A 53-tree plot network from southeastern Peru was used to evaluate the effects of niche-based and dispersal-based processes on the floristic composition of communities from four environmentally distinct but geographically close geomorphological units: floodplain, successional forest, swamp, and terra firme. The tree plots were arranged in floristic space using non-metric multidimensional scaling ordination, and the effects of distance and geomorphology on floristic similarity were evaluated using multiple linear regressions. The effect of habitat was further explored by comparing observed habitat restriction with a null expectation. The geomorphologies were floristically distinct, and species were more habitat-restricted than expected by chance. There was a significant effect of distance on floristic similarity, most of it at very short scales (< 5 km). Geomorphology explained most of the variation at an intermediate scale (< 50 km) and distance explained most of the variation at a larger scale (up to 300 km). Distance explained little (~5%) of the across-habitat similarity, while it explained more than 30% within habitats. While distance affected floristic composition, its effects were overridden by geomorphology. The large scale distance effect may be due to the spatial variation in fluvial dynamics. The short scale distance effect probably reflects dispersal and mass effects. Geomorphology may locally be a valid proxy for environmental variables but seems to fail to summarize environmental variation at large scales.