Decoupling of overstory and understory composition in an African tropical rainforest suggests declining tree diversity

Understanding forest dynamics in the context of global change is a critical issue in ecology and conservation. Particularly in many African rainforests, it remains largely unknown how these forests are changing and what alterations may occur, partly due to a lack of long-term plot data. Here, we analyzed the relationship between the overstory and understory in an African tropical rainforest to understand how tree species diversity and composition might change over time. Specifically, we compared the diversity and composition of seedlings in the understory to that of trees in the overstory using a community-level forest inventory dataset covering 40.3 ha of the Budongo Forest Reserve in Uganda. We contrasted species diversity by calculating ratios linking overstory and understory richness and diversity. We found a strong decoupling between the forest's overstory and understory. Species diversity of tree seedlings in the understory, at both alpha and gamma levels, was lower than that of overstory trees. The composition of the understory differed from that of the overstory, and it consisted of only a subset of tree species present in the overstory. Several tree species we found dominating the overstory were poorly represented as seedlings in the understory. Overall, these findings suggest that Budongo Forest is on a trajectory towards decreased diversity, with potential detrimental consequences for ecosystem functioning. By examining the tree overstory-understory relationship, our study provides early indications of potential changes in species diversity and composition, informing forest management strategies. We recommend using this approach in other tropical forests that lack long-term data to understand possible changes in species diversity and composition over time. • Tree species diversity appears to be declining in an Afrotropical rainforest. • An Afrotropical rainforest is on a trajectory toward a different future composition. • Overstory-understory analysis reveals species-specific recruitment abilities. • Some dominant tree species populations may decrease, whereas others may increase.