Soil hydromorphy and aluminum saturation shape tree community composition in a riparian ecotone, Paraná, Brazil

Riparian ecotones host sharp hydrological-edaphic gradients that can restructure tree communities. We tested how soil hydromorphy and related attributes shape floristic composition in a 7.6-ha ecotone on the eastern boundary of Iguaçu National Park (Paraná, Brazil). We surveyed trees (DBH ≥ 15 cm) in 20 contiguous 10 × 10 m plots, evenly split between hydromorphic and non-hydromorphic soils, and measured groundwater level and surface-soil properties (0-20 cm). Multivariate analyses (PCA, CCA, PERMANOVA) revealed a clear compositional split between environments (PERMANOVA: F = 6.03, p = 0.0002). Soil structure was summarized by two principal components explaining 71.9% of variance; hydromorphic plots were associated with higher aluminum saturation, organic matter, total organic carbon and shallower water tables, whereas non-hydromorphic plots showed higher base status (Ca2+, Mg2+, SB, V) and pH. CCA indicated groundwater depth and aluminum saturation as the main drivers of floristic turnover, with indicator taxa segregating by environment (Vitex megapotamica, Mimosa bimucronata in hydromorphic sites and Ilex paraguariensis, Matayba elaeagnoides in non-hydromorphic sites). Altogether, soil hydromorphy, coupled to aluminum saturation, emerges as a proximal filter structuring tree communities across this subtropical riparian ecotone. These results provide a mechanistic basis for conservation and restoration, including species selection aligned with groundwater regime and soil chemistry.