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

Abstract

AbstractQuestions: What are the interactive effects of flooding and cattle tramplingupon the structural attributes and the floristic composition of a plant commu-nity? Do the effects on the plant community persist over an extended recoveryperiod?Location: Flooding Pampa grasslands, Argentina (36 130 0 S, 58 0 W).Methods: We assessed the effects of 40-d of flooding, trampling and thecombination thereof on plant cover and biomass, vertical distribution of foliageand floristic composition in lowland grassland mesocosms. We considered a120-d recovery period to evaluate the persistence of flooding and tramplingeffects on the plant community.Results: Flooding, with or without trampling, increased cover and biomass ofthe graminoid species, especially marsh grasses, which developed a tallercanopy, whereas most of the forb species were negatively affected. This wasenhanced by trampling, as the aerial biomass of the dominant legume Lotustenuisdecreasedby90%,whilethreemajorforbspeciesdisappeared.Tramplingunder flooding conditions did not reduce the total above-ground biomassproduction, as the growth enhancement of graminoids was enough to com-pensate for the breakdown of the forbs. Below-ground biomass was lowerwhen 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 theend of the growing season.Conclusions: The combination of flooding and trampling shifts the communityco-dominance of graminoids and forbs towards a persistent dominance ofgraminoid species. When both perturbations are combined, the above-groundproduction of the grassland is unaffected and root biomass is rapidly recovered.However, the loss of the legume L. tenuis deserves attention because this is theunique nitrogen-fixing species of the ecosystem, which improves the foragequality for livestock production.IntroductionTrampling and flooding are two major co-occurring per-turbationsinmanyhumidgrasslandecosystemssubjectedto livestock grazing (Soriano 1991; Milchunas & Lauen-roth 1993; Kohler et al. 2004; Altesor et al. 2006). Thoseperturbationsexertantagonisticeffectsuponthestructur-al attributes and the floristic composition of the plantcommunity.Tramplingmodifiesthecommunitystructureby flattening out the vertical distribution of the canopy(Edmond 1964; Pellerin et al. 2006; Van Uytvanck et al.2010), and changes the floristic composition by favouringprostrate stoloniferous graminoids and rosette forbs (Salaet al. 1986; Sun & Liddle 1993; Cole 1995a; Kobayashietal.1997).Incontrast,floodingmodifiesthecommunitystructure by promoting the location of foliage in upperlayers of the canopy (Pehrsson 1988; Insausti et al. 1999;Striker et al. 2005) and subordinating species with rosetteand creeping growth forms, which remain mostly sub-merged and shaded during the flooding period (Insaustietal.1999;VanUytvancketal.2010).Todate,theeffectsof