Investigating the impacts of logging and regeneration burn severity on the soil microbiome and comparing them with natural wildfire disturbance
Creators: name: Ammitzboll, Hans
Publisher: name: University Of Tasmania
Year: 2023

description: The soil microbiome is a fundamental component of terrestrial ecosystems. Bacterial and fungal communities in the soil are responsible for nutrient availability, as well as driving the decomposition and recycling of organic matter and soil formation. Recent advances in DNA sequencing technology have greatly increased our ability to characterise the soil microbiome, allowing researchers novel opportunities to investigate the response of these essential microbiota to environmental disturbances. In the temperate wet eucalypt forests of Tasmania, periodic wildfire is the major type of natural disturbance. Accordingly, the silvicultural management of these forests employs the use of controlled fire after timber harvesting to mimic the natural ecological processes underlying forest regeneration post-wildfire.<br>The objectives of this thesis were to investigate the response of soil bacterial and fungal communities, and the aboveground vegetation, to the clear-fell, burn, and sow method widely employed in the wet eucalypt forests of Tasmania and to compare these findings with the impacts of natural wildfire disturbance. I addressed these objectives using a combination of sequencing techniques and quantitation of bacterial and fungal DNA, vegetation community analyses and soil chemistry measures. By sampling soils from forests that were clear-felled and subjected to a burn severity gradient, ranging from unburnt to those burnt at high severity and comparing them to adjacent unlogged and unburnt reference forests over a period of 12-months I determined the following.<br>First, at one-month post-burn, burn severity was a strong driver of bacterial and fungal community compositions in the upper soil profile (0 – 10 cm depth), with high severity burns significantly reducing the abundance and diversity of these communities compared to unlogged and unburnt reference forests. These fire induced impacts were largely restricted to the upper soil profile, with only weak impacts on the subsoil (20 – 30 cm depth).<br>Second, within 12-months post-burn, bacterial communities in soils burnt at high severity recovered lost abundance and diversity, and their community compositions across all disturbed soils became more similar to the unlogged and unburnt reference soils. Conversely, the impacts of high severity burning on fungal abundance and diversity persisted over the 12-months, and community compositions in disturbed soils did not display increasing similarity towards the unlogged and unburnt reference soils through that time. Despite this, there was evidence for the succession of disturbance-responsive fungal taxa over the study period.<br>Third, burnt soils had stronger positive relationships between the abundance of nitrogen-fixing bacteria and ectomycorrhizal fungi, soil chemistry, total vegetation coverage and Eucalyptus coverage than unburnt soils. These findings suggested post-logging fire helps promote beneficial plant-soil relationships, despite the immediate negative impacts of burning on overall microbial abundance and diversity. Post-logging burning also reduced the abundance and diversity of putative fungal plant pathogens, which may have also benefited plant growth in burnt soils.<br>Finally, areas that were impacted by the 2019 Tasmanian wildfires were sampled and compared to areas subjected to clear-felling and regeneration burning, and unlogged and unburnt reference forests. From this, I determined that wildfire could cause greater decreases to the abundance and diversity, and greater changes to the composition of soil bacterial and fungal communities than logging and regeneration burning. The impacts of wildfire and regeneration burning differ in several aspects, which may influence the recovery of the soil microbiome, including differences in the season of burn, characteristics of fuels and interactions with the overlying vegetation.<br>This thesis demonstrates that soil bacterial and fungal communities are relatively resilient to current silvicultural practices, with post-logging regeneration burning having benefits to both above and belowground community regeneration. I emphasise the importance of maintaining a mosaic of burn severities across a site for the microbial recolonisation of burnt soils, as well as highlighting several putatively functional taxa that respond positively or negatively to the impacts of burning and warrant further study. However, I also show that the impacts of the clear-fell, burn, and sow method differ substantially from those of natural fire disturbance on soil bacterial and fungal communities and the aboveground vegetation. Given the predominance of non-stand replacing fires in most Tasmanian wet eucalypt forests, I argue that the continual adoption of variable retention forestry would be beneficial for maintaining natural ecological processes regarding the post-fire recovery of soil microbial environments and their overlying vegetation communities, particularly in Eucalyptus obliqua dominated forests.