The delta of the Paraná River in Argentina forms islands as it progrades into the Rio de la Plata estuary. In order to understand plant succession patterns, we analyzed cover-abundance and soil data along transects across topographic gradients of three islands. Using multivariate classification (TWINSPAN) and ordination (DCA and CCA), we developed a conceptual model of plant succession for these islands. The general pattern is that the bulrush, Schoenoplectus californicus, first colonizes bars and incipient islands that eventually accumulate sediments to become mature islands with levees dominated by the tree, Erythrina crista-galli, and interior emergent marshes at lower elevations also dominated by a bulrush, Scirpus giganteus. In contrast to earlier descriptions of a unique pathway of primary succession, we used elevation gradients, sediment composition, and inferred hydrologic regime to differentiate two major pathways of species colonization and persistence. During levee formation, colonization of high-energy environments by the bulrush leads to replacement by a mixture of forbs, and finally by a E. crista-galli forest. It is possible that further enrichment of the forest with other tree species is now obscured by Salicaceae plantations (Salix spp. and Populus spp.). Levee formation isolates the interior of islands hydrologically. There, nearly permanent flooding and lack of flushing allow dominance by S. giganteus. The persistence of this species is evidenced by its recovery after fire and its reestablishment after Salicaceae plantations are abandoned. It is noteworthy that two bulrush, S. californicus and S. giganteus, form the simplest stands in terms of richness and structure at highest and lowest energy environments, respectively. At any given site, species composition appears to respond to changes in topographic position, hydrologic regime, and sediment composition.
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Vol. 24 • No. 3