Diagnosing the demographic balance of two light-demanding tree species populations in central Africa from their diameter distribution

•Tree diameter distribution reflects the demographic status of the tree population.•A quantitative tool was defined to diagnose demographic equilibrium.•A long-lived pionner tree species population was diagnosed to decline.•Past peak of recruitment for this species may result from slash-and-burn cultivation.•Natural tropical forests are constantly changing in response to past events.

The diameter distribution of a tree species population in a natural forest reflects its demographic dynamics and the impact of past disturbances. A modal diameter distribution can be the mark of a demographic imbalance or it can be the consequence of a particular growth pattern. By comparing the observed diameter distribution with the one that follows from a reference growth model under the hypothesis of demographic balance, we diagnosed demographic balance. In a forest concession in Gabon before logging, an unbalanced diameter distribution was found for Aucoumea klaineana, a long-lived pioneer species and also a valuable commercial species. The Aucoumea klaineana population consisted of old trees without meaningful recruitment, thus indicating an ageing, declining population. In contrast, in the same concession, a balanced diameter distribution was found for Lophira alata, a pioneer to moderate light-demanding species. The history of this forest concession, once heavily populated and subsequently depleted from human perturbation (shifting cultivation), can be used to interpret these demographic statuses. These results reinforce the view of a forest that is constantly changing and shaped by past human perturbations, with the implication for its conservation that some kind of perturbation should be maintained.