We investigated temporal changes and spatial variation of soil drainage and understory light availability in 2001 and 2002, and small stem (5 ≤ dbh < 10 cm) density, forest successional phase and large stem (dbh ≥ 10 cm) spatial distribution in one ha of tropical swamp forest in SE Brazil. Building patches and treefall gaps comprised, respectively, 69.75% and 7.5% of the area in 2002. Semivariograms indicated spatial segregation of successional phases, with mature areas predominating in the North and gaps aggregated in the South. Exclusion of outliers showed large unpredictability of We tested the hypotheses that: (1) reproductive Geonoma brevispatha, an understorey clonal palm endemic to South American swamps, is most abundant in more brightly lit microsites but the abundance of juveniles is not responsive to light availability; (2) the species is restricted to the transitional zones between flooded and well-drained microsites; (3) if hypotheses (1) and/or(2) are accepted, the microhabitat specialization they represent should be reflected in the spatial distribution of the individuals at distinct scales. The study was carried out in a swamp forest in south-eastern Brazil. Soil moisture was autocorrelated at distances up to ca. 1 m, reflecting a finescale microtopographic pattern of flooded pits and channels delimited by drier mounds. The first hypothesis was rejected, but the second hypothesis was accepted. No genets occurred on flooded microsites. A juvenile emergence experiment showed that seeds were unable to develop into juveniles under flooded conditions, but canopy openness did not influence the number of juveniles emerging. The third hypothesis was accepted. Individual genets were randomly distributed at scales corresponding to soil moisture patches, but were aggregated at larger scales.Juvenile genets were positively associated with reproducer genets. Our results indicate that palms may specialize on narrow parts of moisture gradients in swamp forests, and that this specialization may exclude them from even subtly distinct microhabitats.