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Abstract

Tree distribution, establishment and seedling growth was analysed in the Anavilhanas archipelago (Rio Negro, Brazil). Along a toposequence of the islands, a substitution of communities was observed which is related to the annual flooding duration, depending on the position along the flooding gradient. The palm Astrocaryum jauari was the woody plant with the largest amplitude: it was found on sites flooded from as little as 30 to as much as 340 days a year. The communities of this environment disperse their fruits in synchrony with the high water of the Rio Negro. Some diaspores are buoyant and some are not. Seeds may be eaten by fish and, depending on the species, may be dispersed or destroyed. Also barochory is a frequent dispersal syndrom: the heavy diaspores fall down and may remain close to the adults, or slide down the slopes of the islands. Hydrochory is frequently found and may favour the migration of propagules to colonize distant areas, often associated with ichthyochory. Close to the adults, the number of seedlings was very high, that of older saplings low as a result of competition with other plants and the adult trees. The competition for nutrients and different tolerance of flooding are responsible for the efficiency of seedling establishment. Besides intrinsic aspects of the species, determining specific physiological tolerances to flooding, pluriannual cycles of high water or dry periods explain the distribution of individuals of different species on extreme sites of the floodplains of the Anavilhanas archipelago, whereas the annual cycles influence the growth and local densities of species.