Seed mass and summer drought survival in a Mediterranean-climate ecosystem

Abstract

We related seed mass to summer drought survival in a Mediterranean-climate ecosystem. Previous experimental evidence linking seed mass and survival under drought is limited and at times contradictory. We tracked summer drought survival among four families/subfamilies at a restoration site in southwestern Australia. We coupled these observations with a glasshouse experiment assessing the growth and root morphology of Acacia and Eucalyptus species, with a range of seed masses, under mild and severe drought compared with a well-watered control. Summer drought survival in the field increased with seed mass across all four families/subfamilies. Seedling root biomass and length increased with seed mass consistently across five harvests over 60 days. Initial survival of seedlings in the glasshouse increased with seed mass and decreased with drought, but there was no interaction between the two. Greater absolute root investment provides a mechanism for both short and longer-term drought survival. Within-species variation in root growth may also affect the relative versus absolute survival advantage of large-seeded species. The benefits of large seed mass for establishment under environmental hazards are often considered to be temporary. Our results show that seed mass was correlated with other traits, including root length, which in turn, increased longer-term drought survival. Traits correlated with seed mass should therefore be considered in explanations of the ecological effect of seed mass variation.