Floristics, dominance and diversity within the threatened Themeda grassy headlands of the North Coast Bioregion of New South Wales

Abstract

We surveyed subtropical closed tussock grasslands on headlands within the North Coast Bioregion of New South Wales. The aims of the investigation were to provide baseline data on this listed threatened community. Additionally, we tested the effects of macropod grazing and biomass changes on assemblages. In total, 117 full floristic survey plots were placed within extant natural grassland occurrences on 16 headlands. Data collected included the percentage cover and frequency of each species. In addition, we ranked the three species contributing most to overall biomass. Analysis of the full floristic cover data (percentage cover) was performed using the Kulzynski association measure and Unweighted Pair Group Arithmetic Averaging fusion strategy. Further analysis was performed using constrained and unconstrained ordinations and Generalised Additive Modelling using species frequency data and explanatory variables including overall, and proportional species biomass, and macropod grazing impact. Seven distinct grassland assemblages were derived, of which three were Themeda dominated. Concentrating on the Themeda-dominated assemblages, it was found that increasing biomass depth and a reduction in macropod grazing impact were associated with a reduction in plot species richness and diversity and trait richness and diversity. These changes were associated with a shift in floristic assemblage identity. All three Themeda assemblages are likely to provide a unique combination of functional resources and all should be maintained in order to promote landscape diversity. We predict that use of frequent fire is likely to cause homogenisation (reduced landscape richness) and loss of important components including listed threatened taxa.