Woody encroachment of an East-African savannah ecosystem alters its arbuscular mycorrhizal fungal communities

Aims The aim of the study was to explore whether the encroachment of an East-African savannah ecosystem by the invasive shrub Dichrostachys cinerea L. Wight & Arn has resulted in changes in the Arbuscular Mycorrhizal Fungus (AMF) communities which are associated with roots of the extant herbaceous plant communities. We hypothesized that this could happen either through introducing new AMF taxa, or through inducing changes in the savannah soil environment or plant community composition. Methods We selected five blocks in the savannah plains of the Nech Sar National Park, southern Ethiopia, and in each block, we established nine vegetation plots along a gradient of D. cinerea encroachment. We sampled roots from the encroaching shrub D. cinerea and from three dominant graminoids (Brachiaria deflexa, Chrysopogon plumulosus, Lintonia nutans) and two forbs (Commelina forskaolii, Ruellia prostrata), and used a metagenomics approach to identify the AMF taxa associated with their roots. Results D. cinerea exhibited higher AMF richness and diversity, and a significantly different AMF community composition, compared with the sampled herbaceous plants in the unencroached plots. Herbaceous plants in sparsely encroached plots harbored a higher AMF diversity as compared to the control plots, while those in densely encroached plots exhibited an intermediate AMF diversity. D. cinerea encroachment, host plant lifeform, soil characteristics and extant herbaceous plant species diversity all affected AMF community composition associated with the herbaceous plant roots. Conclusions Woody encroachment might have affected AMF communities of the extant herbaceous vegetation in savannah ecosystems both directly, through introducing novel AMF taxa associated with the encroaching shrub species, and indirectly, through encroachment effects on the environment. Restoration of encroached savannahs should therefore take into account the legacy effects of the invader on the microbial communities.