Unravelling belowground interactions in Aucoumea klaineana Pierre stands: Roles of root anastomoses and root-associated fungi

Aucoumea klaineana, Central Africa’s most harvested timber species, is a light-demanding tree that forms monodominant stands. However, its natural regeneration is constrained by reduced forest disturbance, small logging gaps, and competition from shade-tolerant species. Despite limited light availability in these stands, some suppressed trees persist, suggesting alternative survival mechanisms. While ectomycorrhizal (EcM) fungi often support monodominance in other tree species, their role in A. klaineana remains unexplored. This raises the question: do fungal associations or other belowground interactions contribute to its persistence? To investigate this, we first used high-throughput sequencing on rootlets samples, we identified context-dependent fungal communities rather than stable host associations. No EcM fungi were detected. Instead, arbuscular mycorrhizal fungi were present but in low abundance. Additionally, suppressed trees exhibited no distinct fungal communities, indicating that root-associated fungi do not drive tree survival under low-light conditions. Given these findings, we examined root grafting as an alternative mechanism. Former thinning operations revealed that felled tree stumps remained alive via root grafting with canopy trees, enabling water and carbon exchange. Building on this, we excavated three young stands in Gabon, exposing root systems and analyzing growth rings on collected trunk, root, and graft cross-sections. Dendrochronology confirmed shared growth rings in grafted trees, with post-grafting growth rates either increasing or decreasing, but synchronizing between connected individuals, suggesting they function as a single organism. Importantly, root grafts may also support the survival of suppressed, light-deprived trees by facilitating resource sharing. These results suggest that A. klaineana’s monodominance is driven by belowground integration via root grafting rather than fungal associations. This has key implications for forest management, can inform thinning practices, enhance stand resilience, and improve sustainable harvesting strategies for this key timber species of Central Africa.