The Amazon has great floristic and landscape diversity, which a is result of habitat heterogeneity, high number of species per area, and variation in species composition, even between nearby areas. The aim of this study was to investigate and describe the different vegetation types found in northwestern Rondônia, along a section of the Madeira river. Surveys were conducted in 37 sampling points where data on structure and composition of vegetation, soil samples, measures of the variation of the water table, and herbarium “vouchers” were collected. In the study area a variety of phytophysiognomies with rich and varied flora was found. The most frequent vegetation type was the Open Ombrophylous Forest (Open Terra Firme Forest), divided in three sub-types: with palms (Fp), with sororoca (Fs), and with bamboo (Fb). Other types were also recorded such as Dense Ombrophylous Forest (Fd; Dense Terra Firme Forest), and a small patch of Semideciduous Seasonal Forest (Fm) growing on rock outcrops. The Alluvial Dense Ombrophylous Forest (Fa; Varzea Forest) occurred along the margins of the Madeira river. The Campinaranas form a mosaic in the landscape with a vegetation gradient ranging from forest formations to grasslands and can be divided into four sub-types: Campinarana Florestada (Cf), Arborizada (Cd), Arbustiva (Cv) and Gramíneo-Lenhosa (Cg). These formations occurred only at the right margin, south of the Madeira river, in plains formed by alluvionar sediments subject to seasonal flooding by the rise of the water table. The phytophysiognomies recorded differ in structure, species composition, soil features and water table level. The structure ranged from tall forests (25 m average) to grasslands, with canopy openness ranging from 11% to 70% or even absence of canopy in grasslands. Soils varied from well-drained (with water table reaching 10 m deep during the dry season) to seasonal flooded during the rainy season, and for each physiognomy analysed the water table showed a particular iv behaviour, with different maximum and minimum depth. The soil in the region is acidic, showing variable fertility, being richer in Fa. The first two axes of a principal component analysis (PCA) including nine soil variables (clay, sand, silt, pH, sum of bases, Al, C, N and P) explained 58.7% of the variation in soil parameters between the physiognomies found in the study area. Regarding the floristic composition, some species can be regarded as indicators of vegetation types: Attalea speciosa and Eschweilera coriacea in Fp, Phenakospermum guyannense (sororoca) in Fs, and Ruizterania retusa in Cf and Cv. A detailed description of the vegetation in the study area revealed a great diversity of physiognomies and their particularities. This information should be considered in conservation planning of the natural resources in the region, assuring that attention is paid to all phytophysiognomies, encompassing the diversity of landscapes of the region as a whole.
