Trampling enhances the dominance of graminoids over forbs in flooded grassland mesocosms

Questions What are the interactive effects of flooding and cattle trampling upon the structural attributes and the floristic composition of a plant community? Do the effects on the plant community persist over an extended recovery period? Location Flooding Pampa grasslands, Argentina (36 degrees 30' S, 58 degrees 30' W). Methods We assessed the effects of 40-d of flooding, trampling and the combination thereof on plant cover and biomass, vertical distribution of foliage and floristic composition in lowland grassland mesocosms. We considered a 120-d recovery period to evaluate the persistence of flooding and trampling effects on the plant community. Results Flooding, with or without trampling, increased cover and biomass of the graminoid species, especially marsh grasses, which developed a taller canopy, whereas most of the forb species were negatively affected. This was enhanced by trampling, as the aerial biomass of the dominant legume Lotus tenuis decreased by 90%, while three major forb species disappeared. Trampling under flooding conditions did not reduce the total above-ground biomass production, as the growth enhancement of graminoids was enough to compensate for the breakdown of the forbs. Below-ground biomass was lower when both perturbations occurred simultaneously. After 120-d of recovery, graminoids continued to be dominant while the remaining forbs (including L. tenuis) recovered only partially. Below-ground biomass recovered fully at the end of the growing season. Conclusions The combination of flooding and trampling shifts the community co-dominance of graminoids and forbs towards a persistent dominance of graminoid species. When both perturbations are combined, the above-ground production of the grassland is unaffected and root biomass is rapidly recovered. However, the loss of the legume L. tenuis deserves attention because this is the unique nitrogen-fixing species of the ecosystem, which improves the forage quality for livestock production.