Penelope A. Wurm

Object-based mapping of native vegetation and para grass (Urochloa mutica) on a monsoonal wetland of Kakadu NP using a Landsat 5 TM Dry-season time series

This paper evaluates the use of multi-temporal Landsat 5 TM for object-based classification of native wetland vegetation and the perennial aquatic weed para grass within Kakadu National Park, Northern Territory, Australia. Using identical training data and segmentation, a nearest-neighbour classification produced from a four-image (dry season) time-series was compared with four ‘single-date’ classifications produced from the individual images of the same series. A 15-class vegetation map generated from the multi-date classification produced an overall accuracy of 82 percent (kappa = 0.80). This was an average increase in accuracy of 25 percent (kappa = 0.28) compared to single-date classifications. The multi-date image composite also improved segmentation quality and spectral separability of vegetation classes. Reliable maps of wetland vegetation, potentially useful for strategic conservation, can be produced by integrated, object-based, analysis of multi-temporal Landsat.

Acacia holosericea (Fabaceae) litter has allelopathic and physical effects on mission grass (Cenchrus pedicellatus and C. polystachios) (Poaceae) seedling establishment

Invasion of grass weeds is a major threat for ecosystems. Mission grass (Cenchrus pedicellatus and C. polystachios) vigorously competes with native vegetation and has become a serious problem in northern Australian savanna. A lower density of mission grass has been observed under the canopy of stands of native Acacia holosericea. We used a series of laboratory and shade house experiments to assess the potential for allelopathy and the role of litter on germination, emergence and seedling growth of these two species of mission grass. Different concentrations of aqueous leaf extract of A. holosericea were used to assess allelopathic effects on germination. Various depths and types of litter were used to investigate the allelopathic and physical effects of litter on emergence and growth of mission grass seedlings in the shade house. Results indicate that extracts did not affect germination of either species of mission grass but root growth of seedlings was affected. Emergence of seedlings in the shade house was affected by physical litter treatments but not by allelopathy. After emergence no negative effects on seedling growth were detected. Overall we found that there was no allelopathic effect on germination and that the negative effect on emergence was due to the physical properties of the litter. This effect on emergence increased with increasing depth of litter. Allelopathy slightly inhibited root growth but once seedlings emerged, litter tended to facilitate growth. This has implications for the ecological management of mission grass on disturbed lands, using strategies such as manipulation of litter cover through Acacia establishment.

Temperature affects the dormancy and germination of sympatric annual (Oryza meridionalis) and perennial (O. rufipogon) native Australian rices (Poaceae) and influences their emergence in introduced para grass (Urochloa mutica) swards

The seed biology of two ecologically and genetically important sympatric wild rice species from northern Australia was compared – perennial Oryza rufipogon Griff. and annual Oryza meridionalis N.Q.Ng. The aim was to determine mechanisms of dormancy exhibited at seed shed and to identify factors that trigger or inhibit germination. This information was used to investigate the ecology of in situ Oryza populations in introduced para grass swards (Urochloa mutica (Forssk.) T.Q. Nguyen) and to understand interactions between the two sympatric Oryza species. Primary dormancy in the two species is similar, namely, non-deep physiological dormancy, determined by external maternal structures and broken by warm temperature treatments equivalent to dry season soil temperatures. Light quality, smoke water, gibberellic acid and nitric acid treatments had minor influences on germination. Changes to the soil profile and aboveground biomass structure due to swards of U. mutica significantly affected emergence of O. meridionalis. Thus the influence of soil temperature explains the results of previous field studies in which biomass or litter on the soil surface prevented germination. This has implications for biodiversity management on monsoonal floodplains of northern Australia, where introduced pasture species produce greater biomass than native grasslands, reduce soil temperatures and are displacing native rices. There were differences between the Oryza species – dormancy was more quickly broken in annual O. meridionalis, reflecting the reduced need for investment in seed bank persistence for annual species in annually inundated and climatically reliable wetlands.