Influence of flooding variability on the vegetation development of Amazonian peatlands

Over the past two decades, the existence of intact peatlands in lowland Amazonia has been demonstrated, including a vast area of potential peat-forming vegetation of around 43,617 km² in the Pastaza-Marañón Foreland Basin (PMFB), in the Loreto Department of north-east Peru. Peat sampling and floristic assessments have revealed the presence of peat in four ecosystem types in the PMFB: palm swamps, peatland pole forests, open peatlands, and, more rarely, in seasonally flooded forests. However, recent field investigations and palynological studies show that temporal changes in vegetation in peatlands may have been driven by changes in the degree of flooding, suggesting greater complexity in the controls on peatland development and in the spatiotemporal relationships between ecosystems than current conceptual models allow. Here we present new high-resolution palynological, geochemical and radiocarbon data from an open peatland sampled in the PMFB near San Roque village on the Marañón river. Our study aims to: (1) reconstruct past vegetation changes; (2) investigate the interaction between the fluvial system and the peatland’s vegetation dynamics over time; (3) determine how patterns of ecological and fluvial change at San Roque compare with records from other peatland types in the wider PMFB. Downcore palynology on the dated SAR_T3_03_B peat core provides a vegetation record spanning the last 4,300 years documenting changes in ecological associations over time. Between 4,300 and 3,180 cal yr BP, prior to peat accumulation, our data shows  pronounced fluvial influence with high amounts of inorganic material and pollen assemblages from taxa related to open water conditions. Since the beginning of peat accumulation around 3,180 Cal yr BP, palynological and geochemical data suggest conditions related to predominantly herbaceous communities. Core scanning micro-XRF provides proxy evidence for episodes of fluvially-derived minerogenic input during the period of peat accumulation (3,180–440 Cal yr BP). Simultaneous increases in flood-tolerant taxa (e.g. Symmeria paniculata, Alchornea sp., Myrtaceae) support the inference of intervals with increased frequency and depth of river flooding. From around 440 Cal yr BP, we infer the establishment of M. flexuosa palms linked to lower flooding, which persists today. Compared with other regional sequences, the San Roque record is distinctive because of this notable influence of flooding regime on the ecosystem, which impacted on the vegetation succession through changes in flood levels and nutrient inputs. The dynamics of the sediment-laden Marañón River therefore emerges as a major driver in vegetation change and trajectory of peatland development during the last four millennia.