Tree competition in West African tropical forests mediated by the functional attributes of species and variation in soil moisture

Understanding the ecological processes shaping competitive interactions among forest trees is crucial for predicting ecosystem productivity and climate change resilience. However, few studies have investigated how the biological attributes of tropical species may affect competitive outcomes under varying resource conditions. We collected and analysed a 10-year dataset of radial growth rates in canopy trees from a network of forest inventory plots located in divergent forest types over an extensive meteorological gradient in Ghana, West Africa. We used nonlinear models to estimate the relative reduction in potential growth (basal area increment) of individual target trees of a given species as a consequence of the combined effects of (1) target tree size, (2) variation in crowding levels by neighbouring trees, (3) the functional attributes of those neighbours (wood density and shade tolerance), and (4) local soil moisture levels. Analyses were conducted separately for the 15 most common species in the inventory network. In opposition to neutral theory, our findings indicate that the strength of interactions among competing species was distinctly asymmetric and dynamic. Wood density was an important characteristic that modified competitive outcomes for most species, particularly under varying levels of resource availability. Specifically, dense wood was an attribute that conferred comparatively stronger competitive ability in moisture-limited conditions. Larger individuals were notably less sensitive to the effects of moisture-dependent competition. Our results suggest that attributes such as wood density may reflect divergent life history strategies that differentiate species’ fitness and competitive ability in varying environments. The dynamic nature of competition, influenced by a complex interplay of biological and abiotic factors, implies that more prevalent dry periods, which have been forecast for tropical Africa, may impact the physiognomy and function of future forest communities in the region.