The Neotropical savannas, locally called "cerrado", are Brazil's second largest and diverse biome, constituting a heterogeneous physiognomic and floristic mosaic from which more than 6,000 species have been reported to date. This study describes and analyzes the floras and structures of four tree communities from dry savannas in the municipality of Vilhena, Rondônia, Brazil. The study areas were: savanna with trees "cerradão" (Area 1), savannas wich bushes or "campo sujo" (Area 2), and two savanna woodlands or "cerrado" sensu stricto (Areas 3 and 4). In each area an transect measuring 10 x 1,000 m (1 ha) was placed at random and divided into 100 quadrats of 10 x 10 m. All living and dead woody plants with circumference ³ 10 cm at ground level were included in the survey. "cerradão" was the richest in species, had the greatest plant diversity, and showed the largest basal area. The percentages of dead trees were highest in areas that had been recently burned. The most species-rich and abundant plant families were Vochysiaceae, Ochnaceae, Guttiferae, Malpighiaceae and Melastomataceae. The most important species were Qualea multiflora and Byrsonima crassifolia (area 1), Eugenia sp. and Miconia sp.1. (area 2), Ouratea hexasperma and Caraipa savannarum (area 3) Ouratea hexasperma and Pouteria sp3. (area 4). Most species (53%; N=92 spp.) were found in only one of the four areas, indicating that the selection of priority areas for biological conservation should take into account the whole of the savanna mosaic. Tree mortality caused by fire was different among the four study areas, probably as a function of burning recurrency, the degree of flammability, and historical factors involving land use.
Amazonian; Flora; Phytosociology; Tree