Riqueza e abundância de Poaceae em savanas do estuário amazônico, Brasil.

Abstract

Most studies of savannas in Amazônia focus on the tree stratum, while little is known about the dynamics of the herbaceous plants, especially the grasses (Poaceae). This study presents an inventory of Poaceae in savannas of the Amazon estuary and analyses the environmental factors which influence their grass communities. Besides soil characteristics and climate/seasonality, the human impact on diversity and productivity of these communities is considered. The study area included nine sites within the estuarine region of the Amazon river, situated in the Brazilian states of Amapá and Pará. At each site, 160 plots of 1 m2 were established within an area of 10×100 m and all occurring species of Poaceae within plots were identified.) A superficial soil sample (0-20 cm depth) consisting of 20 subsamples was collected every 10 meters along the parcels. The soil samples were analysed using Embrapa standards. At one site (Campo da Mangaba/Maracanã), the herbaceous plants of five plots were harvested in three months intervals by cutting aerial shoots and leaves directly at the soil surface. Harvested material was separated into three classes (Axonopus polydactylus; Axonopus aureus; other species); biomass and total protein content were determined for each class after drying and grinding. Grass communities presented low species richness (in total 41 species in 19 genera from four subfamilies) and few endemics; the genus Alloteropsis J. Presl (Panicoideae/Paniceae s.l.) is reported for the first time for South America. Identification keys, illustrations and comments about geographic distribution are presented together with taxonomic remarks. Coverage of Trachypogon spicatus increased with soil fertility, while that of Axonopus spp. decreased; organic matter and coarse sand/silt fractions were the most influential soil parameters. Biomass and total protein content showed seasonal variation, with highest/lowest values during rainy/dry season, respectively. Biomass and protein content levels ranged below the recommended viability limits for cattle farming. Human impact could be responsible for low species richness.