SEED DISTRIBUTION CONSTRAINS THE DYNAMICS OF THE PATAGONIAN STEPPE

The Patagonian steppe is formed by tussock grasses and shrubs in a bar e-soil matrix (50% cover), and as in other arid and semiarid systems, vegetation is arranged in patches. Although there is a good understand ing of the probabilities of seedling establishment of Bromus pictus in relation to location within these patches, these probabilities accoun t for only a portion of the spatial dynamics of the community. The obj ective of this paper was to assess the pattern and dynamics of Bromus pictus seed availability in this community, which represent the other portion of the recruitment equation. We first evaluated its seed bank along transects. Plant- or litter-covered areas had 85% of the sampled seeds; however, they accounted for 55% of the area. Bare-soil areas h ad seeds only if they were located close (less than or equal to 10 cm) to a plant or litter microsite. In a second study, we analysed the mo vement of seeds using pitfall traps. Traps located near plants capture d seed amounts similar to those from traps located in bare-ground area s (far from plants). The contrast between the high number of seeds in transit and the low number of seeds retained by bare-ground microsites illustrates the importance of lateral secondary movement of seeds. We used this information on seed distribution and previous data about es tablishment probabilities to estimate the spatial pattern of recruitme nt. Microsites that are vegetated or close to individual plants are ex pected to recruit the greatest number of individuals. Vegetation patch es play an important role in modifying ecological processes in arid an d semiarid communities. Our results highlight the importance of seed d istribution in the formation and maintenance of these patches.