Seed mass and dormancy of annual plant populations and communities decreases with aridity and rainfall predictability

Several theoretical and empirical studies have examined the influence of environmental conditions on seed traits and germination strategies of annual species. A positive relationship between seed mass and dormancy has been described for annuals occupying climatically unpredictable ecosystems. Larger-seeded species tend to have higher seedling survival rates, while dormancy allows a bet-hedging strategy in unpredictable environments. Until now, these ideas have been addressed primarily for only one or a few focal species, without considering differences among populations and communities. The novelty of the present study lies in the population and community-level approach, where a comprehensive seed trait database including 158 annual species occurring along a gradient of rainfall variability and aridity in Israel was used to ask the following question: Does average seed mass and dormancy of annual populations and communities decrease with increasing aridity and rainfall unpredictability? Soil seed bank samples were collected at the end of the summer drought, before the onset of the rains, from four plant communities. Germination was tested under irrigated conditions during three consecutive germination seasons to determine the overall seed germinability in each soil sample. Seed mass was obtained from newly produced seeds collected at the study sites in late spring. The community level results showed that, in contrast to common theoretical knowledge, seed mass and dormancy of the dominant annual species decreased with increasing aridity and rainfall variability. Accordingly, a negative correlation was found between seed mass and seed germination fractions. The present study demonstrates that an analysis of seed traits along climatic gradients is significantly improved by approaches that target both population and community levels simultaneously. A critical evaluation sheds new light upon the selective pressures that act on seed ecology of annuals along a climatic gradient and facilitates formulation of more mechanistic hypotheses about factors governing critical seed traits.