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New Site Formation and Colonizing Vegetation in Primary Succession on the Western Amazon Floodplains

Risto Kalliola, Jukka Salo, Maarit Puhakka and Marjut Rajasilta
Journal of Ecology
Vol. 79, No. 4 (Dec., 1991), pp. 877-901
DOI: 10.2307/2261087
Stable URL: http://www.jstor.org/stable/2261087
Page Count: 25
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New Site Formation and Colonizing Vegetation in Primary Succession on the Western Amazon Floodplains
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Abstract

(1) The major proportion of western Amazon forests grow on fluvial deposits and thus originated in floodplain environments. The fluviodynamic character of the sites initially colonized by plants was studied along different river types, and this information was combined with botanical observations from the same areas. Special emphasis was given to colonizing plant distribution and survival in relation to the abiotic environment. (2) Four frequently occurring landform types, each rich in microforms, were recognized in relation to the colonization process: fluvial bars, swales, abandoned channels and riverbanks. They are affected by seasonal fluctuations in the rivers and tend to be narrow, curved or linear patches. Local site and colonizing vegetation characteristics vary considerably between different river types (meandering or braided, rich or poor in suspended sediment). (3) The newly deposited fluvial sediments are poor in organic carbon and nitrogen. Colonization begins either with immigrant propagules dispersed by wind or water, or with the invasion of species by vegetative means. Usually numbers of individuals are few, and the initial vegetation pattern is a reflection of the small-scale mosaic of microsites. The pioneer flora (125 species recorded) includes many widespread perennial herbs, and seeds of most forest species do not germinate in the dry sediments. Herbaceous colonist species are almost non-existent in suspension-poor rivers of low erosion rate. (4) The persistence of the initial plant assemblage is controlled by the evolution of fluvial landforms. Only a few species among the set of colonists are significant in later forest succession. These include Tessaria integrifolia and Gynerium sagittatum, which resist moderate flood damage and resprout after burial. Mature seed populations of these 10--15-m-tall species are abundant at the outermost margin of the successional forest just beyond the fluvial bars. The low representation of tree species in the colonizing phase suggests that the extreme environmental conditions at the river margins differ from other natural environments along the Amazon. (5) Aquatic succession on lakes affected by suspension-rich waters starts with genuine floating species. The general habits of these vegetation assemblages and their species composition are highly similar in the `white-water' floodplains of the study area. An annual vegetation flush characteristically follows the flood period. On the other hand, macrophyte vegetation is almost non-existent on the floodplain lakes of suspension-poor rivers. (6) The Amazon floodplain corridors provide a good opportunity for ecological studies of plant colonization because basically similar succession is present at each meander along the rivers. Furthermore, the basic physiographic processes which forms the sites and control their further evolution are both well known and measurable.

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