In Amazonian floodplains, plants are subjected to prolonged periodical flooding with a water column of up to 10 m. Seedlings are submerged, except if they establish in the upper parts of the inundation gradient and grow so fast to be able to maintain some leaves above the water surface. The hypothesis of this study is that there are mainly two strategies for seedling establishment in Amazonian floodplains: tolerance of prolonged submergence vs. escape from submergence. Measurements of height increase in relation to floodplain system and nutrient availability, height in the inundation gradient, and seed mass of 31 common species indicated that the two hypothesized strategies enable efficient seedling establishment on different levels in the flooding gradient. Where submergence is unavoidable, on the lower sites in the flooding gradient, seedlings had significantly lower shoot extension than on the higher sites. Large and small seeds produced seedlings with high shoot elongation, enhancing the chances of non-submergence for the seedlings at high elevations. This pattern was evident also within the family of Fabaceae, taking into account the effect of relatedness among species. In igapó high seed mass compensated the lack of nutrients of the environment. In nutrient-rich várzea, nutrients are supplied by the environment and seeds are smaller, but they may produce seedlings which are just as tall. This emphasizes the strong selective pressure for tall seedlings on high elevations in the flooding gradient. It is assumed that the zonation observed in Amazonian floodplains is directly linked to the flooding tolerance and establishment strategy of the seedlings, and less to the tolerance of the adult trees. © 2002 Elsevier Science B.V. All rights reserved.
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