ABSTRACT
This thesis examines the effect of climate change on a high-elevation ecosystem of Australia. It is expected that increases in temperature and changes in rainfall patterns will strongly influence both plant communities and the soil environments that support them. Microbial communities play a central role in these systems by regulating nutrient cycling, supporting plant growth, and maintaining ecosystem stability. However, how climate stressors, especially multiple climate stressors interact to reshape these microbial communities and how such changes modify plant-microbe interactions remain poorly understood in high-elevation ecosystems. My thesis investigates how warming and drought influence soil and root-associated microbial communities in Australian sub-alpine grasslands, and how these changes affect plant performance and root colonisation. A set of field-based manipulations and a controlled greenhouse experiment were used to examine microbial diversity, community structure, functional potential and plant-microbe interactions. 

Across the thesis, combined warming and drought consistently imposed the strongest impacts on both soil and root-associated microbial communities and plant performance, indicating that interacting climatic stressors amplify ecological disruption in sub-alpine ecosystems. Among individual stressors, warming generally exerted broader and more consistent negative effects than drought alone, particularly for fungal and arbuscular mycorrhizal fungal (AMF) communities. In contrast, drought effects were often more variable and appeared especially important for bacterial community restructuring. Moreover, prolonged climatic stress generally reduced symbiotic fungal groups while favouring saprotrophic, opportunistic, and stress-associated taxa across both soil and root-associated communities. These shifts suggest a potential weakening of beneficial plant-microbe associations under climatic stress, which may increase plant vulnerability to environmental stress and disease pressure under future climate change.

Collectively, climate stress led to consistent but group-specific changes in soil microbial communities. These changes extended beyond reductions in diversity to altered microbial interactions and functional potential, indicating that climate stress reshapes not only the composition of microbial communities but also their ecological roles. However, plant responses to warming and drought in control growth chambers were highly species-dependent and links between root fungal community changes and plant performance were often weak, suggesting that above and belowground responses do not always change in parallel. Finally, a field-based transplant experiment demonstrated that climate stress and host species drive changes in root-associated fungal communities during plant establishment by altering which taxa successfully colonise plant roots.

Overall, this thesis demonstrates that climate change restructures soil and root microbiomes in group-specific ways, influencing not only microbial diversity and function but also the assembly of root-associated communities. These findings provide new insights into ecosystem responses to climate change and have important implications for predicting ecosystem resilience and informing restoration strategies in sub-alpine systems.

BIOGRAPHY

Salma Sarker is a plant pathologist and microbial ecologist currently based in Canberra, ACT. She began her PhD at the Australian National University in 2022 in the Linde Group at the Research School of Biology. Her PhD research focuses on understanding how climate change reshapes soil and root-associated microbial communities and plant–microbe interactions in Australian sub-alpine ecosystems. Before moving to Canberra, Salma joined Sher-e-Bangla Agricultural University, Bangladesh, where she currently serves as an Assistant Professor. She also worked at the Bangladesh Agricultural Research Institute (BARI) and in the Bangladesh Civil Service under the Ministry of Agriculture. Salma completed her Bachelor of Science in Agriculture and Master’s in Plant Pathology at Sher-e-Bangla Agricultural University, where she received several awards for academic excellence. During her PhD, she received the Australian Plant Phenomics Facility Postgraduate Internship Award and the Holsworth Wildlife Research Endowment Grant. Her research interests include plant pathology, microbial ecology, mycorrhizae, molecular biology, and climate stress biology.