The Physical Setting of Amazonian Floodplains: History
Please note this is an old version of this entry, which may differ significantly from the current revision.
Contributor: , , , , , ,

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.

  • anoxia
  • seed dispersal
  • life-history traits
  • forest succession

1. Introduction

Large-river wetlands cover approximately 750,000 km2 (~11%) of the area of the Amazon basin, most of which is forested. These seasonally flooded forests are the most species-rich floodplain forests on Earth [1], store significant stocks of carbon [2,3], and provide a variety of valuable ecosystem services in terms of providing habitat, regulating biogeochemical cycles, and provisioning food and material goods for human welfare [4]. The fine-scale habitat heterogeneity of Amazonian floodplain forests has regional implications for the origin and maintenance of Amazonian biodiversity [5]. Floodplain forests also operate as habitat refugia by mitigating climatic stressors, because wetland habitats show reduced temperature and soil moisture variability relative to adjacent uplands i.e., [6,7]). In this sense, wetlands may allow for the survival of populations of species living in constantly changing climatic contexts [8].
Amazonian large-river floodplains are characterized by the contrasting fertility of their flood waters and alluvium. Sediment- and nutrient-rich white-water rivers form floodplains known as várzea. Sediment- and nutrient-poor black- and clear-water rivers form floodplains classified as igapó [4,9,10,11]. Prance [10] and Kubitzki [12] emphasized the floristic differences between várzea and igapó forests based on herbaria collections of characteristic indicator tree species.

2. The Physical Setting of Amazonian Floodplains

Wetlands cover an area of approximately 2.33 Mio km2 (~34%) of the Amazon basin. Out of these, approximately 1.7 Mio km2 are river wetlands, either as episodically flooded riparian areas along the headwaters of upper courses (approximately 1 Mio km2) or as seasonally flooded large-river floodplains along the middle and lower courses (approximately 750,000 km2, [4,13]. Because of seasonal rainfall linked to the yearly shift of the Intertropical Convergence Zone, combined with a flat landscape over most parts of the basin, Amazonian large-river wetlands are subject to a predictable flood pulse in magnitude and timing [14], with one high-water and one low-water period during the year. Seasonal water-level oscillations of major Amazonian rivers, such as the Solimões and lower Negro Rivers, amount to 9–12 m in the central Amazon basin, reaching maxima of up to 12–15 m at the middle to lower courses of the southern Amazon tributaries of the Madeira, Purús and Juruá Rivers [15]. In western and eastern Amazonia, flood pulses decrease to 3–6 m, either because of reduced catchment areas and higher slopes in the west or enlarged riverbeds in the east [16]. The flood pulse is considered the main environmental forcing factor for the biota of Amazonian large-river wetlands (Junk et al., 1989). Many wetland organisms require water-level fluctuations for the survival of their populations, including many fish and invertebrate species with spawning and feeding migrations between rivers and floodplains, i.e., [17,18,19,20,21]. Many aquatic and terrestrial mammals, birds, reptiles, and amphibians synchronize their life cycles with the hydrological cycle, i.e., [22,23,24]. Herbaceous and woody floodplain plant species possess a series of morphoanatomical and physiological adaptations to the seasonal inundations, i.e., [25,26,27].
The Amazonian drainage system is likely as old as the existence of neotropical rainforests, which established in large parts of northern South America since at least the Upper Eocene, approximately 50 Ma BP [28,29,30]. With the Andean uplift, it was profoundly reshaped, mainly during the middle to upper Miocene, between 23 and 10 Ma BP [31]. During the Paleogene, Andean mountain building already generated a system of depressions in the Miocene foreland basin of the western Amazon, where vast freshwater wetland systems established, likely several times influenced by marine transgressions. At approximately 7 Ma BP, the Amazon River reversed to the East with the closure of the Vaupés Arch, and Andean sediments started to reach the Atlantic Ocean [32]. Western Amazonia developed into a landscape of widespread river terrace systems and entrenched rivers with a high sediment load [33]. Today, Andean sediment covers most of the area of the western Amazon, where the Caquetá-Japurá river to the north and the Madeira river to the south form the approximate natural boundaries between the Andean sediment in the west and older, cratonic geological formations of central and eastern Amazonia [34].
Amazonian large-river wetlands display a wide range of environmental settings depending on the geology and geomorphology of basins where rivers originate from. The geochemical differences between white-, black-, and clear-water rivers were first described by Sioli [9]. Old cratonic formations, such as the Guiana and central Brazilian Shields in the N and S of the Amazon basin, are drained by sediment- and nutrient-poor black- and clear-water rivers, while the Andes and sub-Andean regions in NW, W, and SW Amazonia are drained by sediment- and nutrient-rich white-water rivers [9,11]. The ecosystems flooded by white-water rivers were classified as várzea, while those flooded by black- and clear-water rivers were classified as igapó [4,9,11]. Várzeas are formed along the main stem of the Amazon river and its large white-water tributaries Madeira, Purús, Caquetá-Japurá, Juruá, Jutaí, Javaris, and Putumayo-Içá. These rivers transport, deposit, and remobilize large amounts of sediment and dissolved matter from the Andes to the Atlantic Ocean. The sediment load contains large amounts of multilayered clay minerals with elevated cation exchange capacity, such as smectite, illite, and montmorillonite, that release nutrients during the weathering process, resulting in a relatively high fertility of alluvial substrates. The water is slightly acidic to neutral (pH 6–7) and is dominated by Ca, Mg, and carbonates [9]. Electric conductivity decreases from the west to the east and amounts to 140–120 μS cm−1 near the Andes to 50–30 μS cm−1 at the lower Amazon [35]. In contrast, igapó rivers drain old, strongly weathered Tertiary sediments of Paleozoic and pre-Cambrian origin. The water is poor in suspended solids, transparent, and of brown to blackish color when originating from forested regions, or greenish clear when originating from regions predominately covered by savannas, particularly in the eastern parts of the Amazon basin [16]. The most important black-water rivers are the Negro, Coari, and Uatumã rivers in the central Brazilian Amazon. The very low content of dissolved matter results in an electric conductivity of <20 μS cm−1, and the waters are mostly acidic due to the high amounts of dissolved organic material, with a pH of 4–5. The content of alkali-earth metals is low and contributes less than 50% of the total cation content, in which sodium dominates, while the principal anions are Cl and SO42− [36]. The most important clear-water rivers are the Tapajós, Xingu, and Tocantins-Araguaia originating from SE Amazonia, and the Branco, Trombetas, Paru, and Araguari rivers originating from the Guiana Shield. River waters and sediments vary in fertility but are relatively sediment- and nutrient-poor [4].

This entry is adapted from the peer-reviewed paper 10.3390/w14213360

This entry is offline, you can click here to edit this entry!