Introduction

The flooded alluvial environments (Gopal et al. 1980, Décamps & Naiman 1989) are, in essence, periodically overwhelmed or water-soaked, and situated at the boundary between aquatic and terrestrial environments.

The specificity of these environments is emphasized by the fact that their investigation methods and concepts unite with those of limnology and terrestrial ecology, as it was shown by Junk (1980, 1989), and also by Noirfalise & Sougnez (1961) and Hartog & Ségal (1964), concerning the temperate zones.

Because of the extent of their geographical spaces, the distinctiveness of the alluvial Amazonian ecosystems conducted to consider these flooded areas as a specific -neither terrestrial nor aquatic- ecosystem, and to study it such as it is (Junk 1980, 1986, 1989).

Studying stream ecology in temperate and arctic zones (see f.e. Hynes 1975, Minshall et al. 1985, Naiman et al. 1987), Vannote et al. 1980 proposed the fluvial continuum concept. In addition, and in the light of their investigations in Amazonia, Junk et al.(1989) proposed the flood-pulse concept. They emphasized the huge tropical alluvial relief dimensions with what they called the shifting coastline, herewith describing the typical tropical phenomenons affecting the turbid tropical rivers banks.

The flood-pulse concept is based upon the idea of an aquatic-terrestrial transition zone where the huge mass of waters annually running through the hydrographical net, would be “the major force controlling biota” (op.cit.).

Studying wetlands in the high Amazon River basin, from the botanical and morphological points of view, brings some elements into the understanding of this environment, and about the role played by the annual, climatic and hydric restrictions, as well as about the dynamics and structure of the flooded ligneous and herbaceous vegetation.

Fluvial dynamics and vegetation succession

At all latitudes, floodplain geomorphology is characterized by the existence of relatively well-drained longitudinal levees, parallel to the river channels, and separated by long flooded bad drained clayey depressions, (Sternberg 1957, 1960 ; Michel 1963, Hickin 1974, Hickin & Nanson 1975, Nanson 1980, Michel & Sall 1984, Kalliola & Puhakka 1988, Lamotte 1990).

For the lateral displacement of the meanders, the ancient levees and depressions are located inland, while the recent ones are located near or on the river edges.

At high latitudes, large scale variations of the vegetation cover are controlled by a complex of gradients associated with time and relief elevation [1]. The levees and the depressions are topographically and edaphically distinct enough so that the vegetation can develop in distinct plant community belts, which can be easily spotted on aerial photographs. Nevertheless, here the levees edification is slow, deposits are few, and all takes place in a little-active context, because of the weakness of the currents and sedimentary load. The levees and depressions are narrow; flood duration is not very different from one station to another, so the relief is progressively hidden by the rapidly homogeneous vegetation development (Nanson op. cit.). The depressions can be identified with their characteristic poor drainage, but the levees are just mentioned as "well-drained floodplain areas" (Kalliola & Puhakka, op. cit.).

Along a cross-section perpendicular to a river Ucayali bend located at 200 km upstream Iquitos, (Peruvian Amazon), the study of ligneous vegetation structure shows the existence, on the recent relief, of two well distinct vegetation types, characterized by distinct flora and structure, and corresponding to the alternating levees and depressions (Lamotte 1993).

On the occasionally flooded recent levees (flooded less than 3 months) a succession of three stages occurs:

  a graminean stage dominated by Gynerium sagittatum (Aubl.) Beauv. (Poaceae)

  a pioneer shrubby stage dominated by Cecropia membranacea Trécul (Cecropiaceae)

  an arborescent stage, dominated by five species : Ficus insipida ssp. insipida Willd. (Moraceae), Maquira coriacea (Karst.) C.C. Berg (Moraceae), Calycophyllum spruceanum Benth. (Rubiaceae), Guarea guidonia (L.) Sleum. (Meliaceae), and Sloanea sp. (Elaeocarpaceae) (fig.).

FIG (please see French version)

Here it is interesting to note that F. insipida, C. spruceanum and M. coriacea can form a part of the emergent trees in the whole alluvial Amazonian forests (Huber 1910, Ducke & Black 1953).

In the low lying areas, the flooding period varies between 3 and 7 months per year. These deeper areas are crossed by strong currents during high water time, and Paspalum fasciculatum Willd. ex Flügge (Poaceae) develops after each flooding.

In case of strong currents, vegetation successions are stopped because the plants are uprooted annually. Cecropia latiloba Miquel (Cecropiaceae) and Echinochloa polystachya (HBK) Hitchc. var. spectabilis (Poaceae) take place in the relatively deep depressions where the currents are moderate, or near the main channels: in fact, these two species seed dispersal is insured by waters, and also by fish in the case of C. latiloba. (Rankin & Merona 1988, Lamotte op.cit.). Maclura tinctoria (L.) D. Don ex Steud develops in conjunction with C. latiloba, and these two species constitute a pioneer ligneous vegetation in the older depressions, far from the actual currents.

When the sedimentation rate is poor, the vegetation succession is also stopped. So the pioneer populations -paradoxically- regenerate on the site.

These pioneer trees, for their axis proliferation due to a strong reiterative ability (Lamotte 1993) and the C. latiloba stilt-roots, participate in retaining the vegetal debris and the sediments, transported by the waters. So they play a non-indifferent part in the sedimentation process, and in a long-term prospect they largely contribute to the installation of less flood tolerant species, mainly: Annona hypoglauca Mart. (Annonaceae), Nectandra inundabilis Rohwer (Lauraceae), Laetia corymbulosa Spruce ex. Benth. (Flacourtiaceae).

In the annual and medium flooding areas (3 to 5 months), the successional model varies in accordance with the drainage conditions. Gynerium sagittatum develops in the shallow, isolated depressions, where the drainage is sufficient.

This species is not replaced neither by Cecropia membranacea, whose young seedlings do not survive submersion on the annual flooding sites, nor by Cecropia latiloba, which only develops where fish dispersal is possible.

It can be replaced by a graminean stage of semi-aquatic Poaceae: Echinochloa polystachya and Paspalum fasciculatum. The graminean stage degeneration is replaced by heliophilous seedlings development, among them most numerous are of Ficus insipida, Annona hypoglauca, Nectandra inundabilis, Laetia corymbulosa, Maquira coriacea, and Maclura tinctoria. Then F. insipida, gregarious species characterized by a fast growing, constitutes the pioneer shrubby stage. If the internal drainage is good, Calycophyllum spruceanum abundantly develops below the Ficus insipida cover. If the internal drainage is poor, F. insipida develops together with Maquira coriacea. Moreover, Guarea guidonia, Sloanea sp. and Pseudobombax munguba (Mart. & Zucc.) Dugand are scattered.

The development and vegetation colonization in the annually flooded areas, are submitted to the following hydro-geomorphological factors: flood duration, currents circulation, drainage quality and sedimentation rate.

Vertical structure of the tropical flooded forests

The observation of high Amazonian flooded vegetation brings to the forefront three characteristic situations (fig. 2):

1. In occasionally flooded levees vegetation, succession processes lead to the constitution of a stratified forest, composed by emergent trees (up to 40 to 50 meters in the ancient forests), jointly together with a dense medium layer (20-30 meters) and an under layer.

2. On the annually flooded low levees, these processes are disturbed, and expressed by the presence of competitive and gregarious arborescent species, which also are emergent in the ancient forests of the alluvial Amazonian plain. Here these species play the role of pioneer ones, for their abundance and competitiveness. There are no emergent trees in this forest type, whose canopy is composed by a medium high continuous layer (20-30 m).

3. In the areas flooded for a long time succession processes are slackened, sometimes to the point of being indiscernible. The pioneer species paradoxically regenerate, for the lack of rapidly growing competitors. There are no emergent trees, no under layer, and the height is less than 20 m.

To summarize, and following a flooding gradient parallel to the abiotic factors, the high arborescent species disappear, being replaced by lower forests where the under layer becomes scarce until it disappears as well, and finally the ligneous vegetation cannot develop any more (fig. 2). According to edaphic and hydric restrictions, the structural differentiation of the forests is confirmed by Oldeman (1974), Jeník (1976), Kahn (1983) and Laumonier (1983) in the humid tropical zone. It can be considered as the manifestation of a precise ecological context, where edaphic constraints provoke the simplification of the forest structure as well as a biodiversity reduction (Lamotte 1993), in a way which can be compared with the influence of the climatic conditions applied on the forests at high latitudes (Richards 1952).

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