State and transitions model for grasslands at the Rio Dulce flooding plain (depresion de Mar Chiquita, Cordoba, Argentina)

Changes in diversity and biomass of higrophyllous and halophyllous plant communities in a wetland system at 30 degree S y 63 degree W are analyzed. Based on structural and functional characteristics, nine vegetation states grouped within emergent hydrophytic communities, prairies and grasslands are described, and five transitions are considered. One of the transitions is just a speculation about the impact on the vegetation and ecosystem of flooding suspension for periods longer than those observed. The flooding variability (annual temporary flooding, or its absence for two or three years), and grassland management (extensive grazing and different frequency of burnts) which occurred from 1989 to 1997 promoted vegetation changes which are interpretated as fluctuations of the same states, with magnitude and characteristics variables according to the analyzed vegetation. These trends suggest different local and global stability of major vegetation types against long drougth periods and increasing salinity. It is proposed that the diversity and productivity of the ecosystem could be affected at different levels (landscape, habitats, communities, plant and animal species). Wetlands should shrink to smaller areas, feeded only by rain and groundwater outcrops; the aquatic communities would be reduced, and the Spartina argentinensis grasslands could replace the praires. We discuss about the impact of these landscape changes on the forage quality, free water and management technique proper of areas usually regulated by fluvial flooding.