Effects of seasonal flooding and grazing on the vegetation of former ricefields in the Rhône delta (Southern France)

Abstract

Six management regimes were tested during 5 years in 18 abandoned ricefields in the Rh^one delta, France: two artificial floodings for 6 months (winter and summer flooding, 10 cm deep) and a control only flooded by rain, each flooding treatment either with or without grazing by cattle and horses. In the absence of artificial flooding and in presence of grazing by domestic herbivores (i.e., maintaining the initial management since the abandonment) no significant change in plant communities was recorded after 5 years. The vegetation was mainly composed of halophytes (Salicornia fruticosa and Inula crithmoides). The removal of grazing led to the dominance of a salt tolerant grass: Aeluropus littoralis. Flooding favoured the dominance of clonal plants and led to a decrease in the number of species. In the ungrazed fields, changes in plant communities were related to the height of species with Bolboschoenus maritimus and Phragmites australis becoming dominant. When grazing was combined with summer flooding, B. maritimus dominated the first two years of the experiment, but with a low cover, and was replaced in the 3rd year by Typha angustifolia. When grazing was combined with winter and early spring flooding the competitive exclusion of B. maritimus by Juncus gerardii slowed the establishment of the former. The management of former ricefields led to the establishment and dominance of emergent species common to Mediterranean wetlands. Although it is subordinate to the maintenance of artificial flooding, the project may be considered a restoration (or a rehabilitation) of seasonally flooded marshes as original functions existing before the land was put under cultivation are re-established.