Unraveling Belowground Interactions in Okoumé (Aucoumea klaineana Pierre) stands: the roles of Root Anastomoses and Mycorrhizal Associations

Okoumé (Aucoumea klaineana Pierre) is the most harvested timber species in Central Africa. This pioneering light-demanding species favors open environments to form monospecific stands. During forest management activities, thinning operations in these stands were carried out. Through subsequent operations, the stumps of felled trees were discovered to be alive. Their survival was found to be assisted by the graft (anastomose) of their root steles with close conspecifics reaching the canopy. This enabled hydromineral and carbon exchanges between individuals and extension of the root system of canopy trees.
Despite the widespread occurrence of root anastomose among woody species, physiological mechanisms involved and their importance are poorly understood. For the first time in the tropics, we investigated it in young interconnected natural stands. We clearcut and excavated three stands using fire hose to expose roots. Cross-sections of stems, roots, and root grafts were collected to observe growth rings patterns. The impact of root grafts on suppressed trees survival and tree growth through dendrochronology was then analyzed. Preliminary findings show the presence of shared growth rings within root grafts, proving exchanges between individuals.
Knowing that mycorrhizal associations are key components of tropical forests, the belowground interactions among A. klaineana trees are unlikely to be exclusively the result of root anastomoses. For that reason, we also investigated the mycorrhizal status of the species to gain understanding of these interactions influence their growth, health, and ecological roles. By collecting fine roots, we aimed to elucidate the links between fungal communities (through specific richness and functional diversity) and growth dynamics across populations of varying ages and structures through metabarcoding methods. Among 4.818 OTU, preliminary results show a low pathotroph (4%) but high saprotroph load (36%) compared to symbiotrophic fungi (26%, of which 80% of arbuscular mycorrhizal fungi, AMF), 34% uncertain.
Considering two belowground flows, we shed light on the relative importance of AMF in fungal assemblages in A. klaineana populations and could reveal the impact of root anastomoses on variations in tree growth. This contributes to fill knowledge gaps regarding A. klaineana ecology and interactions at the stand scale to improve management strategies by finding a balance between productivity and sustainable resource use