Samantha Setterfield

Background/Question/Methods The social, economic and environmental impacts of invasive plants are well recognized. However, the social and economic costs of managing and eradicating invasive plants are rarely accounted for in the spatial prioritization of funding for weed management. We examine how current spatially explicit prioritization methods can be extended to identify optimal budget allocations to both eradication and control measures of invasive species to minimize the costs and likelihood of reinvasion. Our framework extends recent approaches to systematic prioritization of weed management to account for spatially variable environmental, social and cultural assets that are threatened by weed invasions. We apply our method to the Daly catchment in the Northern Territory which has significant conservation and development values which are threatened by gamba grass (Andropogon gayanus), a highly invasive species recognized by the Australian government as a Weed of National Sign...

Financial mechanisms such as offsets are one strategy to abate greenhouse gas emissions. However, in the case of carbon offsets, if the carbon is released due to intentional or unintentional reversal through environmental events such as fire, the finan-cial liability to replace lost offsets will likely fall on the provider. In order to manage this risk, an understanding of the spatial and temporal distributions of threats is needed. We use the case study of savanna burning, an approved greenhouse gas abatement methodology under the Carbon Farming Initiative in Australia, to examine the risks posed by high biomass invasive grasses, such as gamba grass (Andropogon gayanus Kunth), to carbon markets in northern Australia. We focus our analysis on the threat of gamba grass to savanna burning due to its documented impacts of increased fuel loads and altered fire regimes. We build on an initial assessment of the spatial and financial extent to which gamba grass poses a risk to savanna burn...

Comparative studies of plant resource use and ecophysiological traits of invasive and native resident plant species can elucidate mechanisms of invasion success and ecosystem impacts. In the seasonal tropics of north Australia, the alien C4 perennial grass Andropogon gayanus (gamba grass) has transformed diverse, mixed tree-grass savanna ecosystems into dense monocultures. To better understand the mechanisms of invasion, we compared resource acquisition and usage efficiency using leaf-scale ecophysiological and stand-scale growth traits of A. gayanus with a co-habiting native C4 perennial grass Alloteropsis semialata. Under wet season conditions, A. gayanus had higher rates of stomatal conductance, assimilation, and water use, plus a longer daily assimilation period than the native species A. semialata. Growing season length was also ~2 months longer for the invader. Wet season measures of leaf scale water use efficiency (WUE) and light use efficiency (LUE) did not differ between the two species, although photosynthetic nitrogen use efficiency (PNUE) was significantly higher in A. gayanus. By May (dry season) the drought avoiding native species A. semialata had senesced. In contrast, rates of A. gayanus gas exchange was maintained into the dry season, albeit at lower rates that the wet season, but at higher WUE and PNUE, evidence of significant physiological plasticity. High PNUE and leaf (15)N isotope values suggested that A. gayanus was also capable of preferential uptake of soil ammonium, with utilization occurring into the dry season. High PNUE and fire tolerance in an N-limited and highly flammable ecosystem confers a significant competitive advantage over native grass species and a broader niche width. As a result A. gayanus is rapidly spreading across north Australia with significant consequences for biodiversity and carbon and retention.

ABSTRACT Background/Question/Methods Tropical savannas are the dominant vegetation across northern Australia, covering approximately ¼ of the continent. The region is characterised by frequent fire, with approximately 50% burnt each year. During the 20th Century, hundreds of grass cultivars were introduced to northern Australia for the purpose of improving agricultural potential. Many of these species are now major weeds with some species of particular concern due to their effects on fire behaviour. The most serious is Andropogon gayanus Kunth. (gamba grass), which was introduced from Africa for pastoral production but has now invaded significant areas of the Australian savanna region, including land managed for conservation, rural/residential purposes, agriculture, defence and mining. The objective of this research was to quantify the changes in fire regimes following broadscale invasion, and document environmental, cultural and economic consequences. Results/Conclusions Compared with native grasses, gamba grass forms substantially taller and denser stands that cure later in the dry season. The large fuel loads resulting from gamba grass invasion support fires 5 to 10 times more intense than those fuelled by native grasses. The change in fire fuel properties means that flame height is significantly increased and we have recorded the first passive crown fires in the region in the gamba invaded areas. The change in fire regime caused a major decline in woody vegetation, which results in a major loss of above-ground carbon store. Fire risk has increased dramatically and this presentation describes the transformation in changes in fire management practices required to mitigate this threat, resulting in a 10 times increase in the cost of fire-management resources.

... ACKNOWLEDGMENTS This project was funded by the Tropical Savannas Management CRC. CSIRO Sustainable Ecosystems provided Natalie Rossiter with Ph.D. funding. ... Functional Ecol-ogy 14, 183-94. Moore, PHR, Gill, AM and Kohnert, R. (1995). ...

ABSTRACT 1. The understanding of large-scale patterns in expanding populations of alien invasive plants can be used to infer the environmental limiting factors, habitat heterogeneity and, ultimately, the range expansion potential of invasive plants. 2. We used multivariate analysis and a novel quantile regression technique accounting for spatial autocorrelation to compare and contrast factors influencing the abundance and distribution of the African grass Andropogon gayanus (gamba grass) at two large-scale invasion sites in the tropical savanna region of Australia. We collected data using aerial and ground surveys and tested for limiting factors using three landscape-scale indices related to soil quality, soil moisture and invasion history. 3. In one site, gamba grass was principally colonising drainage lines and riparian areas. Occupation of these areas was limited in proportion to the distance from the original gamba grass source. In the second site, gamba grass abundance was independent of distance from the original source and was well established in all vegetation communities, although abundance was also limited in higher elevation sites away from drainage lines. 4. Comparisons between these sites with different patterns of invasion enabled the estimation of both the invasion pathways and range expansion potential of gamba grass. Our results indicated that gamba grass spreads from riparian communities to invade upland sites and has the potential to invade 70% of north Australia's upland savanna communities. 5. Aerial surveys comprehensively assessed patterns over a larger area than ground surveys and were much more economical. 6. Synthesis and applications. Large-scale surveys across multiple sites are critical to understanding the dynamics of recent alien species invasions where little is known about the pattern and potential range of spread. The application of quantile regression and aerial surveys shows promise as aerial surveys are efficient at capturing a large amount of data. The novel quantile regression technique we demonstrate here can account for both spatial autocorrelation and noisy ecological data from aerial surveys while returning robust results. We were thus able to demonstrate widespread colonisation of creek lines by gamba grass and recommend that management focuses on detection and eradication along drainage lines in addition to the present focus on transport corridors.

... Samantha A. Setterfield1,2, Sean Bellairs1,2, Michael M. Douglas1,2 and Taegan Calnan1 1 School of Science and Primary Industries, Charles Darwin University, Darwin, Northern Territory 0909, Australia 2 ... Ewart at Shoal Bay and the perennial Themeda traindra Forssk. ...

Abstract This study investigated the effect of three experimental fire regimes on the fecundity, ovule development and seedfall of two common wet-dry tropical savanna eucalypts, Eucalyptus minima and Eucalyptus tetrodonta, in northern Australia. Both species flower early in the dry season and ovule development occurs during the dry season. This coincides with a period of frequent fires. The three fire regimes considered were applied for four years between 1990 and 1994. These regimes were (i) Unburnt, (ii) Early, fires lit early in the dry season, and (iii) Late, fires lit late in the dry season. The treatments were applied to nine catchments (15–20 km2) with each fire regime replicated three times. Fire intensity typically increases as the dry season proceeds. Therefore, early dry season fires generally differ from late dry season fires in both their intensity and their timing in relation to the reproductive phenology of the eucalypts. Late dry season burning significantly reduced the fecundity of both species, whereas Early burning had no significant effect. Ovule success was significantly reduced by the Early burning for both species. The Late burning significantly reduced ovule success in E. tetrodonta, but not in E. miniata. The results suggest that fire intensity and fire timing may both be important determinants of seed supply. Fire intensity may be a determinant of fecundity, whereas fire timing in relation to the reproduction phenology may have a significant impact on ovule survival. Both fire regimes resulted in a substantial reduction in seed supply compared with the Unburnt treatment. This may have a significant impact on seedling regeneration of these tropical savanna eucalypts.