Along the floodplain there is a flooding gradient that increases in height and length, from the highest to the lower portions of the topography. Differences in height and in time of flooding are reflected in the structure and plant diversity and distinguish low várzea forests of high várzea forests. However, the position that individuals of a species occupy is related both, flooding tolerance and competitive ability of species. The point-of-no-return hypothesis proposes that tree species highly adapted to flooding are not able to compete successfully in non-flooded. In addition, the stress-gradient hypothesis redicts that facilitation is more common in high abiotic stress conditions in relation to abiotic conditions more benign. This study aimed to understand the mechanisms that determining the distribution of tree species in two different forest types of the várzea the Central Amazon. For this, we selected the tree species Crataeva benthamii typical of low várzea and Hura crepitans, typical of high várzea. Seeds of each species were characterized according to size, fresh weight, content of reserves and germination. To evaluate the occurrence of the species in the two forest types was done a field survey of seedlings, young individuals and adults in areas three of 0.5 ha in low várzea and more three in high várzea of a várzea forest of Central Amazonian. Then an experiment was conducted in greenhouse, which evaluated the effect of flooding in combination with interspecific interaction (two flood treatments [not flooded with daily watering; flooding, apical meristem submerged for 60 days] x two interspecific interaction treatments [without interspecific interaction; in interspecific interaction] x 20 replicates), on the growth and biomass of the species. To access the intensity and importance of the interaction were calculated from the total biomass, respectively, the Relative Interaction Index and the Importance Index. In the field, we measured the effect of forest type in combination with interspecific interaction (two treatments of forest type [low várzea, high várzea] x two treatments of interspecific interaction [no interspecific interaction; in interspecific interaction] x 20 replicates) on the growth of the species. Hura crepitans, typical of high várzea, presented seeds larger, heavier and with greater total amount in all reserve substances evaluated. However, Crataeva benthamii showed higher germination, shorter mean germination time, higher coefficient of uniformity of germination, higher germination index, lower mean emergence time and higher emergence. Seedlings of Crataeva benthamii and Hura crepitans were found in both low várzea and in high várzea, but in different densities. In the greenhouse the fastest growing and largest incorporation in biomass of Crataeva benthamii occurred in the flooded treatment, unlike Hura crepitans. Under flood the low várzea species, Crataeva benthamii favored incorporation into biomass of the high várzea species, Hura crepitans. In the field there was no effect or interaction or the type of habiat to Crataeva benthamii. However, seedlings Crataeva benthamii, especially in high várzea, had the apical meristem and stem predated, compromising survival. Already Hura crepitans, despite not having shown an interaction effect, had higher growth in low várzea. The results related to biometrics, the amount of reserve substances and germination seeds of both species assist in the understanding of the characteristics related to the growth and establishment of the same. We found evidence to support the point-of-no-return hypothesis, however, it is believed that herbivory is important as flooding and competition acting to determining the distribution of species in high várzea. The results also showed that the species more tolerant to flooding, Crataeva benthamii facilitated the incorporation into biomass of Hura crepitans, thus confirming the stress-gradient hypothesis.
