SEASONAL-VARIATION IN THE SEED BANK OF A PATAGONIAN GRASSLAND IN RELATION TO GRAZING AND TOPOGRAPHY

The germinable seed bank of Festuca spp., Carex patagonica, and other annuals and perennials in a semiarid Patagonian grassland was analyzed every three months. The effects of grazing, topography and microsites with respect to established grass tussocks on the germinable-seed bank were also analyzed. The total germinable-seed bank was larger in summer after seed rain. At this time of the year most of the seeds were from annuals and perennial dicots. Seeds of the dominant Festuca spp. were the main components of the graminoid seed bank, which was homogeneously distributed in patches of bare soil. In spring, i.e. some months after the seed rain, the germinable-seed bank of most of the species was greatly reduced, while the seed bank of Carex patogonica did not change significantly. At this time of the year, the graminoid seed bank was heterogeneously distributed in space, with the seeds accumulating in wind-protected microsites. In the grazing treatments, the seed bank of the perennial grasses was reduced proportionally to the decrease of the plant cover. On the other hand, the germinable-seed bank of C. patagonica increased with the grazing treatments, in relation to the increase of vegetation cover. Topographical position had two types of effect on the seed bank of perennial grasses. One was the increase of the germinable-seed bank, after the seed rain, on the relatively warmer northeast facing slopes, which was related to a higher seed rain in these places. The other was an increased seed loss on slopes. The different seed-bank strategies observed in this grassland are compared and discussed in relation to strategies described for other grasslands.