Vegetation and soils of tidal floodplains of the Amazon estuary: a comparison of várzea and terra firme forests in Pará, Brasil

Abstract

The floodplains of sediment-rich rivers comprise 200 000 km2 of the Amazon basin. Varying flood regimes within this region contribute to different forest formations along the basing. This study compared tree species compositions, forest-floor litter biomass, and soil chemistry of three sequentially-aged secondary forest stands of tidal "várzea" (5,15, and 35 years old) and one "terra firme", (non-inundated) stand of mature forest in the southern Amazon estuary. Tree species diversity was much lower in the várzeas than in the terra firme. Total tree basal area was highest in the terra firme, followed by the 35-, 15-, and 5-year várzeas respectively. In each várzea site, the Leguminosae and Palmae families comprised more than 50 percent of total basal area. Forest-floor litter was significantly higher in the terra firme than in the várzea sites. Water drainage was an important influence on vegetation and soil chemistry among the várzea sites. Concentrations of basic cations (Ca, Mg and K) and extractable P were higher in the soils of the three várzeas than of the terra firme. The pH of the top soil in the terra firme was significantly lower than in all other sites except the oldest várzea. Total C concentration in the soil was highest in the 5- and 35-year várzeas. Total soil N also was higher in the tidal várzeas than in the terra firme site. Interpretation of these findings was limited due to lack of proper site replication, and because age and inundation regimes both varied among sites. However, the results tend to confirm the expectation that soils of tidal várzeas are more fertile than those of terra firme sites, and that tidal várzeas present a more limited species composition than terra firme forests.