Gap disturbance triggers the recolonization of the clonal plant Ambrosia tenuifolia in a flooding grassland of Argentina

Flooding provokes the death of many dicotyledonous species in grazed grasslands of the Flooding Pampa in Argentina, including the clonal plant Ambrosia tenuifolia, which produce the opening of numerous gaps. The objective of this study was to investigate the recolonization of grassland by A. tenuifolia after this species disappeared due to the occurrence of prolonged flooding events. To this end, responses of seed germination to environmental factors associated with gaps, such as light quality and temperature regime, conditions related to seedling survival, and clonal growth of ramets outside the gaps were studied in two different experiments in the field. Environmental factors related to gaps promoted the recruitment of new genets. The combined effect of alternating temperatures and the high red : far-red ratio set off germination from the soil seed bank; germination also was enhanced when signals were generated artificially under the intact canopy in the field. Higher resource availabilities and maximum seedling survival were recorded in canopy gaps, which were the focus of invasion. Grassland recolonization outside the gaps continued rapidly by clonal growth, from small gaps and large ones, even within the dense surrounding canopy. This provoked an intense competition with the other species. Gap opening by disturbances, seed germination in gaps and clonal growth were decisive for the recolonization of A. tenuifolia populations. This sequence of events triggered the recolonization of the plant community by this species, in sites where it had been eliminated by prolonged flooding. This process represents one of the most significant fluctuations in the vegetation dynamics of the Flooding Pampa Grasslands.