Two contrasting swamp forest succession pathways in central Congo Basin peatlands

Abstract

The central Congo Basin (CCB) contains one of the world's most extensive regions of tropical peat swamp forest, occupying interfluves and floodplains surrounding the Congo River and its tributaries. The region is dominated by hardwood and Raphia-palm forests. Little is known about how and when these forests developed. We investigate peatland ecosystem and forest development via pollen and δ¹³C_TOC analysis on peat cores from four study sites, two on floodplains to the east of the Congo River, and two on interfluves to the west of the Congo River. We present 81 ¹⁴C dates on bulk peat samples from four peat cores to show that peat initiation and forest establishment occurred at three sites before or during the Last Glacial Maximum (LGM), at 26,750 calendar years before present (cal yr BP) (Bolengo), 19,600 cal yr BP (Ekolongouma) and 19,400 cal yr BP (Bondamba), and later at 13,350 cal yr BP (Boboka) at a fourth site. The three oldest dates support earlier hypotheses that the CCB may have been a dynamic refuge for rainforest taxa during the LGM. The pollen records from the four sites show two contrasting patterns of vegetation succession following peat initiation: the interfluvial peatlands developed from an open herbaceous wetland; the river floodplain peatlands developed from a hardwood swamp forest. All four sites eventually transitioned to a mixed swamp forest containing Raphia and Uapaca trees. Differences in the timing of vegetation transitions between sites suggest that autogenic (within-site) processes are important drivers of vegetation succession in these peatlands, overlain to varying degrees by allogenic (climatic) drivers. Overall, the comparative analysis of pollen records from four sites within the CCB reveals two contrasting trajectories of forest development—one associated with interfluves and the other with river floodplains—driven by a combination of autogenic and allogenic processes. Our findings highlight the biological diversity and complexity of the CCB peatlands, and their sensitivity to climatic change. Reduced precipitation in the Late Holocene affected peat accumulation and forest composition at all sites, with some sites being more sensitive than others. This study provides important context for efforts to protect and conserve the CCB peat swamp forests by demonstrating their sensitivity to environmental stress in the past.