Identifying a seasonally inundated, nutrient-poor peat swamp forest type in the central Congo Basin

The Cuvette Centrale wetland area in the central Congo Basin is home to the most extensive peatland complex in the tropics, which covers an estimated 167,600 km² and stores approximately 29.0 petagrams of carbon belowground (Crezee et al., 2022). However, relatively little is known about these peat swamp forests, which span both the Republic of the Congo (ROC) and the Democratic Republic of the Congo (DRC). In ROC, peatlands are mostly found in interfluvial basins, where they form wide but shallow peat domes (Davenport et al., 2020), which are largely rain-fed, have ombrotrophic (low-nutrient) status and are covered by distinct hardwood- or palm-dominated forest vegetation types (Dargie et al., 2017). In contrast, peatlands in DRC can be found in different hydro-geomorphological settings, including relatively narrow river valleys along the Congo River’s eastern tributaries.

Here, we present insights from in situ measurements of water table depths, peat and water chemistry, and vegetation characteristics along three of these tributaries, the Ruki, Busira and Ikelemba Rivers. Peat deposits (≥ 30 cm of ≥ 65% organic matter) were found in seasonally inundated swamp forests close to rivers, which are characterised by large (>1 m) wet season inundations. This shows that peat formation in the central Congo Basin is not only confined to permanently waterlogged swamps with relatively stable water tables year-round, predominating in ROC’s interfluvial basins, but also occurs in swamp forests that experience greater water table seasonality. Seasonal inundations are due to riverbank overflow during the wet season, and potentially upland runoff from higher terra firme ground. Since the Congo River’s eastern tributaries are highly acidic (blackwater) rivers, the seasonally inundated, river-fed swamps are still of low nutrient status, allowing peat to form. Nonetheless, they are typically characterised by shallower peat deposits than permanently waterlogged swamp forests. Comparison of vegetation plot data identifies a distinct forest type in the seasonally inundated, river-fed peat swamps: a hardwood swamp forest dominated by Oubanguia africana and Guibourtia demeusei. This vegetation type was previously described by Evrard (1968), but was not known to overlie peat soils.

Our findings identify distinct rain-fed and river-fed peatlands in the central Congo Basin. These two peatland types may face different threats to their integrity. While rain-fed peatlands will be susceptible to future climate change if rainfall levels decrease, those that are also maintained by riverbank overflow, may be susceptible to changes in upstream rainfall patterns. This, as well as infrastructure projects that affect river dynamics, will likely have larger impacts on peatland functioning than previously thought.