Determining the Efficacy of Fire as a Management Tool in Grasslands Invaded by Exotic Plant Species

Fire has long been used to manage the environment. In recent times, the use of fire for management requires additional consideration of the impacts of the changing climate and habitat disruption. Additionally, the effect of fire management is greatly altered by the invasion of exotic plant species. Much of the research to date has focused on the effects of fire and exotic species on the plant communities. Few studies have considered their individual and combined effects on the plant community and the soil microbial community. To better understand the use of fire as a management tool for habitats invaded by exotic plants, two field experiments focusing on Victorian grasslands that have been invaded by exotic plant species were implemented. Several of the exotic grasses from this research appear in both sites, such as the Avena spp., Bromus spp., and Lolium rigidum, however there is limited overlap in the native plant communities. Each offered a unique insight into the some of the factors which influence changes in native plant species metrics after fire. In the semi-arid Torrumberry grassland, the influence of invasion extent (i.e., the abundance of exotic plants) and of fire on the post-fire recovery of the native and exotic plant communities were measured directly, through changes in plant species diversity and abundance, and, indirectly, through changes in the soil seed bank, soil chemistry, the soil microbial community, and fire characteristics (Chapters 2 and 3). In the temperate Nicola Davis Conservation Grassland, the direct effects of fire frequency in conjunction with herbicide management were measured by determining native and exotic species diversity and abundance, and the indirect effects of fire frequency were measured through analysis of soil chemistry and the soil microbial community (Chapters 4 and 5). By measuring plant community, soil chemistry, and microbial community components within these grasslands, a holistic view of the role of fire in contributing to community shifts was created. In Torrumberry, the effect of fire management across two contrasting soil types within a grassland that differed in their invasion extent was tested. The effects of fire on the plant community were highly context dependant. In the less productive, nutrient-poor red soil, fire benefitted natives and inhibited exotic plants recovery and their seed germination. In the nutrient-rich grey soil, fire facilitated exotic species invasion and negatively affected native species. Finally, fire temperature increased with exotic species biomass, however, this did not impact the native plant community. The effect of fire on the soil microbial community, the richness, diversity, evenness, and community composition of both bacteria and fungi were minimally influenced by fire. Instead, soil type (red versus grey) was a strong determinant of the community composition of both bacteria and fungi in these soils. Combined, these results showed that the delivery of fire management should take into consideration the soil environment and extent of invasion to gauge its efficacy. In Nicola Davis, the effect of fire frequencies on plant and soil microbial communities across a habitat moderately invaded by exotic plant species was tested. Annual fire was shown to have a stronger impact on both the native and the exotic plant communities than one fire event. Additionally, increases in the abundance of exotics did not impede on the recovery of natives after fire. Fire in general, irrespective of its frequency, altered the composition of soil bacterial and fungal communities. For fungal communities, changes to the community composition also varied through time. Results from Chapters 4 and 5 show that increasing the frequency of fire management can positively affect the plant community, but that additional weed management, and the ongoing effects that high fire frequency might have on the microbial community need to be considered. Together these results show that 1) fire benefits native species when invasion by exotic plant species is low, 2) fire has little impact on soil chemistry, but soil characteristics (such as soil type in Torrumberry) strongly influence plant and microbial communities, and subsequently influence how plant communities respond to fire, and 3) soil microbial communities are resilient to fire impacts. As such, fire can be used by managers to the benefit of the native plant community despite the positive effects fire may have on exotic species, and because the resilient microbial community can persist alongside fire management.<p></p>