  Ecological theory predicts that the soil seed bank stabilises the composition
of annual plant communities in the face of environmental variability. However,
long-term data on the community dynamics in the seed bank and the standing
vegetation are needed to test this prediction. We tested the hypothesis that
the composition of the seed bank undergoes lower temporal variability than the
standing vegetation in a nine-year study in Mediterranean, semi-arid, and arid
ecosystems. The composition of the seed bank was estimated by collecting soil
cores from the studied sites on an annual basis. Seedling emergence under
optimal watering conditions was measured in each soil core for three
consecutive years, to account for seed dormancy. In all sites, the composition
of the seed bank differed from the vegetation throughout the years.
Small-seeded and dormant-seeded species had a higher frequency in the seed bank
than in the standing vegetation. In contrast, functional group membership
(grasses vs. forbs) did not explain differences in species frequency between
the seed bank and the vegetation after controlling for differences between
grasses and forbs in seed mass and seed dormancy. Contrary to predictions, the
magnitude of year-to-year variability (the mean compositional dissimilarity
between consecutive years) was not lower in the seed bank than in the
vegetation in all sites. However, long-term compositional trends in the seed
bank were weaker than in the vegetation in the Mediterranean and semi-arid
sites. In the arid site where year-to-year variability was highest, no
long-term trends were observed. Overall, the effect of the seed bank on the
temporal variability of the vegetation in annual communities depends on site
conditions and time scale. While the year-to-year variability of the seed bank
is similar to the vegetation, the soil seed bank can buffer long-term trends.
