Soil seed banks in the semi-arid Prepuna of Bolivia

The Bolivian Prepuna is a high-altitude, summer-rain semidesert. The aim of this work was to sample, for the first time, the composition, abundance, and distribution of its seed bank in two different seasons. Soil samples were collected at four sites with a corer (3.5 cm diameter, 2.5 cm deep), 80 in January (summer) and 80 in May (autumn). On each date, half the samples came from under shrubs and half from between shrubs. Seeds were separated from the soil by a flotation-filtering technique and compared with a reference collection established previously. Seeds belonging to a few species, mostly summer annuals, were found in the seed bank. These had heterogeneous distribution in the soil. However, there were more seeds in areas under shrubs. The relatively low seed bank density (1150-1650 seeds/m²) may have been due mainly to precipitation values well below the average in the previous growing season. Despite its low density, seeds > 0.25 mg represented 92% of seed bank's total dry weight. Long-term studies are needed to assess true seasonal and annual fluctuations of Prepuna's seed bank.