Resource acquisition strategies facilitate Gilbertiodendron dewevrei monodominance in African lowland forests

Abstract

Tropical forests are hyperdiverse, yet extensive areas of monodominant forest occur in the tropics worldwide. Most long-lived and persistent monodominant tree species form ectomycorrhizal fungi symbioses, allowing them to obtain nutrients directly from soil organic matter. This might promote monodominance by reducing nutrient availability to co-occurring species, the majority of which form associations with arbuscular mycorrhizal fungi. Gilbertiodendron dewevrei forest is the most widespread monodominant forest in tropical Africa. Its distribution appears determined in part by moisture availability, but its monodominance is not thought to be driven by its fungal partner or soil fertility. Here, we compare soil fertility of 20 G. dewevrei stands to mixed forest from three sites across an 8,400 km2 region of the Central African Republic and the Republic of Congo. In contrast to previous studies, we find monodominant G. dewevrei stands associated with infertile soils, as base cations (calcium, magnesium, total exchangeable bases) and extractable manganese are extremely low, and significantly lower in soils under G. dewevrei forest compared to mixed forest. Further, and consistent with ectomycorrhizal forests globally, soil carbon to nitrogen and carbon to phosphorus ratios are significantly higher in G. dewevrei stands than in mixed forest stands, providing evidence in support of direct acquisition of nitrogen and phosphorus from soil organic matter by ectomycorrhizal fungi. Gilbertiodendron dewevrei recruits from the seedling bank, with its large seedlings surviving in high densities for over a decade. We tested whether light plasticity could facilitate monodominance by growing seedlings of G. dewevrei under controlled light conditions. We found that its seedlings grow well under a wide range of irradiance levels and conclude that this plasticity affords a competitive advantage. Synthesis. We reframe the discussion of factors contributing to monodominance of Gilbertiodendron dewevrei into one of resource acquisition and use efficiency. In particular, G. dewevrei is associated with moist and infertile soils and competes well under a variety of light conditions. Our data are consistent with a model where root associations with ectomycorrhizal fungi drive monodominance through the direct acquisition of nutrients from soil organic matter, promoting nutrient limitation of co-occurring species.