Original Framework for Understanding the Role of Mycorrhizal Fungi in Driving Tree Species Coexistence in the West African Guinea Savanna

Mycorrhizal fungi mediate nutrient acquisition and interplant competition in savannas, and their role in enabling tree species coexistence through soil-mediated facilitation and niche partitioning represents a theoretical frontier that has not been explored for the West African Guinea savanna, including Nigeria's extensive Guinea savanna belt. This study develops and empirically tests an original theoretical framework for the role of mycorrhizal networks in tree species coexistence in the Nigerian Guinea savanna, integrating mycorrhizal network theory, competition-colonisation trade-off models, and soil heterogeneity theory into a unified coexistence model. The model predicts that arbuscular mycorrhizal network connectivity promotes coexistence among subordinate tree species by equalising competitive outcomes through fungal-mediated phosphorus sharing, and that ectomycorrhizal specialists exclude arbuscular mycorrhizal generalists at microsites with high nutrient homogeneity. Empirical testing was conducted at three Guinea savanna sites in Kwara, Nasarawa, and Kogi States. Root colonisation rates, arbuscular mycorrhizal spore density, ectomycorrhizal community composition (using 18S rRNA amplicon sequencing), and plant community composition were assessed across soil heterogeneity gradients. Reciprocal transplant experiments using seedlings of 8 target tree species with and without mycorrhizal inocula tested coexistence predictions. Results showed that arbuscular mycorrhizal colonisation rate differed significantly among co-occurring tree species in ways consistent with competitive equalisation predictions. Ectomycorrhizal communities differed significantly between soil patch types and hosted distinct tree seedling assemblages. The reciprocal transplant experiment confirmed that mycorrhizal inoculation significantly improved survival of subordinate species in competition with dominant Isoberlinia trees. These findings provide the first mechanistic evidence for mycorrhizal-mediated coexistence in West African Guinea savanna, making an original contribution to savanna ecology theory.

Keywords: mycorrhizal fungi, tree species coexistence, Guinea savanna, arbuscular mycorrhizae, West Africa