Fire-induced colonization of a Flooding Pampa grassland by thistles: Remnant litter and interference effects

Winter and spring burnings constitute a traditional management practice of the Flooding Pampa grasslands which are dominated by the tussock grass Paspalum quadrifarium. In addition to increasing the primary productivity and the nutrient quality of the regrowth, this burning favours the establishment of opportunistic species, especially the legume Lotus glaber and the thistles Cirsium vulgare and Carduus acanthoides. The aims of the present study were to assess the effects of burning and those of the remnant litter on L. glaber and thistle recruitment, as well as the effects of L. glaber density on the emergence, survival and flowering of thistles. Two field experiments were carried out: 1. A completely randomized factorial design with occurrence of L. glaber and fire was applied in a mature Paspalum stand; 2. A completely randomized design with four L. glaber seed densities oversown on a burned Paspalum stand, as the treatments. Thistle and L. glaber recruitment within the Paspalum stand was dependent on fire, but their emergence in burnt plots was reduced by the presence of remnant litter. Remnant litter and interference from L. glaber provide complementary mechanisms to reduce fire-cued colonization by thistles. While seedling emergence of thistles was mostly affected by the presence of remnant litter, seedling survival depended on local density of L. glaber. By the end of the first post-fire growing season, the survivorship of established thistles was linearly reduced with increasing sowing density and above-ground biomass of L. glaber. The maintenance of a significant litter coverage and a high density of L. glaber in the pre-fire seed bank, as well as the oversowing of L. glaber after fire, may be useful tools in fire management of Paspalum stands aimed to improve their forage value with reduced dependence on chemical control of weeds.Nomenclature: Cabrera (1953).