Managing Tropical Woodlands to Control Exotic Woody Weeds

Summary

This project concentrated on developing practices and strategies that are effective in containing and controlling invasive shrubs in northern Australia and based on an understanding of their ecology and population biology. It has built on the work completed during MRC Project CS219. Three of the four species that were the focus of this project are listed among the twenty Weeds of National Significance (WONS) identified in conjunction with the National Weeds Strategy. These are rubber vine (Cryptostegia grandiflora), prickly acacia (Acacia nilotica) and mesquite (Prosopis spp.). The fourth species, chinee apple or Indian jujube (Ziziphus mauritiana) is a declared weed in Queensland and the Northern Territory and is present in northern Western Australia. Because each of these species is predominantly a weed of extensive rangelands, there are severe economic constraints to their management. In extensive grazing situations, it is especially important to identify practices and strategies of weed control that are low-cost per unit area of land threatened or infested. Crucial to achieving this goal are questions relating to the timing of weed control activities as well as cost-effective broadacre control techniques.

There have been very significant developments in the management of these species since the early 1990s, in part a result research conducted through NAP Projects. First, it was demonstrated by MRC Project CS219 and other work that fire is very effective against rubber vine, especially small individuals. Mortality rates after a single fire may exceed 90% for plants under 1m high and 50% for plants over 2m high. Second, two biological control agents have become firmly established. These are the now well-known rubber vine rust (Maravalia cryptostegiae) and the moth Euclasta gigantalis. Both agents repeatedly defoliate rubber vine plants during the growing season. Third, the National Weed Strategy was released in 1997, providing stimuli and mechanisms for encouraging action against weeds and prompting the development of national strategies against specific weeds, most especially against those that were identified as WONS. Each of these developments has been important in relation to MLA Project NAP3.206.

There has been a consolidation of the promising results with prescribed fire against rubber vine. This has involved a continuation of research as well as larger scale demonstrations of fire’s effectiveness. Importantly, it has been noted that there are likely to be several important interactions between fire and biological control agents. On the one hand, the defoliating agents are likely to alter the dynamics of herbaceous and litter fuels and so change the frequency and intensity of fires. Fuel loads are likely to be greater where defoliation has increased the availability of light at ground level and fallen rubber vine leaves will add to this, especially in riparian areas. Second, while the effect has not been quantified, repeated defoliation apparently decreases fruit production by rubber vine. This is likely to reduce the rate of spread of rubber vine into new areas (and whatever scale) and the rate of recolonisation of treated areas. Higher probabilities of effective fire and reduced rates of population increase will probably diminish the frequency with which prescribed fires are required and so alter the economic consequences.