A Review of the Ecological and Biogeographic Differences of Amazonian Floodplain Forests

Abstract

Amazonian floodplain forests along large rivers consist of two distinct floras that are traced to their differentiated sediment- and nutrient-rich (várzea) or sediment- and nutrient-poor (igapó) environments. While tree species in both ecosystems have adapted to seasonal floods that may last up to 270–300 days year−1, ecosystem fertility, hydrogeomorphic disturbance regimes, water shortage and drought, fire, and even specific microclimates are distinct between both ecosystems and largely explain the differences in forest productivity and taxonomic composition and diversity. Here, we review existing knowledge about the influence of these environmental factors on the tree flora of both ecosystems, compare species composition and diversity between central Amazonian várzeas and igapós, and show that both ecosystems track distinct species life-history traits. The ecosystem-level and taxonomic differences also largely explain the biogeographic connections of várzeas and igapós to other Amazonian and extra-Amazonian ecosystems. We highlight the major evolutionary force of large-river wetlands for Amazonian tree diversity and explore the scenarios by which the large number of Amazonian floodplain specialist tree species might even contribute to the gamma diversity of the Amazon by generating new species. Finally, we call attention to the urgent need of an improved conservation of Amazonian várzea and igapó ecosystems and their tree species.

Habitat specialization predicts demographic response and vulnerability of floodplain birds in Amazonia

“…In the floodplains these terrains represent seasonally flooded forests and seem to have existed for even longer than registered. In fact, mature floodplain forests harbour trees with ages of hundreds to a thousand years and some of the older islands date to several thousand years (Passos et al., 2020; Sawakuchi et al., 2022; Wittmann et al., 2022). Except for Synallaxis albigularis and Ochthornis littoralis that occupy open environments, the Generalists in our classification are mainly floodplain forests dwellers, which might explain their use of these stable islands (Remsen & Parker, 1983; Ridgely & Tudor, 2009).…”

“…Except for Synallaxis albigularis and Ochthornis littoralis that occupy open environments, the Generalists in our classification are mainly floodplain forests dwellers, which might explain their use of these stable islands (Remsen & Parker, 1983; Ridgely & Tudor, 2009). Moreover, large stable islands are often formed by anabranches that transform floodplains margins in islands and they tend to have environments more similar to the ones found on the margins (Wittmann et al., 2022).…”

“…In the floodplains these terrains represent seasonally flooded forests and seem to have existed for even longer than registered. In fact, mature floodplain forests harbour trees with ages of hundreds to a thousand years and some of the older islands date to several thousand years (Passos et al, 2020;Sawakuchi et al, 2022;Wittmann et al, 2022). Except…”

“…In the Amazonian floodplains, a specialized biota occupies a myriad of distinct environments created by the annual flood cycle of the rivers. These environments, that cover approximately 750,000 km 2 (~11%) of the area of the Amazon basin, are deeply influenced by the geomorphology of the rivers and the geological provinces they drain (Wittmann et al., 2022). While clear and black water rivers drain old cratons with low amounts of sediments and suspended matter, leading to slow‐growing floodplain vegetation, white water rivers originate on Andean slopes and carry high amounts of nutrients and sediments that continually reshape their floodplains, which sustain a more stratified and dynamic vegetation (Junk et al., 2011).…”

“…Furthermore, floodplain habitats away from the main river channel tend to be more stable than the ones closer to the river edges and on river islands, which are more impacted by varying rates of sediment deposition and erosion. Higher substrate stability results in floodplain forests with tall trees adapted to shorter periods of inundation, while dynamic environments are mostly dominated by shorter, herbaceous vegetation that stays under water for many months every year (Figure 1; Junk et al., 2011; Tamura et al., 2019; Wittmann et al., 2022). This environmental complexity admits an avifauna with rather distinct habitat affinities within the floodplains, including a remarkable diversity of river islands specialists (Remsen & Parker, 1983; Rosenberg, 1990).…”