Lotus tenuis seedling establishment and biomass production in Flooding Pa mpa grasslands (Buenos Aires, Argentina)

Biomass and plant density of Lotus tenuis Waldst. & Kit. ex Willd. have been reported in decreasing in grasslands and pastures. Our objective was to determine if L. tenuis biomass and plant density can be increased in grassland through seed addition. Two separated experiments under cattle grazing exclusion were conducted in three paddocks of a Flooding Pampa grassland. The first experiment was from autumn 2004 to autumn 2006 and the second from autumn 2005 to autumn 2007. Different L. tenuis seed additions (0, 57, 229, 917 and 1833 seeds m^sup -2^) were broadcast into experimental plots. In the second experiment, besides seed additions there was a reseeding of approximately 900 seed m^sup -2^ from seed rain produced by plants of grassland. Seed density explained the 81% and 19% of the variation in seedling density and L. tenuis biomass, respectively. Seedling emergence occurred mainly between autumn and early spring, while seedling mortality was mainly between late spring and early summer. Lotus tenuis adult plant density and biomass production increased with seed additions. Total biomass production in the plant community varied between 589.94 ± 26.89 and 1042.44 ± 54.39 g m^sup -2^ yr^sup -1^ and the differences were principally attributed to precipitations. Lotus tenuis biomass contribution was of approximately 10%. The results suggest that L. tenuis seedling and plant establishment and biomass production can be increased through seed addition and/or seed rain through grazing exclusion during reproductive period. In the first period seed addition explained the 81% and 19% of the variation in seedling density and L. tenuis biomass, respectively. Lotus tenuis response to seed addition could have been conditioned by its grazing history and climate variables, for example, precipitations. Under continuous grazing the removal of reproductive and vegetative organs of L. tenuis can negatively affect the seed production and consequently the soil seed bank size ([Bologna] et al., 1996; [Sternberg] et al., 2003; [Osvaldo N. Fern]ández et al., 2008). Lotus tenuis soil seed bank recorded in the same grassland, near of the paddocks, was estimated in approximately 50 seed m-2 (Fernández et al., 2008). Therefore, the increment of L. tenuis seedling with the seed additions could be in response to the low soil seed bank. In the second experiment, L. tenuis seed rain produced during the period 30 December 2004 to 31 March 2005 increased the soil seed bank and determining l