Abstract The incision of major Amazonian white-water rivers, driven by Andean uplift, shaped regional hydrological systems and generated distinct terrace levels above the Caquetá-Japurá floodplains during the upper Pleniglacial period. On poorly drained low terraces (PDLT), flooded by rainfall and secondary watercourses, diverse swamp forest types developed that remain poorly understood in western Amazonia. We aimed to identify and characterize these forest types by evaluating alpha diversity, vegetation structure, and their relationships with edaphic conditions. Tree height and diameter at breast height, together with 16 soil variables, were measured in 30 plots distributed along four transects. Taxonomic and phylogenetic beta diversity between PDLT forest types and adjacent várzea and terra firme forests were also assessed. Hierarchical cluster analysis identified four forest types: paleo- várzea , Mauritia -dominated, igapó , and transitional forests toward white-sand systems. Ordination analyses showed that floristic composition varied along gradients of species diversity and dominance and was strongly associated with a transition from organic to mineral soils and from phosphorus-rich organic to aluminum-rich mineral soils. Floristic composition was significantly correlated with soil properties and geographic position. High taxonomic and phylogenetic beta diversity revealed pronounced species turnover between PDLT forests and adjacent várzea and terra firme forests, driven primarily by edaphic and hydrological gradients and stronger evolutionary affinities between PDLT and várzea forests than with terra firme forests. Our results show that fine-scale edaphic heterogeneity and long-term hydrological dynamics, rather than geographic proximity alone, play a central role in structuring forest communities and maintaining the exceptional biodiversity of western Amazonia.
Urrego et al. (2026) studied this question.