Compositional patterns of overstorey and understorey woody communities in a forest–savanna boundary in Ghana

Background: Forest and savanna vegetation in the zone of transition (ZOT) contain distinct woody species due to fire, drought and herbivory barriers that constrain forest species from invading adjacent savannas and vice-versa. Little is known if these barriers cause divergence in species composition between the overstorey and understorey strata in these vegetation types. Aim: We investigated woody species composition across overstorey and understorey strata in the ZOT and explored the relationship between soil fertility and species composition patterns. Methods: We sampled overstorey and understorey woody species and determined soil nutrient concentrations in twenty-five 20 m × 20 m plots in a ZOT in Ghana. Results: Forest and savanna species dominated the overstorey and understorey of their respective environments. However, species composition was decoupled between the overstorey and understorey strata in both forest and savanna vegetations. Few savanna and forest species had individuals co-occurring in both overstorey and understorey such that ~65% of the dominant species was limited to only one stratum. Soil fertility had little effect on these patterns. Conclusion: These patterns indicate that, forest and savanna species face significant recruitment barriers in their respective environments, suggesting that requirements for juvenile establishment may differ from recruitments to the canopy layer.