Stability of monodominant and mixed tropical forests: Implications from a biomass‐based competition model

Tropical rainforests generally exhibit high tree species diversity, yet some stands are dominated by a single species that makes up more than half of the canopy. How these outcomes arise remains uncertain. Here we employ a biomass‐based Lotka‐Volterra competition model to analyse community dynamics and stability in monodominant and mixed‐species forests.

We use census data from four 10‐ha permanent plots in Ituri, Democratic Republic of Congo, where monodominant Gilbertiodendron dewevrei stands occur alongside mixed stands with co‐dominant Cynometra alexandri and Julbernardia seretii —both known to also form monodominant stands elsewhere in the region—under similar environmental conditions. The model estimates responses of species‐specific biomass productivity to con‐ and heterospecific leaf biomass.

The model successfully predicts observed monodominance and species biomass abundances in stable equilibrium for both forest types. Dominance arises through the escape from intraspecific self‐limitation rather than from superior interspecific competitive ability, challenging classical interpretations. The monodominant species exhibits a high single‐species leaf biomass asymptote (~4 Mg ha ⁻¹ ), allowing dominance under conspecific competition. This high carrying capacity reflects weak sensitivity to conspecific leaf biomass relative to intrinsic growth. Other species show low single‐species asymptotes (<1 Mg ha ⁻¹ ) and coexist stably in low abundances when able to invade the monodominant stand.

Monodominant species accumulate biomass slowly (~1500 years) and require prolonged disturbance‐free conditions. Excluding G. dewevrei from the species pool enhances equilibrium species richness and diversity while causing a decline in community‐level biomass and productivity. This suggests that a positive diversity–productivity relationship is not universal.

Synthesis . Our findings suggest that diverse forests are potentially vulnerable to becoming monodominant, driven by species that experience weak intraspecific self‐limitation. The present modelling approach offers a simple, quantitative tool to predict tropical forest dynamics and guide restoration and management efforts.