Contrasts in forest structure, floristic diversity and aboveground carbon stocks between peat swamp forest and terra firme forest in northern Congo

The Congo Basin plays a central role in global climate regulation and biodiversity conservation, yet plot-based comparisons among contrasting forest types remain limited. This study compares forest structure, floristic diversity and aboveground carbon stocks between peat swamp forest and terra firme forest in Likouala, northern Republic of the Congo. The analysis is based on 23 one-hectare plots, comprising 12 plots in peat swamp forest and 11 plots in terra firme forest. Forest structure was assessed using stem density, mean diameter at breast height (DBH), basal area and diameter-class distributions. Floristic diversity was analysed using species richness, family richness, Shannon, Simpson and Pielou indices, and floristic turnover. Aboveground biomass was estimated from DBH and wood specific gravity using a regional allometric equation developed for Congo Basin forests, and converted to carbon using a carbon fraction of 0.47. The results show that peat swamp forest has a more developed structure, with a mean DBH of 22.63 cm, a basal area of 29.34 m² ha⁻¹ and an aboveground carbon stock of 166.16 Mg C ha⁻¹. Terra firme forest in Likouala has a slightly higher stem density, a basal area of 24.47 m² ha⁻¹ and an aboveground carbon stock of 125.80 Mg C ha⁻¹, but it supports higher species richness, with 155 species compared with 129 in peat swamp forest. Floristic overlap between the two forest types is extremely low, with only two shared species and a Jaccard index of 0.007. Large trees play a disproportionate role in carbon storage: trees with DBH ≥ 50 cm contribute 48.2% of aboveground carbon in peat swamp forest and 68.7% in terra firme forest. These findings indicate that peat swamp forest and terra firme forest in Likouala are not interchangeable: the former represents a major aboveground carbon reservoir, whereas the latter makes a substantial contribution to regional floristic diversity.