Study area

This study was conducted in the Cuvette Centrale, an area in the Congo Basin rainforest biome characterised by primary tropical moist forest that forms extensive peatlands (mean estimated peatland area: 167,600 km², mean estimated thickness 1.7 m¹⁸. Our study site was located in the Likouala district, Republic of the Congo. The annual temperature is 25.6 °C, and rainfall is approximately 1700 mm year⁻¹, characterised by a bimodal regime with two peaks between March to May and September to November³⁹. Three long-term 1 ha forest inventory plots were installed in January to March 2019 by the CongoPeat project (https://congopeat.net) in three different ecosystem types (Fig. 1, Supplementary Table ^S4): (i) terra firme forest, (ii) hardwood-dominated peat swamp forest, and (iii) palm-dominated peat swamp forest.

The terra firme plot is characterised by closed canopy-forming species (mean tree height: 26.5 m, personal observation) such as Angylocalyx sp. Taubert, Strombosiopsis tetrandra Engl. & Prantl, and Symphonia globulifera L.f. The number of trees with diameter larger than 10 cm at breast height was 378 ha⁻¹, while the basal area of all trees in the plot was 25.3 m² ha⁻¹ in 2020–2021.

The hardwood and palm peat swamp forest plots were installed in forested peatlands and both sites have the water table at or above the peat surface for most of the year. The hardwood peat swamp forest contains peat to a maximum depth of 1.8 m; minimum and maximum water table levels at this site are − 0.60 m (subsurface) and 0.22 m (ponding), respectively⁴⁰. This ecosystem type is characterised by the dominance of high-stem, hardwood species such as Carapa procera DC., Grossera macrantha Pax, and Uapaca paludosa Aubrév. & Leandri, a relatively high (mean tree height: 18.2 m, personal observation) and closed canopy. The number of trees with diameter larger than 10 cm at breast height was 382 ha⁻¹ and the basal area of all trees in the plot was 29.7 m² ha⁻¹ in 2020–2021.

The palm-dominated peat swamp forest has a maximum peat depth of 3.0 m; minimum and maximum water-table levels are recorded between 0.14 m below and 0.24 m above the ground surface, respectively⁴⁰. This forest is dominated by the palm Raphia laurentii De Wild., which has a low stature (~ 9 m maximum palm height, personal observation), and a non-continuous tree canopy with mean tree height of 17.6 m (personal observation). The numbers of trees having a diameter larger than 10 cm at breast height was 85 ha⁻¹ and the basal area of all trees in the plot was 7.6 m² ha^-1 in 2020–2021.