1. Savannas, spanning 20% of the Earth’s surface, are characterized by a continuous grass matrix interspersed with woody patches, supporting high biodiversity and providing ecological and economic services. Coexistence is maintained by interactions among climate, soil nutrients, and disturbance, but can be destabilized by invasion, land-use changes, and climate shifts. Although climatic and edaphic controls on savanna structure are well-studied, the contribution of soil microbial communities in maintaining spatial heterogeneity and coexistence remains poorly understood. We investigated whether savanna heterogeneity is mirrored belowground and how disturbance and invasion by Megathyrsis maximus modify these relationships. 2. We used a factorial field sampling design in a mesquite savanna to compare woody patches and adjacent grasslands with and without mechanical disturbance and invasion. We quantified soil physiochemical properties, plant community composition, and bacterial and fungal communities to evaluate linkages among vegetation and soils. 3. Grasslands and woody patches supported distinct soil and microbial assemblages, consistent with differences in vegetation inputs and nutrient regimes. Grasslands exhibited relatively simple assembly patterns, with plant diversity closely associated with soil nutrients and comparatively homogeneous microbial communities. In contrast, woody patches displayed more complex assembly dynamics, with microbial communities structured by plant diversity, soil nutrients, precipitation, and spatial distance, indicating greater environmental heterogeneity and stronger dispersal limitation. 4. Invasion by M. maximus increased soil nutrient availability and altered microbial community composition in grasslands, while mechanical disturbance produced similar but weaker effects. These impacts reduced grassland microbial distinctiveness and disrupted linkages between vegetation, soils, and microbes. 5. Synthesis. Aboveground patch structure in savannas is tightly coupled to belowground microbial assembly and nutrient dynamics. By demonstrating that invasion and disturbance weaken these spatial linkages, this study indicates that soil microbial communities contribute to savanna coexistence and resilience and reveals how belowground community assembly underpins landscape-scale heterogeneity and its destabilization under multiple stressors.

Plants host diverse assemblages of fungi on their foliar tissues, both in internal compartments and on exterior surfaces. When plant distributions shift, they can move with their fungal associates (i.e. co-introduction) or acquire new associates present in the novel environment (host-jumping). The fungal communities that plants acquire influence a plant’s ability to establish and spread in this new environment. Here, we aimed to assess whether invasive Cenchrus ciliaris hosts similar groups of fungi in its native and introduced ranges and to evaluate community overlap of fungi associated with foliar tissue of C. ciliaris and native and non-native plants within the introduced range. In the introduced range, C. ciliaris associated with a majority of novel OTUs, although 3.2% of OTUs were common to both ranges. Of these shared OTU, 77.6% were found on co-occurring natives and non-natives in the introduced range, whereas 22.4% were completely unique to C. ciliaris indicating a possible co-introduction. Fungal communities within the introduced range contain a higher proportion of generalist symbionts and an increase in heterogeneity of foliar communities than in its native range. Within the introduced range, host phylogenetic distance explained more variation in foliar communities than native status. Our findings provide evidence that non-natives acquire fungi opportunistically from their environment, although host and environmental filtering is present suggesting that successful invasive plants may be able to limit the effect of poor symbionts and select for better ones. Future experimental work will be needed to confirm the occurrence of host selection and identify its mechanisms.