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

Abstract

<p>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 km<sup>2</sup> 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.</p> <p>Here, we present insights from <em>in situ</em> 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 <em>terra firme</em> 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 <em>Oubanguia africana</em> and <em>Guibourtia demeusei</em>. This vegetation type was previously described by Evrard (1968), but was not known to overlie peat soils.</p> <p>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.</p>

Mapping Water Levels across a Region of the Cuvette Centrale Peatland Complex

“…The orange outline is the footprint of the peatland complex, derived from Crezee et al [29]. Highlighted are the transects where water table depth measurements were taken during either the dry (red), wet (blue) or both dry and wet (green) seasons [33]. Additionally, two water loggers are located in the Ekolongouma region and three are in the Lokolama region.…”

“…We used water table depth measurements taken at 250 m intervals along transects, collected during multiple field sampling trips [29] (see Figure 1), to arrive at a transfer function between the SAR backscatter and water level that we can then apply across the full extent of the imagery. Further details about the transect sampling dates are included in Table A1, and details about the field campaigns are provided by Crezee et al [29] and in the PhD thesis by Crezee [33].…”

“…Altimeters primarily measure water levels over permanent water bodies, including lakes and rivers, and interpolation of these data is, therefore, best suited for flood-prone regions, and would not be representative of ombrotrophic (rainfed) peatland regions. There remains a gap in our knowledge relating to inundation dynamics across the ombrotrophic peatland regions [32,33].…”

“…Alternatively, direct in situ water table data may be used, and some are available from water loggers installed in the Cuvette Centrale region [32,33]; however, they are limited in their spatial and temporal coverage. The peatlands are difficult to access to first place water loggers and then to adequately service them, and the logger's lifespan can be short due to the harsh swamp conditions.…”

“…To address this, we used the high temporal (daily) pattern of net water input (rainfall minus evapotranspiration, not accounting for run-off or groundwater inputs) together with the less frequent but higher spatial resolution (100 m) L-band ALOS-2 PALSAR-2 ScanSAR data, which, at approximately 23.5 cm wavelength, can effectively see through the canopy to the peat surface, to interpolate the SAR imagery to daily resolution. We then used in situ water table depth measurements, recorded during expeditions along multiple transects in the RoC and DRC [29,33], to develop a transfer equation between the SAR backscatter and water level, enabling us to estimate the daily evolution of water level across a sub-region of the Cuvette Centrale. In a rainfed hydrological system, the level of inundation correlates directly with spatial and temporal variations in rainfall, while water level changes in flood-prone regions will decorrelate quickly with the accumulated rainfall input at that location due to the additional time-lag component of water input from upstream rainfall.…”