Fluvial sediment export from pristine forested headwater catchments in the Congo Basin

Abstract

Studies on sediment export from tropical forest watersheds are scarce. Of the assessments that do exist, most are of larger rivers or are model-based and lack validation with measured data. Understanding the mechanisms of sediment export dynamics in forested headwaters is important for assessing downstream effects and as a baseline for net impacts of land-use change. To that end, we quantified annual total suspended sediment (TSS) yields in forested headwater catchments of two major forest types in central Africa (tropical lowland forest and subtropical Miombo woodland) and analyzed turbidity-discharge hysteresis over one hydrological year. We measured TSS yields of 0.24 ± 0.09 t ha−1 yr−1 in the Miombo woodland and 0.25 ± 0.05 t ha−1 yr−1 in the lowland forest catchment. The Miombo woodland experienced similar TSS yields as the lowland forest despite a shorter, five-month, rainy season and lower annual precipitation. In the Miombo forest, sparser vegetation cover, seasonal fires that remove understory vegetation and high rainfall intensity during the rainy season therefore resulted in similar TSS yields. As a result of these differences in vegetation and rainfall, approximately 68% of TSS was exported during storm events in the Miombo woodland and 30% in the lowland forest. Both sites showed mainly clockwise hysteresis (positive hysteresis index) patterns of sediment export. In the Miombo woodland, the hysteresis index (i.e., the magnitude and direction of hysteresis) increased with the ongoing rainy season, indicating source limitation already after one month of rain. In the lowland forest, the predominant clockwise hysteresis was more likely caused by the increasing contribution of baseflow during the falling limb of an event, whereas during the rising limb there is a quick flushing of surface material available in the forest. These findings based on hysteresis analysis were further supported by C:N ratio and δ13C analyses of particulate organic matter (POM). POM C:N ratios increased and δ13C signatures decreased with increased discharge in the lowland forest, indicating the mobilization of topsoil sediments during rain events. In contrast, the Miombo exhibited no shifts in C:N ratios nor in the δ13C signature. Despite the pristine nature of these forests and their assumed negligible sediment yields, our results demonstrate that erosion is a significant loss process in tropical forests and call for future research to examine its role in forest functioning and biogeochemical cycling.