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Effects of livestock grazing on the structures and composition of jarrah (Eucalyptus marginata) woodland remnants

Pettit, Neil (1995) Effects of livestock grazing on the structures and composition of jarrah (Eucalyptus marginata) woodland remnants. Masters by Research thesis, Murdoch University.

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Grazing by livestock has led to passive clearing of the majority of remaining areas of native vegetation on farmland (remnants) in the Collie river catchment in the south-west of Western Australia. Livestock grazing in these areas removes most of the understorey and prevents recruitment. This thesis documents the impact of livestock grazing on the vegetation dynamics of these remnants through changes to floristics and structure and effects on the soil. It also looks at the possibility of rehabilitation of degraded areas by assessing seed reserves in the soil seedbank and the regeneration of species with the exclusion of grazing. A review of the literature provided a background to this study and looks at the broader issues of the significance of remnants of native vegetation as well as the ecology of disturbance in terms of vegetation response and resilience and deals with the problems of management. Species diversity and richness have decreased with increased grazing intensity and these were negatively correlated with proportion of exotic species. Site ordination of a large sample of remnants placed sites in two major groups based on grazing intensity, with position of sites influenced by proportion of exotic species, proportion of perennial herbs and shrubs and species diversity and richness. Cover and abundance of native perennial species decreased, but increased for exotic annual species with a gradient of response between heavily grazed, lightly grazed and ungrazed sites. Other life form groups such as native annuals, geophytes and native grasses were not significantly affected by grazing. Perennial species that are able to resprout from an underground storage organ as well as germinate readily from seed appear the most resilient to grazing disturbance. Effects of grazing disturbance on the soil included increased surface soil compaction and water repellency as well as significant increases in concentrations of soil nutrients, particularly N and P. Age structure of overstorey species indicated that there has been a lack of recruitment at the heavily grazed sites for some time. Germination of overstorey species took place each year of the study but mortality of seedlings was high, with no seedlings surviving after one summer at the heavily grazed sites. Experiments on the soil seedbank showed a dominance of seed from exotic annual species and a lack of seed from native perennial species within the heavily grazed sites, indicating that natural regeneration is unlikely from this source. Heat treatment of soil samples showed a decrease in germination of exotic species and an increase in the germination of native species. After l to 3 years there was a significant increase in number and cover of native perennial species in exclosure plots, mainly from resprouting. The greatest increase in cover in exclosures was seen for native perennial grasses, while abundance of exotic annuals did not increase significantly compared to adjacent open plots. Time series ordination showed the movement of exclosure plots towards the ungrazed plots after three years, indicating the increase in floristic similarity between the exclosures and the ungrazed plots. This study has shown that grazing has resulted in a shift from a community dominated by native perennial species to one dominated by exotic annual species. High grazing intensity and short grazing history, climatic variability and effects on the soil are the major factors affecting the observed ·responses of the vegetation to grazing. Natural regeneration in degraded remnants is possible if livestock are excluded. Rehabilitation of some sites is also required and a procedure is suggested.

Item Type: Thesis (Masters by Research)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Ladd, Phil
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