Early-season Drought Favors Delayed Germination Strategies in Two Grass Species

The highly adaptive trait of seed germination timing is influenced by cues such as climate and phenology. In turn, timing of seed germination plays an important role in a plant’s overall fitness by temporally influencing access to resources and exposure to environmental conditions. Existing research demonstrates that early emerging individuals often have a fitness advantage. However, despite potential selective pressures favoring early emergence, a wide variation in germination timing persists. We hypothesized that unfavorable environmental conditions at the start of the growing season, such as drought, could result in mortality favoring later emerging individuals. Furthermore, clinal variation in emergence time may exist, whereby populations in areas with greater interannual climate variation might harbor later emerging genotypes. We evaluated how germination timing affects plant fitness by exposing seeds from grass species Stipa pulchra and Bromus diandrus to early-season drought conditions, then recording survival and biomass as proxies for fitness. To study potential clinal variation in the trait, we used populations of seeds collected across the North-South environmental gradient in California. Overall, we found an association between later germination and higher fitness as measured by survival, but no support for clinal variation in germination timing. Our observations suggest that under mortality-event circumstances such as early season drought, temporal variation in germination affects seedling survival, but further research is required to evaluate clinal trait variation. Ultimately, our findings provide evidence that variable environmental conditions, such as early-season drought, may play a part in maintaining variation in germination timing