Evolutionary and plastic changes in a native annual plant after a historic drought

Severe droughts are forecast to increase with global change. Approaches that enable the study of contemporary evolution, such as resurrection studies, are valuable for providing insights into the responses of populations to global change. In this study, we used a resurrection approach to study the evolution of the California native Leptosiphon bicolor (true babystars, Polemoniaceae) across populations differing in precipitation in response to the state’s recent prolonged drought (2011-2017). In the Mediterranean-climate region in which L. bicolor grows, this historic drought effectively shortened its growing season. We used seeds collected both before and after this drought from three populations found along a moisture availability gradient to assess contemporary evolution in a common garden greenhouse study. We coupled this with a drought experiment to examine plasticity. We found evolution toward earlier flowering after the historic drought in the wettest of the three populations, while plasticity to experimental drought was observed across all three. We also observed trade-offs associated with earlier flowering. In the driest population, plants that flowered earlier had lower intrinsic water-use efficiency than those flowering later, which was an expected pattern. Unexpectedly, earlier flowering plants had larger flowers. Two populations exhibited evolution and plasticity toward smaller flowers with drought. The third exhibited evolution toward larger flowers, but displayed no plasticity. Our results provide valuable insights into differences among native plant populations in response to drought.

Methods
These data were collected as part of a resurrection type study to examine contemporary evolution in response to drought. Seeds were collected from three field populations of Leptosiphon bicolor (Polemoniaceae) varying in moisture availability both before and after a multi-year historic drought in California. Seeds were grown together in a greenhouse in a "refresher" generation to remove maternal and storage effects, and allowed to set seed. These seeds were then planted in a greenhouse experiment in which plants were given one of two watering treatments (low- and well-watered). We monitored plants daily to record germination date and flowering date. On the first flowering date, flower size was measured and plants were collected to measure aboveground biomass and perform stable carbon isotope analyses to assess intrinsic water-use efficiency. Date of death was recorded for any plants that died before flowering