Aquatic plants in the rice fields of the Tagus Valley, Portugal

Abstract

In 1997, in 50 surveys of irrigated rice fields (Oryza sativa L.), 47 species of vascular plants were recorded in 19 families. Most of the species belonged to the Gramineae and Cyperaceae families. Seven species of algae were found, mostly Hydrodictyon reticulatum (42%) and Chara vulgaris (38%). Echinochloa crus-galli and Paspalum paspalodes showed a relative frequency of 98% and 82%, respectively. An interpretation of the major factors is presented influencing weed distribution. Amongst the weeds recorded, 13 were alien species. For these alien species the probable date of introduction into Portugal and into rice fields was determined. The recently introduced neophytes Heteranthera limosa and Heteranthera reniformis and the increasing presence of red rice (Oryza sativa), represent a potential danger to rice culture.