  The soil seed bank is a major component of plant communities. However,
long-term analyses of the dynamics of the seed bank and the ensuing vegetation
are rare. Here, we studied the dynamics in plant communities with high
dominance of annuals in Mediterranean, semiarid, and arid ecosystems for nine
consecutive years. For annuals, we hypothesized that the density of the seed
bank would be more stable than the density of the standing herbaceous
vegetation. Moreover, we predicted that differences in temporal variability
between the seed bank and the vegetation would increase with aridity, where
year-to-year rainfall variability is higher. We found that the temporal
variability at the population level (assessed as the standard deviation of the
loge-transformed density) of the nine dominant annuals in each site did not
differ between the seed bank and the ensuing vegetation in any of the sites.
For the total density of annuals, patterns depended on aridity. In the
Mediterranean site, the temporal variability was similar in the seed bank and
the vegetation (0.40 vs. 0.40). Still, in the semiarid and arid sites,
variability in the seed bank was lower than in the vegetation (0.49 vs. 1.01
and 0.63 vs. 1.38, respectively). This difference between the population-level
patterns and the total density of annuals can be related to the lower
population synchrony in their seed bank. In contrast, for the herbaceous
perennials (all species combined), the seed bank variability was higher than in
the vegetation. Overall, our results highlight the role of the seed bank in
buffering the annual vegetation density with increasing climatic uncertainty
typical in aridity gradients. This role is crucial under the increasing
uncertainty imposed by climatic change in the region.
