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

Abstract

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 (361300 S, 581300 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.

Keywords: Community structure; Flooding; Grassland recovery; Grazing; Lotus tenuis; Trampling.

This abstract was originally published in Applied Vegetation Science 14 (2011) 95–106. Doi: 10.1111/j.1654-109X.2010.01093.x