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ABSTRACT Aim Forest plots are a benchmark tool to quantify aboveground carbon (AGC) stocks and biodiversity in tropical forests. However, atypical edaphic forest types such as seasonally flooded forests or forests on sandy soil are scarce in the plot networks, despite their widespread occurrence. This study assessed how edaphic conditions shape tropical forests, in terms of forest structure, tree diversity and composition. Location Lomami National Park in the Democratic Republic of Congo. Methods We installed 89 plots of 0.25 ha each in the understudied southeastern Congo basin. Plots are spread over eight different combinations of flooding regime (seasonally flooded vs. terra firme ), soil texture (clay vs. sand) and region (north vs. south). We tested the influence of flooding regime, soil texture and region on structural attributes, tree diversity, and tree species composition. We also assessed relationships between structural attributes, diversity indices, and compositional gradients. Results We inventoried 9156 trees and identified a total of 416 tree species, with 93 species (22.4%) belonging to the Fabaceae family. Species diversity is much higher in terra firme than in seasonally flooded forests. The latter also shows a different species composition but holds similar AGC (163 Mg C ha −1 on average). We also found that region influences both species diversity and structural attributes (diversity and AGC highest in the north), while soil texture only influences structural attributes (AGC highest on clay). Finally, we found that structural attributes are independent of diversity indices at different scales. Main Conclusions This study describes for the first time the forests in the southeastern Congo basin, which show a high degree of structural variability, high tree biodiversity and important carbon stocks. The region merits further exploration, with specific attention to seasonally flooded forests which harbor a less diverse but different tree flora, and similar carbon stocks as compared to terra firme forests.