Forest islands in West African savannas: multi-scale evidence of soil and vegetation differentiation across soil profiles and ecosystems

Abstract

Forest islands embedded within West African savanna landscapes represent striking examples of fine-scale ecological heterogeneity, yet the extent to which they differ from surrounding savannas remains unclear. This study evaluated soil physicochemical properties and vegetation characteristics across paired forest island and savanna sites in Nigeria, Ghana, and Burkina Faso, using a combination of profile-based sampling, mixed-effects modelling, and multivariate analysis. Topsoil (0–20 cm) comparisons showed higher mean organic carbon and total nitrogen in forest islands relative to savannas, although these differences were not statistically significant. In contrast, analysis across soil profiles revealed a significant effect of ecosystem type on soil organic carbon, with forest island soils exhibiting higher values across depths. Organic carbon declined significantly with depth, particularly below 50 cm, but this decline occurred at similar rates in both ecosystems. Multivariate analysis indicated partial differentiation between forest and savanna soils, with ecosystem type explaining 12.6% of the variation in soil chemical composition. Vegetation analyses showed clearer contrasts between ecosystems. Forest islands exhibited significantly lower Shannon diversity but significantly larger stem diameters compared to savannas, while differences in species richness, stem density, and basal area were not statistically significant. Relationships between soil properties and vegetation metrics were generally weak, with soil pH showing the only significant association, indicating a negative relationship with Shannon diversity. Overall, the results demonstrate that differences between forest islands and savannas are scale-dependent. While topsoil properties show high variability and limited statistical separation, profile-level and multivariate analyses reveal more consistent ecosystem differentiation. These findings highlight the importance of multi-scale approaches for understanding soil–vegetation dynamics in heterogeneous tropical landscapes.