Theme Leader: John Ludwig, CSIRO Sustainable Ecosystems, Atherton
Theme focus | Achievements | Indicators | Healthy savannas | Change in vegetation cover | Future directions |
This theme aims to integrate and synthesise knowledge gained from projects that study how healthy savanna landscapes function. Our working definition of healthy savannas is where landscapes maintain processes to:
- retain and use limited resources such as water and nutrients
- provide food and shelter for fauna
- support viable populations of native plants and animals
- meet the material, aesthetic and spiritual needs of people.
If we are to achieve a sustainable use and conservation of savannas (our Centre's mission), we need to understand these basic landscape processes and functions.
Progress towards our aim of synthesis and integration was advanced in 1999 by a series of Theme workshops held in early November. At these workshops, our definition of savanna health was refined by specifically looking at indicators of landscape function. That is, what surrogates of landscape attributes are useful as measures of how well a savanna landscape is functioning? What are the signals that indicate that a savanna is healthy?
Useful soil surface condition and vegetation indicators were identified, along with potentially useful vertebrate and invertebrate indicators. Workshop participants listed the strengths and weaknesses of proposed indicators and discussed the studies needed to validate these indicators.
Progress was made in a number of research projects during 1999-2000 that significantly contributed to the aim of this theme by increasing our knowledge on important processes found in healthy savanna landscapes. For example, Water and Carbon Exchange of Savannas found that both Eucalyptus savannas and Melaleuca swamp forests in the Howard River catchment of the Northern Territory are unlikely to be vulnerable to damage from groundwater extracted during the wet season. However, they could be negatively impacted by such extractions during the dry season when plants in these savanna systems are most sensitive to water availability. These plant ecophysiology and hydrology studies used new techniques such as the ratios of naturally occurring stable isotopes of oxygen.
Another project that is using ratios of stable isotopes is Savanna Form and Function . It was found that savanna fires significantly impact on critical nitrogen cycles. Fires change the pools of nitrogen available for plant growth from forms of nitrogen that are readily available (ammonium) to forms that are less available (nitrate). The result is that trees and grasses in savanna that are frequently burnt have lower concentrations of nitrogen in their leaves. This has significant flow-on effects for fauna dependent on these leaves as food.
Assessing Structural Change in Tropical Woodlands used historical pairs of aerial photographs to demonstrate widespread increases in canopy cover of woody vegetation in the VRD, particularly on alluvial flats. This work was complemented by Ben Sharp's PhD study which analysed vegetation change on Bradshaw station in the VRD and Darrell Lewis's series of historical photo pairs, part of Project 2.4.1 Fire in Savanna Landscapes. Both studies reinforce the findings of project 1.1.3, while Ben's study also shows more complex changes in vegetation at the local scale.
A project that is specifically integrating our knowledge about savanna processes, such as the findings described above, is Modelling Landscape Change . This project used the Savanna simulation model to quantify the trade-off between fire and grazing. This trade-off is observed when pasture grasses are used for forage by livestock so that the opportunity for using this grass for fuel for fires to control tree and shrub thickening is lost. From these model predictions, forage versus fuel biomass guidelines for pasture management can be established. The development of such guidelines will be a focus for the theme in 2000-2001
