics in an arid ecosystem. Ecolgy 80 :150-160.

Evans, R. D., Rimer, R., Sperry, L., and Belnap, J. (2001). Exotic plant invasion alters nitrogen dynamics in an arid grassland. Ecological Applications 11 :1301-1310.

Gill, A. M., and Moore, P. H. R. (1994). Some ecological research perspectives on the disastrous Sydney fires of January 1994. In: The Second International Conference on Forest Fire Research, Coimbra, Portugal 1994 pp.63-72.

Gordon, D. R. (1998). Effects of invasive, non-indigenous plant species on ecosystem processes: Lessons from Florida. Ecological Applications 8 :975-989.

Heywood, V. H. (1989). Patterns, extent and modes of invasions by terrestrial plants. In: Biological Invasions: a Global Perspective (eds.J. A. Drake, H. A. Mooney, F. de Castri, R. H. Groves, F. J. Kruger, M. Rejmanek, and M. H. Williamson).pp.31-55. John Wiley & Sons, Chichester.

Howard, T. (2001). Living with Gamba grass: the Latest News on Research, Extension and Management. Living with Gamba grass newsletter April 2001 .

Howard, T. (2002). Exotic grasses and fire. In: Savanna Burning: Understanding and Using Fire in Northern Australia (eds.R. Dyer, P. Jacklyn, I. Partidge, J. Russell-Smith, and R. J. Williams).p.25. Tropical Savannas CRC, Darwin.

Hurst, D. F., Griffith, D. W. T., and Cook, G. D. (1994). Trace gas emmissions from biomass burning in tropical Australian savannas. Journal of Geophysical Research 99 :3619-3634.

Hutley, L. B., O'Grady, A. P., and Eamus, D. (2000). Evapotranspiration from Eucalypt open-forest savanna of Northern Australia. Functional Ecology 14 :183-194.

Ibarra-Flores, F., Cox, J. R., Martin-Riveria, M., Crowl, T. A., Norton, B. E., Banner, R. E., and Miller, R. W. (1999). Soil physicochemical changes following Buffelgrass establishment in Mexico. Arid Soil Research and Rehabilitation 13 :39-52.

Mack, M. C., D'Antonio, C. M., and Ley, R. E. (2001). Alteration of ecosystem nitrogen dynamics by exotic plants: a case study of C4 grasses in Hawaii. Ecological Applications 11 :1323-1335.

Medina, E., and Silva, J. F. (1990). Savannas of northern South America: a steady state regulated by water-fire interactions on a background of low nutrient availability. Journal of Biogeography 17 :403-413.

Moore, P. H. R., Gill, A. M., and Kohnert, R. (1995). Quantifying bushfires for ecology using two electronic devices and biological indicators. CALMScience Supplement 4 :83-87.

Rossiter, N. A., Setterfield, S. A., Douglas, M. M., and Hutley, L. B. (2003). Testing the grass-fire cycle: alien grass invasion in the tropical savannas of northern Australia. Diversity and distributions 9 :169-176.

Russell-Smith, J. S., Hoare, J. R. L., Whitehead, P. J., Cook, G., D., and Brooks, K. (in review). Response of Eucalyptus open forest and woodland savanna to four experimental fire regimes, 1973-1996, at Munmarlary, monsoonal northern Australia: a final assessment. Ecological Monographs .

Solbrig, O. T., Medina, E., and Silva, J. F. (1996). Determinants of Tropical Savannas. In: Biodiversity and Savanna Ecosystem Processes: A Global Perspective (eds.O. T. Solbrig, E. Medina, and J. F. Silva).pp.31-39. Springer-Verlag, Berlin.

Tilman, D. G. (1982). Resource Competition and Community Structure . Princeton Univeristy Press, Princeton.

Vitousek, P. M., D'Antonio, C. M., Loope, L. L., and Westbrooks, R. (1996). Biological Invasions as global environmental change. American Scientist 84 :468-478.

Williams, D. G., and Baruch, Z. (2000). African grass invasion in the Americas: ecosystem consequences and the role of ecophysiology. Biological Invasions 2 :123-140.

Williams, R. J., Cook, G. D., Ludwig, J. A., and Tongway, D. T. (1997). Torch, Teeth, Trees and Tussocks: Disturbance in the Tropical Savannas of the Northern Territory, Australia. In: Frontiers in Ecology: Building the Links pp.55-66. Elsevier Science, Oxford.

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.

Contacts

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

Fax: 08 8946 6847

Faculty of Science, Bld 42
DARWIN, NT 0909