Seed production timing influences seedling fitness in the tropical live oak Quercus oleoides of Costa Rican dry forests

PREMISE OF THE STUDY: Reproductive phenology is important for tree species that occur in seasonally dry environments, particularly for those with desiccation-sensitive, nondormant seeds. In this study, we compared germination, growth, and survival of seeds of the evergreen tropical live oak Quercus oleoides produced at different times during the wet season at two sites that differ in rainfall along an elevation gradient. Our goal was to determine the effects of reproductive timing on germination and juvenile fitness for this widespread species in seasonally dry forests of northwestern Costa Rica. METHODS: We collected seeds early and late in a single wet season from two populations with contrasting rainfall and reciprocally planted them into common gardens. Two watering treatments (ambient and supplemental watering) were established at the drier low-elevation garden. Seeds were exposed to ambient rainfall at the wetter high-elevation garden. We conducted selection analyses using aster models to examine variation in selection on seed size and timing of germination. KEY RESULTS: Trees of Q. oleoides had higher fitness when seeds were produced, dispersed and germinated late in the wet season. Postgermination, water limitation during the dry season reduced seedling fitness by decreasing survival but not growth. CONCLUSIONS: In contrast to studies in temperate climates where earlier germination is typically favored, we show that selection on days to germination is temporally and spatially heterogeneous. Selection was found to favor either rapid or delayed germination depending on seed cohort and habitat.