Background and aims <p>Tree dieback is increasing worldwide, threatening the biodiversity and functioning of many terrestrial ecosystems. Tree dieback is associated with multiple interconnected changes in community composition and ecosystem processes. These changes affect plant, fauna and soil microbial communities, and soil physical and chemical processes. However, these ecosystem changes associated with tree dieback have received little attention.</p> Methods <p>We characterised ecosystem changes associated with dieback in the Australian tree, <i>Eucalyptus viminalis</i>. Dieback of <i>E. viminalis</i> is widespread and of unknown cause. Surveys were conducted in sites with a range of dieback severity to assess native and exotic plant communities. Soil was collected to assess soil chemistry and communities of arbuscular mycorrhizal fungi (AMF), ectomycorrhizal fungi (EMF), fungal plant pathogens and oomycetes.</p> Results <p>Dieback in <i>E. viminalis</i> was associated with an increase in nitrate–N, soil pH, exotic plant cover and richness, AMF and plant pathogen richness (both fungi and oomycetes). Conversely, there was a negative relationship between dieback and ammonium-N, native plant richness and EMF richness. Dieback severity was also associated with reduced heterogeneity in community composition of both native and exotic plants as well as all microbial groups assessed.</p> Conclusions <p>Ecosystem changes associated with <i>E. viminalis</i> dieback are complex and likely interconnected via feedbacks between plants, soils, and microorganisms. If highly disturbed landscapes suffering tree dieback are depleted of healthy soil microbial communities and physicochemical properties, then natural or assisted revegetation programmes may have limited success unless microbial mutualists are reinstated through intervention.</p>