Suppression of germination and establishment of native annual rice by introduced para grass on an Australian monsoonal floodplain

The native annual wild rice (Oryza meridionalis) underpins the vertebrate food chain on the monsoonal floodplains of northern Australia. It is being displaced by the exotic perennial para grass (Urochloa mutica). This study reports on a field experiment, where wild rice seed was sown into 1 m 2 quadrats of established para grass. Para grass cover was manipulated above the wild rice seed bed, including clipping and herbicide application. The behaviour of wild rice seed under para grass cover in this study was then compared to its behaviour under wild rice cover in a previous study. Oryza meridionalis plants did not establish under para grass treatments, including herbicide treatments that successfully killed para grass. Retrieval of buried bags of wild rice seeds revealed that germination was suppressed. Although 40% of seeds remained viable in the soil for more than 2.5 years, suppressed germination prevented establishment of O. meridionalis populations. Under wild rice cover most seed germinated by this time. High biomass and complex architecture of para grass cover may modify the seedbed, preventing wild rice seeds receiving dormancy-breaking or germination cues. Simply spraying established para grass with herbicide may not allow re-establishment of wild rice. Additional site treatments may be required for reestablishment by O. meridionalis in wetlands managed for biodiversity.