Disturbance severity and seed availability as controls of invasibility in Flooding Pampa grasslands.

GARIBALDI, L. A.; SEIDLER, T.; CHANETON, E. J.

Mérida, México.

Congreso; International Conference, Ecological Society of America.; 2006

We tested how disturbance and seed availability of exotic plants interacted in determining patterns of grassland invasibility in the Flooding Pampa of Argentina. A disturbance severity gradient was created by randomly removing 0, 5, 20, 40, 80 or 100% of the aboveground plant biomass from 3x6m plots. In each of 36 plots (6 per disturbance level), we added seeds of 12 species to 1x1m subplots, including the most important exotic grasses and forbs (900 seeds/m2 per species), and compared seedling recruitment rates with unsowed subplots. In the 100% removal plots, we further delimited 5 subplots and randomly sowed them with 0, 5400,10800 or 32400 seeds/m2. The remaining subplot was sowed with an equal biomass of seeds per species (i.e. different numbers of seeds per species). Disturbance, seed addition and seed density all affected overall recruitment dynamics. Yet the importance of each factor varied according to plant functional group and season. Exotic forbs were strongly seed-limited, showing positive responses to increased seed sowing densities, and non-linear (quadratic) or linearly positive responses to increased disturbance severity (at the earlier and later stages of the growing season, respectively). Whereas recruitment of exotic grasses did not vary with seed addition or sowing density, it also showed a non-linear response to disturbance severity with a peak at intermediate disturbance levels. Our results suggest that seed availability, microhabitat constraints, and biotic interactions (competition and facilitation) all contribute to determine the dynamics of exotic species invasion in this grassland. The relative importance of these factors may depend on the functional identity of invading species.