Impacts of exotic grasses

Leaders: Dr Samantha Setterfield, Dr Michael Douglas,Charles Darwin University, Darwin
Mr John Clarkson,  Queensland Parks and Wildlife Service, Mareeba

Full title: The impacts of exotic grass establishment on basic savanna function: using ecological information for better management

Project 1.1.5

Summary | Objectives | Modelling | Weed Management CRC | Links to CRC research themes and projects | Outputs | Project team |

Summary

The aim of Theme 1 is to provide an understanding of the landscape ecosystem processes and functions of the tropical savannas. Although savanna ecosystems are defined by the presence of a grass-dominated understorey, we have a poor understanding of the functional role of grasses, as most ecological research in the Australian savannas has focused on woody vegetation. The limited information available has, however, highlighted the importance of native grasses in maintaining healthy landscapes. For example, native grasses have been shown to play an important functional role in catchment water use and carbon uptake (Hutley et al. 2000, Eamus et al. 2001) and characteristics of the grass understorey are important determinants of fire regimes (Williams 1999). Grass also constitutes a food resource for a variety of savanna fauna ranging from grasshoppers to macropods and grass seeds are a vital food resource for granivorous birds, many of which are in decline (Garnett and Crowley 1995; Franklin 1999).

Coupled with the increasing awareness of the functional significance of native grasses in savanna landscapes is the growing concern among savanna land managers worldwide about the displacement of native grasses by exotic grasses (Williams and Baruch 2000; Whitehead 2000). Research elsewhere has shown that the displacement of native savanna grass species by exotic African grasses has affected ecosystem function by

1. altering productivity,
2. altering nitrogen and carbon cycling,
3. altering ecosystem water balance,
4. increasing the intensity and frequency of fire,
5. altering stand structure and composition, and
6. compromising ecosystem stability (Williams and Baruch 2000).

Exotic grass establishment is currently seen as a significant threat to the integrity of Australia’s tropical savannas (Whitehead 2000; Bowman 1999; Russell-Smith et al. in review). However, the impact of the exotic grasses is impossible to determine due to the paucity of data on the functional role of both native and introduced grass species.

This project will focus on the functional ecology of selected native and exotic grass species in the mesic savannas of the NT. To elucidate the effects of invaders, researchers recommend comparative research on the invaders and native species (Mack 1996). This project will use a comparative approach to determine how the invaders alter:

• hydrology
• the pools and fluxes of nitrogen,
• fire regimes (fire intensity and frequency), and
• community structure

This project will focus on the functional ecology of selected native and exotic savanna grass species. We will use a comparative approach to determine how the invaders alter hydrology, the pools and fluxes of nitrogen, fire regimes (fire intensity and frequency), and community structure.

Data will be used to calibrate components of the Flames and Savanna Au models (Project 1.1.1), which will be used for scenario modelling of vegetation management options. A collaborative link will be developed with the Weed Management CRC, which will enable economic and social implications of grass management to be incorporated.

Objectives

To compare basic savanna function under native and exotic grasses. Specifically, we will investigate:

• water use
• carbon assimilation
• nitrogen relations
• fire regimes, and
• stand structure, including recruitment processes.
• To provide vegetation management options based on simulation modelling of project results

Modelling

To determine appropriate savanna land management actions, it is necessary to understand and then evaluate the impacts of changes in grass composition and abundance. For example, will land-management actions that result in changes in native grass composition result in changes in ecosystem function? Will such changes have off-site effects at broader temporal and spatial scales? Is control of exotic grasses necessary if they have little impact on ecosystem processes?

The tools that will be used to predict the effects of altering grass species composition on ecosystem function at the landscape scale are the FLAMES and SAVANNA AU models. Therefore planning of this project was undertaken in collaboration with CRC staff in Project 1.1.1 to ensure that it generates data that will be used to calibrate components of the FLAMES and SAVANNA AU models. For example, measures of shoot:root ratios, phenology, root distribution, water use, soil available nitrogen and fuel moisture patterns of grass species have been identified as important parameters for the models, and the data will be collected in this project. Therefore, this project will enhance the capability of these two important models currently being developed by the TS-CRC and its partner agencies.

Collaboration with the Weed Management CRC

Collaborative links between this project and the Weed Management CRC (WM-CRC) are currently being developed. Some of the research and management outcomes of this project are similar to those identified by the ‘Unpalatable Grasses’ project in the WM-CRC. There is commitment from both CRC’s for collaboration and a joint project is currently being explored with Tony Grice and staff from the Weed Management CRC. A project development workshop is planned for June 2002, and the joint proposal will be developed during 2002 and will commence in 2003. Particular areas being investigated for collaboration are plant population modelling, ecophysiological modelling, and economic modelling of weed impacts. Tony Grice will be a Project Leader on this proposal. He will be undertaking this role representing the Weed Management CRC, and his primary responsibility will be to facilitate the development of the collaborative link between CRCs.

Links to CRC research themes and projects

The objective of Theme 1 is to provide up-to-date and scientifically sound information to underpin management of the tropical savannas for sustainable use and for conservation.Currently there is limited conceptual knowledge on the ecology and function of the grass component of the savanna ecosystem, and models of savanna ecology are reliant upon extrapolating from the very limited available data. The project outcomes will therefore substantially improve the conceptual knowledge on the ecology and function of the tropical savannas, and significantly improve the robustness of savanna models. The project will provide scientifically sound recommendations for managing the savanna landscape.

This project has a strong link with Project 1.1.1, and will also contribute to the outcomes of Project 1.1.4, particularly information on the role of grasses in hydrology and nitrogen and carbon cycling.

Project staff will ensure that outcomes will be taken up by Theme 4 projects for communication to stakeholders and incorporation into the Education products, particularly the GD/MTEM.

Outputs

• Knowledge on the ecology and function of native grass species in the Australian savannas, particularly their role in determining fire regimes and nitrogen, carbon and water cycling.
• Knowledge on the shifts in function caused by the expansion of exotic grasses on fire regimes and nitrogen, carbon and water cycling.
• Contribution to the development of a component of the Savanna AU model incorporating the effects of altering grass species on carbon, water and nutrient cycling.
• Component of ‘Flames’ model incorporating effects of altering grass species on fire regimes.
• Recommendations for management of native and exotic grasses based on an understanding of their functional role.
• A suite of communication articles on the outcomes of this research including scientific manuscripts, postgraduate theses, conference presentations, popular articles and targeted reports to management agencies on vegetation management options based on simulation modelling results.
• A workshop with stakeholders to communicate project results.

Project team

S. Setterfield, CDU
M. Douglas, CDU
L. Hutley, CDU
S. Bellairs, CDU
D. Parry, CDU
N. Rossiter, CDU
S. Schmidt, UQ
G. Cook, CSE
T. Grice, CSE
WM-CRC
R. Williams, CSE
C. Wilson, PWCNT
Weed management CRC staff