Year

2004

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Biological Sciences - Faculty of Science

Abstract

On the southeast coast of Australia, there are extensive infestations of the environmental weed Chrysanthemoides monilifera ssp. rotundata (L.) T. Norl. (Bitou bush). This weed is highly invasive and persistent, yet little is known about how it impacts on coastal ecosystems. Plant invasions can greatly alter the area they invade, and can modify primary productivity, plant species composition, species diversity, decrease ecosystem stability and disrupt ecosystem processes. This research aimed to determine if C. monilifera invasion changes leaf litter decomposition and nutrient cycling. Leaf litter invertebrates were also investigated as they play key roles in functioning of forest ecosystems and can greatly enhance litter decomposition and nutrient cycling. The herbicide glyphosate is widely used to control C. monilifera, however there have been few studies examining the effects of this herbicide on invertebrate communities in the field, especially on sand dunes. I aimed to determine if glyphosate application impacts leaf litter invertebrate involved in decomposition. Field studies and experiments were undertaken in five sites along the NSW coastline, each with an area heavily infested with C. monilifera and in native un-infested area. A litterbag decomposition study found that the succulent C. monilifera leaves decomposed at least three times faster than a sclerophyllous native leaf mix (Acacia longifolia var. longifolia, Banksia integrifolia and Leptospermum laevigatum) with the decomposition rates being related to the physical properties of the leaves. Nutrients were leached and mineralised quicker from the C. monilifera leaves, mostly due to the rapid decay. There was some immobilisation of sulphur and phosphorous in the native leaves when placed in the native sites and more immobilisation is expected with time. C. monoilifera leaves decayed significantly faster in coarse mesh litterbags compared to the fine mesh, indicating leaf littler invertebrates positively influenced their decomposition. Mesh size had little effect on the native leaf decomposition rate. All C. monilifera litterbags and the coarse native litterbags decomposed faster and generally had greater nutrient loss within the C. monilifera infestations. This is due to an increase in invertebrate detritivores within the C. monilifera, and dense infestations creating a protected environment with an altered microclimate. There was a greater input of litter to the native sites, and this was highly seasonal compared to the C. monilifera areas. Maximal litter fall corresponded with period of peak flowering and growth (spring-summer). Due to the higher leaf fall rate there is a greater input of nutrients to the forest floor in the native sites, even though large amounts of nutrients were withdrawn from the leaves before abscission. The slow decomposition and high litter fall resulted in greater amounts of litter accumulating on the native forest floor, which appears to act as a nutrient sink. Invasion by C. monilifera in coastal areas does not appear to change the nitrogen budget, but there are differences in where nitrogen is stored in comparison to uninvaded areas. The total soil nitrogen and ammonia nitrogen were significantly greater in most of the bitou bush sites, whereas in the native vegetation there was more nitrogen held within the leaf litter layer. Nitrogen appears to be cycled faster in the weedy areas, mainly as result of the increase in leaf quality and the speed of breakdown. Leaf litter invertebrate abundance and assemblage were compared between habitats over twelve months. The total abundance was not significantly reduced in the weedy habitat but the abundance of mites, thrips, spiders, ants, and centipedes was reduced at many sites. The invertebrate assemblages also differed between habitats, with the C. monilifera supporting a lower diversity of beetles. However, the millipedes, amphipods, earthworms, pseudoscorpions and isopods appeared to respond positively to the invasion, occurring in higher abundance and detected more frequently in the weedy areas. This has been attributed to the change in microclimate within C. monilifera infestations, which are moister and darker, which these invertebrates tend to prefer. Furthermore as the leaf litter is of lower quantity and higher quality, and possibly higher palatability, than the native sclerophyllous vegetation, the detritivores assemblages seems to have responded positively. To determine the impact of the herbicide glyphosate on the non-target litter invertebrates control and impact sites were selected in coastal hind dunes heavily infested with C. monilifera. The impact sites were sprayed with a 1:100 dilution of Roundup(superscript R) Biactive (superscript TM). The herbicide application had no direct or indirect effect on leaf litter invertebrate abundance or community composition in the four months following application. The litter invertebrate assemblages were highly variable on a small spatial scale with abiotic factors more strongly regulating leaf litter invertebrate numbers than glyphosate application. These results conflict with previous studies, indicating the detrimental indirect effects herbicide application has on non-target litter invertebrates may depend upon the application rate, the vegetation community and structure and post spray weather. Invasion by C. monilifera has changed the movement of leaf litter and the cycling of nutrients within coastal ecosystems, mainly through a change in quality and quantity of leaf litter. It has impacted on the litter invertebrates, which has amplified the increase in decomposition rate. The change in soil nutrient availability could increase the competitive superiority of C. monilifera directly by increasing growth rate, or indirectly by impairing the establishment of native species.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.