Weed community dynamics in two pastures grown after clearing Brazilian Amazonian rainforest

To explain the dynamics of the weed vegetation in the Amazonian pastures after clearing of the rainforest, the diversity and floristic composition of 15 Brachiaria brizantha pastures and seven Panicum maximum pastures of various ages were studied. These pastures reflect the original forest, from which they retain a high species richness (from 20 to 73 species on average per 50 m2), a high proportion of forest woody species (from 26% to 65%) and rare species (> 50%). The four ecological groups of plants, namely forest woody species, pioneer/secondary woody species, forest herbaceous species and secondary herbaceous species, are present in all pastures immediately after the clearing and burning of the forest and have to compete with the two sown forage species, B. brizantha and P. maximum. Because of its erect habit, P. maximum is rapidly invaded by secondary herbaceous weeds, whereas B. brizantha with its spreading habit slows down their proliferation, which therefore takes about 10 years to complete. The study showed the importance of factors such as the initial vegetation, manual-clearing practices, duration of cropping and species grown in influencing the floristic composition of the pastures and on the diversity of weeds. These results show the response of weeds faced with human disturbance. The results also demonstrate that farmers can manage pastures in a spirit of conservation of diversity or for weed control